KillBack EXPOSED: .killback Files, README.TXT, and a 24-Hour Inbox

You open Documents the way you did yesterday. The first photo should still be 1.jpg. The second shot should still be 2.png. The invoice should still be invoice-2026.pdf. The tax sheet should still be taxes.xlsx. Instead every familiar name has a long ID in braces and a second ending tacked on. 1.jpg is now 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. 2.png is now 2.png.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. 3.exe is now 3.exe.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. A small text file named README.TXT is sitting in the same folder, as if it has always belonged there.

That is the trap, not a glitch.

This page is for the hour those names appear. It is not a miracle key. It is a map of the trap, so the note does not become your incident plan.

Overview

KillBack ransomware locks the files first. Then it keeps the old name and the old type, hangs a victim ID in braces on the end, and finishes with .killback. In the sample used for this page that ID is {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}. Then it drops README.TXT and talks like a recovery desk. The note is a storefront. It shouts YOUR FILES ARE ENCRYPTED. It claims the lock happened because of weak security. It says only the writers of the note can restore the files. It tells you to write killback@mailum.com within 24 hours. It offers to decrypt one simple file for free as a test. It warns you not to rename the locked files, not to try third-party programs, and not to call recovery companies because those companies, it says, will raise the price. It says payment has to be Bitcoin. There is no wallet printed. There is no Session ID. There is no onion site. There is no printed $ figure. This page will not invent any of those blanks.

If you are staring at a pile of names that end in {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback and that text note, you are in a ransom incident, not a broken disk and not a Windows pop-up. The folder is the evidence. The note is a checkout. Stay with the folder, not with the script.

This is KillBack from the mid-2020s, the locker that brands files with a brace ID and .killback, then leaves README.TXT pointing at a Mailum inbox. It is not the older 2019 family covered in the published guide How To Remove {Killback@protonmail.com}KBK Ransomware. That older post is a different contact, a different ending, and a different decade. A search result that says Killback is not a match just because six letters look the same. If someone later quotes that older guide as if it already solved your folder, they mixed the names. Keep the Mailum address. Keep the brace ID. Keep .killback. Those three marks are how you tell them apart.

The first tell is the rename. In the sample used for this page, 1.jpg becomes 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. 2.png becomes 2.png.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. 3.exe becomes 3.exe.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. invoice-2026.pdf becomes invoice-2026.pdf.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. taxes.xlsx becomes taxes.xlsx.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. vacation.jpg becomes vacation.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. You can still read what the file used to be. You cannot open it. Windows may call the type a KILLBACK File and offer a useless “choose an app” box. An app cannot talk locked bytes back into a JPEG.

The second tell is the note. After the files are locked, the malware drops README.TXT into the same folders. README sounds like documentation. TXT sounds like something Notepad should handle. Put them together and the first English you can still read after the photos break is a sales flyer. The flyer is meant to be opened. It is not meant to be trusted.

The body of that sample is short on purpose. YOUR FILES ARE ENCRYPTED. All your files have been encrypted due to weak security. Only we can recover your files. You have 24 hours to contact us. Write killback@mailum.com. Send a simple file and they will decrypt one for free. Do not rename. Do not try third-party programs. Do not contact file recovery companies. Negotiate on your own. We accept Bitcoin cryptocurrency for payment. Email us at killback@mailum.com again. That is the whole costume. There is no printed wallet. There is no printed $ amount. There is no Session ID. There is no onion. The note does not need extra theater. A locked folder plus a 24-hour inbox plus a free test tease is already a shop.

There is no wallet string to redact, because the sample never printed one. This page will not invent a Bitcoin address to finish the sentence. A missing wallet is not a puzzle. It is how this flyer actually looks. If a later message shows you a long string and calls it the KillBack wallet, that string did not come from the original note. Treat it as a second shop, not as the missing line.

That is the whole pitch. A locked folder of names that carry {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E} and end in .killback. A README.TXT file that talks like a recovery brief. Weak security as a blame line. A monopoly claim. One inbox. A 24-hour fuse. A free test file. Warnings against rename, third-party tools, and recovery companies. Bitcoin with no wallet printed. There is no public free decryptor known for this strain. There is no reason to mail that inbox today because a text file told you to.

The ID plus .killback name is the first warning

Most lockers pick an extension so you notice the change in seconds. KillBack uses a victim ID in braces, then .killback. In this sample the ID is {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}. The extra cruelty is how readable the listing stays. You do not have to decode anything. You can still see 1.jpg inside 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. You can still see 2.png. You can still see the invoice year. You can still see the word taxes. The familiar words are still in the folder. The files are not familiar anymore.

Do not treat that readable name as a fix. Do not strip .killback off the end and expect Photos or Excel to open the file. Do not strip the brace ID and hope Windows forgets. Do not run a bulk “extension fixer” you found in an ad. The name is a label. The lock is in the content. Editing the label can make a later trusted tool have a harder time matching the file to what it was. Leave the locked copies as they are until you have a clean machine and a plan that does not start in a criminal inbox.

The extra ending also does advertising. Anyone who sees vacation.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback next to README.TXT can pretend they already have your case. A later fake helper will quote the ID back to you as if that were expertise. The ID is public. It is sitting in the folder. Quoting it proves nothing except that they can read a listing. Do not treat a stranger who recites {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E} as someone who already called the crew.

The ID is also a tag. It is meant to make you feel inventoried. A long brace string looks like a ticket. Tickets make people wait in lines. You do not have a ticket at a help desk. You have a filename the malware wrote. Copy that string into the incident record the way you would copy a serial. Then leave the files named as they are. Do not invent a second ID because a later form asks for one. This sample already printed the only ID it wanted you to see.

README.TXT is a sales floor, not a help file

The ransom note is a plain text file with a name that sounds like documentation. README.TXT wants the first click after the panic. Plenty of real software drops a README. This one uses that habit against you. It is meant to be the first English you can still read when the photos will not open.

Double-clicking it opens Notepad. That is why a panicked first click feels like opening the help page. It is not help. It is the demand, written so it looks official enough to follow. The filename is built to look like paperwork. README. TXT. An ending Windows already trusts. Paperwork is easier to obey than a threat.

Keep the file on disk. Then close it. Do not treat it as a set of steps you should follow. Do not tidy the filename. Do not shorten it to “the readme” when you write the incident down. Copy README.TXT the way you would copy a serial off a stolen laptop. That string is how a responder matches this locker. It is not a coupon you redeem by mailing killback@mailum.com.

If the same text is sitting in more than one folder, that is normal for this family. Lockers drop the same page in many places so you cannot miss it. Finding five copies is not five incidents. It is one incident with a loud flyer. Keep one clean copy as evidence. You do not need to open every copy. You do not need to compare them in mail to killback@mailum.com.

YOUR FILES ARE ENCRYPTED is a shout, not a lab report

The first line of the sample is not a diagnosis. It is a headline. YOUR FILES ARE ENCRYPTED does the same job a fake breaking-news banner does on a supplement page. It names the disaster in capital letters so the rest of the flyer feels like the only adult in the room. You are supposed to stop thinking and start obeying.

The shout is also a brand. Later search ads will quote it. Later helpers will paste it so they sound close to your folder. Anyone can copy a headline. Copying YOUR FILES ARE ENCRYPTED does not mean they can reverse .killback. It means they read the same note you did, or they read a screenshot of it.

The sample does not name a cipher. It does not print AES, RSA, or a key length. It just shouts that the files are encrypted. That is enough for the pitch. You do not need to argue cryptography with a text file. Leave the locked names as they are. The shout in that note is part of the costume. It is not a recovery procedure.

Weak security is a blame line, not a diagnosis

Right under the shout, the sample says all your files have been encrypted due to weak security. That sentence is not a pentest. It is a shove. You are supposed to feel that you caused this, so paying feels like cleaning up your own mess. Blame is cheaper than a printed $ amount. Blame makes a 24-hour inbox feel like adult supervision.

Do not accept that verdict from the people who locked the disk. Weak security is a slogan they can drop on every machine. Maybe the box was unpatched. Maybe it was not. You cannot audit their claim from Documents. You also cannot disprove it by mailing them. Writing killback@mailum.com does not get you a security report. It gets them a visitor.

Keep the line as an identifier. Encrypted due to weak security is how this flyer talks. A later helper who recites it is reading, not repairing. Do not let the blame line become the incident plan. The files are locked because malware ran. The note wants you to treat that as a personal failure you can buy your way out of. That is the product.

killback@mailum.com is checkout, not support

The recovery step in the sample is mail. Contact killback@mailum.com. The address appears twice, once next to the free test tease and once at the end. That is the whole door. It looks like support because support uses email. Criminal shops use email too, because a thread is where a 24-hour fuse becomes a negotiation and a locked photo becomes a sample they asked you to attach.

Mailum is a mailbox service. A Mailum address is not a company, a refund desk, or a registered office. It is a door they can abandon. Do not treat the domain as proof of a real shop. Do not look up Mailum just to see if the crew is still there. Looking it up is how you start their errand.

Do not write that inbox. Do not create a throwaway account just to see. Do not send a screenshot of Explorer to prove the lock. They already chose the extension. They already chose the note name. They already chose {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}. A photo of README.TXT sitting next to 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback does not make you a verified customer. It makes you a person still following the flyer.

24 hours is theater, not a clock

The sample says you have 24 hours to contact us. That is a fuse printed in plain text. It is not a timer your PC is running. It is not a court date. It is not a key that evaporates at hour 25. It is pressure. A one-day window is short enough to skip sleep and long enough to feel like you still have a chance if you write right now.

Do not treat 24 hours as a real clock. Do not set an alarm. Do not start a family countdown. Do not let a later helper say the price already went up because you missed the window. The original note never printed a second price. It never printed a first $ amount. A missed 24-hour line is still just a line. It is not a reason to mail killback@mailum.com at 23:59.

Silence is not rude here. Silence is how you stay out of their queue. A relative, an insurer, or an incident lead can still talk on a phone. The people who locked the files do not get a seat in that call. The 24-hour sentence is how they try to steal that seat before morning.

Decrypt one file for free is bait

The sample offers a demo. Send an email to killback@mailum.com and decrypt one file for free. We accept simple files as a test. They do not have to be important. That is a shop window. It is meant to feel scientific. It is meant to feel low risk. It is how they get the first attachment.

A demo that unlocks one junk file is not proof they can unlock the vacation photo. Anyone can open one file they control. Anyone can send back a picture you already had. Sending a simple file still sends them a file. Sending an unimportant file still starts a thread. The note already told you the test does not have to be important. That is how they lower the bar until you attach something.

Do not send a test file. Do not send 1.jpg from a copy you still have. Do not send a blank document to see. Do not let a later helper collect the test for you and mail it to killback@mailum.com on your behalf. The free decrypt line is bait. It is not a lab. It is not a reason to start their 24-hour clock by being punctual.

Do not rename, do not third-party tools, do not recovery companies

The warning block is three punches. Do not rename your encrypted files. Do not try to decrypt your data with third-party programs, it may cause irreversible data loss. Decrypting files with third-party programs may result in higher prices, they add their fees to ours, or you may become a victim of fraud. Then a fourth punch: do not contact file recovery companies. Negotiate on your own. No one but us can get your files back to you. If you contact a file recovery company, they will contact us. This will cost you dearly, because such companies take commissions.

Read that block as sales protection, not as a safety briefing. They want exclusive negotiation. They want you scared of every other adult in the room. They want the recovery-company warning to sound like they are protecting your wallet, when they are protecting their checkout. Higher prices is their phrase. Commissions is their phrase. Negotiate on your own is how they keep you in the Mailum thread.

The rename warning happens to match good evidence hygiene, and that is useful to them. Leaving .killback names alone is the right move for a later trusted tool. It is not the right move because a criminal flyer said so as a path to payment. Do not rename. Do not run a random decryptor from an ad. Do not hire a recovery company that found you through a search ad and then mails killback@mailum.com for you. Do not negotiate on your own with the people who locked the disk. Those are four different doors. The note wants you to pick theirs.

Bitcoin with no wallet printed

The sample ends the money talk in one sentence. We accept Bitcoin cryptocurrency for payment. That is the whole checkout. There is no wallet string. There is no 0.5 BTC line. There is no printed $ amount. There is no QR code. Public copies do not redact a wallet, because the flyer never showed one. This page will not invent a Bitcoin address, a Session ID, an onion, or a victim $ figure to finish that sentence.

A payment method without an address is a hook. You are supposed to write the inbox to collect the missing string. The missing string is the product. Do not ask an exchange what KillBack charges. Do not let a later helper generate a wallet and call it the original. Do not convert an invented $ amount in your head and decide it might be worth it. The published flyer did not print a price. Leave the price uninvented.

If a later message quotes README.TXT and then pastes a long wallet, that wallet is new. New is not original. New is how copycats and brokers finish a flyer that was left unfinished on purpose. Keep the original note as it is. Do not tidy a screenshot so a wallet looks like it always belonged there.

This is not the old KBK protonmail locker

The name is the other trap, and it lives in search. KillBack, brace ID, .killback, README.TXT, killback@mailum.com is this locker. Years earlier, a different family used Killback@protonmail.com and a KBK mark. That older strain already has a published guide titled How To Remove {Killback@protonmail.com}KBK Ransomware. That older post is not this incident. Mixing them is how you follow the wrong steps and miss the Mailum note sitting in your folder.

If a search ad, a forum reply, or a paid helper says they already covered Killback and then quotes a Proton Mail address, a KBK ending, or a decryptor that never mentions README.TXT and {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}, they are not talking about this folder. Ask for the Mailum inbox. Ask for the brace ID. Ask for .killback. If they cannot say those three things, they are reading the old guide, or they are guessing.

Do not let the older title become a comfort. How To Remove {Killback@protonmail.com}KBK Ransomware is a real published page about a different strain. It is not a free key for this locker. It is not a reason to ignore the 24-hour Mailum pitch in this note. Keep the marks when you write the incident down. KillBack, .killback, README.TXT, killback@mailum.com. That is the label a responder can match. KBK plus protonmail is the other story.

How The Attack Works

The shop is simple once you stop reading it as a rescue. Lock the files. Keep the old names so you recognize the loss. Brand each name with {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E} and .killback. Drop a README that looks like paperwork. Shout that the files are encrypted. Blame weak security. Claim only they can restore. Point the scared person at killback@mailum.com. Print a 24-hour fuse. Offer one free test file. Warn against rename, third-party tools, and recovery companies. Ask for Bitcoin. Skip the wallet. Skip the $ amount. Skip the Session ID. Skip the onion. Then wait for someone else to sell the missing string.

1. The lock keeps the old name and brands it with an ID

KillBack encrypts the files it wants as leverage, then rebrands them just enough. That is why 1.jpg is still readable inside 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. The operators want you to inventory the damage in the same breath you panic. They want the first minutes to feel like a rename you could undo. A coded filename with no original left can shock you into stopping. A readable filename plus a brace ID lets you keep clicking.

If only one folder looks wrong, do not assume the rest of the disk is safe. Ransomware walks trees. It hits Documents, Desktop, Pictures, Downloads, and any drive letter it can reach, including a USB stick you forgot was plugged in. Unplug extra disks. Do not plug in a copy just to check. Checking is how the extra copy gets the same ID and the same .killback ending.

Double-clicking a branded file does not bring the photo back. Windows may offer to pick an app for the KILLBACK type. Decline it. Excel may ask for a workbook that is still sitting in the same folder, only the bytes inside are no longer a workbook. That broken open is the first proof. The second proof is README.TXT in the same listing. Together they are enough. You do not need a Mailum thread to confirm what you are looking at.

Leave 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. Leave 2.png.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. Leave 3.exe.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. Leave invoice-2026.pdf.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. Leave taxes.xlsx.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. Leave vacation.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. The original name is already sitting in front of the ID. You do not need to edit it to know what you lost.

2. README.TXT turns a break-in into a ticket

Once the names are branded, the malware needs a voice. README.TXT is that voice. It sits in the same folders as the locked files so the first sort puts help next to harm. On many machines a text file named README sorts near the top. You open it because it looks like the only thing that still speaks English.

The body is short enough to finish before a relative walks into the room. Encrypted. Weak security. Only we can recover. 24 hours. killback@mailum.com. One free test file. Do not rename. Do not third-party tools. Do not recovery companies. Bitcoin. Each beat closes a door. The shout closes maybe Windows just glitched. The blame line closes maybe this is random. The monopoly closes maybe a local tool exists. The 24-hour line closes maybe I should wait overnight. The free test closes maybe I can check without paying. The Bitcoin line closes the money talk without even printing a wallet.

Notice what the note does not give you. It does not give you a company, a refund policy, or a way to prove the other side will ever send a key. It does not give you a wallet. It does not give you a $ amount. It does not give you a Session ID. It does not give you an onion. It gives you a tone that says this is already decided. That tone is the product. Read the file as a sales flyer. Then close it. Keep it on disk.

3. YOUR FILES ARE ENCRYPTED is the opening shout

The first line of the sample is a headline, not a technician. YOUR FILES ARE ENCRYPTED names the disaster in capital letters before you have even reached the contact line. You are supposed to feel outmatched. Capital letters do that work. Together with the branded filenames they make the inbox feel like the only adult move left.

The shout also travels. Search ads will recycle it. Commenters will paste it. A later KillBack helper will recite YOUR FILES ARE ENCRYPTED so they sound close to your Documents folder. Reciting a headline is not a decryptor. It is proof they can read. Keep the line as an identifier when you describe the incident. Do not treat it as a diagnosis you have to obey by mailing a Mailum address.

4. Weak security and only we can recover close the argument

After the shout, the note rests on two claims. All your files have been encrypted due to weak security. Only we can recover your files. That pair is meant to end the conversation you have not even started. Blame plus monopoly sounds like a problem you cannot beat at the kitchen table. It is still a sales paragraph. It is not a lab report. It is not a reason to skip copies that were never mounted. It is not a reason to skip a trusted decryptor project later if one appears.

You do not need to argue with a text file about who is allowed to restore a JPEG. The sample does not prove exclusive control. It asserts it. Leave 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback named as it is. Leave README.TXT where it is. Only we can recover is part of the pitch. It is not a recovery procedure.

The monopoly line also does advertising for later brokers. Anyone who says they already spoke to the only people who can restore KillBack files is selling access to a door you should not walk through. Exclusive is their word. It is not a reason to hire a middleman. It is not a reason to write the inbox yourself.

5. killback@mailum.com is the door they want you to walk through

The recovery step in the sample is mail. To contact us, you need to write to the mailbox below, then killback@mailum.com, then the same address again at the bottom. That is the whole door. It looks like support because support uses email. Criminal shops use email too, because a thread is where a 24-hour fuse becomes a login and a free test becomes an attachment they control.

Do not write that inbox. Do not create a throwaway identity just to see. Do not send a screenshot of Explorer to prove the lock. They already chose .killback. They already chose README.TXT. They already chose {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}. A photo of those marks does not make you a verified customer. It makes you a person still following the flyer.

If someone later says they will talk to KillBack for you through that same address, that is a broker pitch. Brokers need you in the same funnel. You do not owe them a seat. Report the incident through official desks. Let counsel decide if contact is ever required for a specific legal reason. That decision is not yours to make from the Documents folder at midnight.

6. You have 24 hours is a fuse they printed

Plenty of lockers hide the timer until you write. This one prints 24 hours up front. That is not kindness. A printed fuse removes the last excuse for delay. You do not have to ask how long you have. You already know what they want you to feel. The remaining step is the mail. The note has already done the quoting.

Short fuses also travel well. A 24-hour ask is easy to repeat in a family group, easy to treat like a missed bill, easy to round into maybe we should just write them tonight. It is still a countdown the criminals invented. It still does not produce a warranty. Fast is how they get people who think of themselves as too careful to wait.

Do not start that clock. Do not treat hour 24 as a cliff. Do not let a later message say the files will be deleted, leaked, or repriced because you missed a fuse the original note never connected to a second penalty. The published flyer printed 24 hours to contact us. It did not print a leak site. It did not print a second $ price. Leave those missing penalties uninvented.

7. Decrypt one file for free is how they get the first attachment

The sample wants a parcel. To make sure we have a decryptor and it works, you can send an email to killback@mailum.com and decrypt one file for free. We accept simple files as a test. They do not have to be important. That paragraph is a loading dock. Simple is the word that makes a locked photo feel safe to send. Free is the word that makes the 24-hour fuse feel like a trial, not a ransom.

Do not send the test. Do not pick the least important file as if that were clever. Do not send a file you still have in email, hoping they will prove themselves against a copy you already control. Proof is their word. A returned file is not Photos opening vacation.jpg again. It is a conversation. Conversations are how prices appear. The original note never printed a price. A thread is where they get to invent one.

If a later helper offers to run the test for you, they still need the attachment. Do not hand it over. Do not hand over README.TXT plus a locked sample to a stranger who found you through a search ad, even if they promise not to mail the crew. The sample already told you they will offer to check your files as proof. That offer is in the warning block next to the recovery-company scare. It is still bait.

8. Do not rename, do not third-party programs, do not recovery companies

The warning list is written like safety. It is sales. Do not rename your encrypted files keeps the brand intact so a later tool, or a later broker, can still see .killback. Do not try third-party programs, it may cause irreversible data loss, plus higher prices because they add their fees to ours, is how they poison every other option. Do not contact file recovery companies. Negotiate on your own. No one but us can get your files back. If you contact a file recovery company, they will contact us. This will cost you dearly. Because such companies take commissions.

They even promise to check your files as proof, so you will not need those companies. That proof is the same free test in a different sentence. It is still an attachment. It is still their inbox. It is still the 24-hour door.

Do not hire the recovery company the note is scared of, and do not take that fear as a reason to negotiate alone. The note wants a private Mailum thread. A search-ad recovery shop wants a deposit. A trusted incident lead wants the folder left as evidence. Those are not the same people. Stay with the folder. Do not complete anyone’s checkout because a flyer mentioned commissions and higher prices.

9. Bitcoin with no wallet, no Session ID, and no onion

The money line is one sentence. We accept Bitcoin cryptocurrency for payment. Then the inbox again. That is the whole register. There is no wallet to copy. There is no QR. There is no 0.5 BTC. There is no printed $ amount. There is no Session ID. There is no onion site. This page will not invent those marks to make the flyer look complete.

That gap is where copycats work. Someone will publish a long wallet string and call it the KillBack address. Someone will sell a connector that is just more malware. Someone will ask you to run a command so they can reach the operator on your behalf. None of that is a reason to finish their sentence. The sample used for this page does not reprint a wallet, because the sample never had one. Do not invent one.

A Bitcoin line without an address is an identifier for a responder. It tells them this flyer stops at the method. It is not a payment instruction you should carry out. It is not a password you redeem by mailing killback@mailum.com. Copy the sentence into the incident record. Then leave it alone. Do not type Bitcoin into an exchange search next to the word KillBack. Do not ask a nephew to price a ransom the note never numbered.

10. Third parties will sell a KillBack conversation

After a ransomware incident, search results fill up with companies that say they can decrypt KillBack or all .killback files. Some of those shops are ordinary overpriced consultants. Plenty are a second scam. They will quote README.TXT so they sound close to the case. They will mention weak security. They will mention 24 hours. They will mention killback@mailum.com as if they already called the original crew. They will ask for a sample file, a remote-access session, or a deposit.

A demo that unlocks one file is not proof they can unlock the vacation photo. Anyone can open one junk file they control. Anyone can send you a wallet string that makes an exchange window appear. An exchange window is not Photos opening vacation.jpg again. If a later message skips the original inbox and asks for coin over ordinary mail, that is still the same market, or a copycat standing in the same doorway. Do not answer. Do not help them set a wallet the original note never printed.

A real public decryptor, when one exists, shows up on an official project page that already existed before your incident. It does not need a Bitcoin send. It does not need killback@mailum.com. If the pitch is we can decrypt, pay us first, you are still in the market the ransomware created. There is no public free decryptor known for this strain. Until a trusted project publishes one, treat any site that says we already have the KillBack key as a second sales pitch.

Watch the name mix in those pitches. Helpers who only say Killback and then quote Killback@protonmail.com or a KBK ending are recycling the old published guide. Helpers who cannot show README.TXT, killback@mailum.com, and a brace ID plus .killback are not close to this folder. Quote those three marks back to them and stop. If they change the subject, they were never matching your disk.

11. Payment and mailing that inbox close nothing

Paying a stranger for a private key is not a guarantee. The people who send the note can take the Bitcoin and disappear. They can send a string that only makes a decryptor show an error. They can come back later and lock the same machine again, still hanging {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E} on the names. A mail thread makes that outcome easier, not harder. You cannot leave a review on a Mailum inbox.

Mailing killback@mailum.com is the free version of the same dead end. There is no public free decryptor known for this strain. Starting a thread does not rebuild a JPEG. It only keeps you in their story while the rest of the disk still needs a real plan. Paying a $ amount they never printed in the original note is still a payment to the people who locked vacation.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. It still funds the next copy of the same locker. It still does not produce a warranty.

If this is a company machine, tell counsel and your incident lead. If this is a home PC, tell the people whose files lived on it. Change passwords from a clean device. Watch bank and email alerts. Opening that inbox again is not useful work. It is the checkout, even when the wallet was never printed and the 24-hour line was only theater.

What To Do

Do not pay. Do not mail killback@mailum.com. Do not treat 24 hours as a real clock. Do not invent a Bitcoin address to finish README.TXT. Do not invent a Session ID. Do not invent an onion. Do not invent a victim $ figure. Do not send a test file. Do not send proof of payment. If you already sent a message before you found this page, stop there. Do not pay a deposit to hold a key for anyone who quotes the note. Do not treat the older How To Remove {Killback@protonmail.com}KBK Ransomware guide as a fix for this folder.

• STEP 1: Use Rkill to terminate suspicious programs.
• STEP 2: Use Malwarebytes to remove for Ransomware and Unwanted Programs
• STEP 3: Use HitmanPro to remove Trojans and other Malware
• STEP 4: Use AdwCleaner to remove Malicious Browser Extensions and Adware
• STEP 5: Perform a final check with ESET Online Scanner
• STEP 6: Restore the files encrypted by ransomware

STEP 1: Use Rkill to terminate suspicious programs.

In this first step, we will download and run Rkill to terminate suspicious programs that may be running on your computer.

RKill is a program that was developed at BleepingComputer.com that attempts to terminate known malware processes so that your normal security software can then run and clean your computer of infections. When RKill runs it will kill malware processes and then removes incorrect executable associations and fixes policies that stop us from using certain tools.

1. Download Rkill.

   You can download RKill to your computer from the below link. When at the download page, click on the Download Now button labeled iExplore.exe. We are downloading a renamed version of Rkill (iExplore.exe) because some malware will not allow processes to run unless they have a certain filename.

   

   RKILL DOWNLOAD LINK
   (The above link will open a new page from where you can download Rkill)

2. Run RKill.

   After downloading, double-click the iExplore.exe icon to kill malicious processes. In most cases, downloaded files are saved to the Downloads folder.
   The program may take some time to search for and end various malware programs.

   

   When it is finished, the black window will close automatically and a log file will open. Do not restart your computer. Proceed to the next step in this guide.

STEP 2: Use Malwarebytes to remove Ransomware and Unwanted Programs

In this second step, we will install Malwarebytes to scan and remove any infections, adware, or potentially unwanted programs that may be present on your computer.

Malwarebytes is one of the most popular and trusted anti-malware tools for Windows — and it’s completely free for removing infections. It catches threats that many antivirus programs miss, including adware, browser hijackers, and trojans. Follow the steps below to scan and clean your PC in just a few minutes.

1. Download Malwarebytes

   Click the button below to download the latest version of Malwarebytes for Windows from the official source. The free version is all you need — it will scan your computer and remove adware, browser hijackers, and other malicious software at no cost.

   

   DOWNLOAD MALWAREBYTES FOR WINDOWS (FREE)
   
   (The link opens in a new page where your download will start)

2. Install Malwarebytes

   When the download finishes, open your Downloads folder and double-click the MBSetup file. If Windows shows a User Account Control pop-up, click “Yes” to allow the installation.

   

3. Follow the On-Screen Prompts to Install Malwarebytes

   The setup wizard will walk you through a few quick screens:

   • Choose where you’re installing the program — “Personal Computer” or “Work Computer” — then click Next.

     

   • Malwarebytes will now install on your device. This usually takes under a minute.

     

   • When installation is complete, the “Welcome to Malwarebytes” screen will open automatically.

     

   • On the final screen, click Open Malwarebytes to launch the program.

     

4. Enable “Scan for Rootkits”

   Before scanning, turn on rootkit detection so Malwarebytes can find even the most hidden threats. Click the Settings gear icon on the left side of the screen.

   

   In the settings menu, find “Scan for rootkits” and click the toggle so it turns blue.

   

   Done? Click “Dashboard” in the left pane to return to the main screen.

5. Start the Scan

   Click the blue Scan button. Malwarebytes will automatically update its virus database and start checking your computer for malware.

   

6. Wait for the Scan to Finish

   The scan checks your entire system for browser hijackers and other malicious programs, so it can take several minutes. Feel free to do something else — just check back occasionally to see the progress.

   

7. Quarantine the Detected Threats

   When the scan is done, you’ll see a list of everything Malwarebytes found — malware, adware, and potentially unwanted programs. Click the “Quarantine” button to remove all of them at once.

   

   Malwarebytes will now remove the malicious files and registry entries and move them safely into quarantine.

   

8. Restart Your Computer

   Some threats can only be fully removed after a reboot. If Malwarebytes asks you to restart, click Yes. Once you’re logged back in, your PC is clean and you can continue with the next steps in this guide.

   

STEP 3: Use HitmanPro to remove Rootkits and other Malware

In this third step, while the computer is in normal back, we will download and run a scan with HitmanPro to remove Trojans, rootkits, and other malicious programs.

HitmanPro is a second-opinion scanner — it’s designed to catch what your main antivirus might have missed. Instead of relying on a single detection engine, it checks the behavior of files in the locations where malware usually hides. Anything suspicious gets sent to the cloud, where it’s analyzed by two of the best antivirus engines available: Bitdefender and Kaspersky.

Good news: scanning is completely free, with no limits. You only need a license when it’s time to remove what was found — and even then, you can activate a free one-time 30-day trial to clean your PC at no cost. (A full license is $24.95 per year for 1 PC.)

1. Download HitmanPro

   Click the button below to download HitmanPro. Remember — the scan is free, so you have nothing to lose by checking your PC.

   DOWNLOAD HITMANPRO (FREE SCAN)
   (The link opens in a new page where your download will start)

2. Install HitmanPro

   When the download finishes, open your Downloads folder and double-click the file: “hitmanpro.exe” on 32-bit Windows, or “hitmanpro_x64.exe” on 64-bit Windows.

   

   If a User Account Control pop-up asks whether HitmanPro can make changes to your device, click “Yes” to continue.

   

3. Follow the On-Screen Prompts

   On the HitmanPro start screen, click “Next” to begin the system scan. No lengthy setup required — it goes straight to work.

   

   

4. Wait for the Scan to Finish

   HitmanPro will now check your computer for malicious programs. This usually takes just a few minutes thanks to its cloud-based scanning.
   

5. Review the Results and Click “Next”

   When the scan is done, HitmanPro will show you everything it found. Click “Next” to remove the detected threats.

   

6. Click “Activate Free License”

   To remove the malicious files, click the “Activate free license” button. This starts your free 30-day trial — no payment details needed — and unlocks the full cleanup.
   

   When the removal is complete, HitmanPro will show a summary of everything it cleaned. Click Next, then click Reboot if prompted. If there’s no reboot prompt, just click Close — your PC is clean.

STEP 4: Use AdwCleaner to remove Malicious Browser Extensions and Adware

In this next step, we will use AdwCleaner to remove malicious browser policies and unwanted browser extensions from your computer.

AdwCleaner is a free on-demand scanner that specializes in adware, browser hijackers, and unwanted toolbars — the exact threats that mainstream antivirus programs often miss. It also includes tools that repair the damage malware leaves behind, like hijacked browser settings and malicious policies. It’s a quick scan that’s well worth running.

1. Download AdwCleaner

   Click the button below to download AdwCleaner — it’s free, portable, and requires no installation.

   

   DOWNLOAD ADWCLEANER (FREE)
   (The link opens in a new page where your download will start)

2. Run AdwCleaner

   Open your Downloads folder and double-click the file named “adwcleaner_x.x.x.exe“. There’s no installation — the program starts right away.
   

   If Windows asks whether you want to allow AdwCleaner to run, click “Yes“. When the license agreement appears, click I agree to continue.

   

3. Enable “Reset Chrome policies”

   This setting removes malicious browser policies — a trick malware uses to lock your browser settings so you can’t change them back. Click “Settings” on the left side of the window, then turn on “Reset Chrome policies“.

   

4. Start the Scan

   Click “Dashboard” on the left side of the window, then click the “Scan” button.

   

5. Wait for the Scan to Finish

   AdwCleaner will now check your computer for adware and other malware. This usually takes only a few minutes — it’s one of the fastest scanners around.

   

6. Quarantine the Detected Threats

   When the scan finishes, AdwCleaner will list everything it found. Click the “Quarantine” button to remove all the malicious items at once.

   

7. Click “Continue” to Finish the Cleanup

   Save any open work first — AdwCleaner needs to close your open programs before it can clean. When you’re ready, click the “Continue” button.
   

   AdwCleaner will now delete all detected malware from your computer. If it asks you to restart your PC, allow it — your computer will be clean when you log back in.

STEP 5: Perform a final check with ESET Online Scanner

This final step involves installing and running a scan with ESET Online Scanner to check for any additional malicious programs that may be installed on the computer..

ESET Online Scanner is a free second-opinion scanner that performs a deep, full-system check for viruses, trojans, rootkits, and other malware. We use it as the final step because it’s thorough — if anything slipped past the previous scans, ESET will find it. A clean result here means your computer is malware-free.

1. Download ESET Online Scanner

   Click the button below to download ESET Online Scanner.

   

   DOWNLOAD ESET ONLINE SCANNER (FREE)
   (The link opens in a new page where your download will start)

2. Run the Installer

   When the download finishes, open your Downloads folder and double-click “esetonlinescanner.exe“.
   

3. Install ESET Online Scanner

   On the start screen, select your language from the drop-down menu and click Get started.

   

   On the Terms of use screen, click Accept.
   

   Choose your preferences for the Customer Experience Improvement Program and the Detection feedback system (either choice is fine), then click Continue.
   

4. Start a Full Scan

   Click Full Scan — this checks your entire computer, not just the common hiding spots.

   

   Select Enable for Detection of Potentially Unwanted Applications — this lets ESET catch adware and bundled junk programs, not just viruses. Then click Start scan.

   

5. Wait for the Scan to Finish

   ESET will now check every file on your computer. Because it’s a full scan, this can take a while — often an hour or more, depending on how much data you have. Leave it running in the background and check on it from time to time.

   

6. Review the Results

   When the scan completes, the Found and resolved detections screen appears. Any threats found were automatically cleaned and quarantined — there’s nothing extra you need to do. Click View detailed results if you want to see exactly what was removed.
   

   If ESET found nothing — congratulations, your computer has passed the final check and is malware-free.

STEP 6: Restore the files encrypted by ransomware

Unfortunately, in most cases, it’s not possible to recover the files encrypted by this ransomware virus because the private key which is needed to unlock the encrypted files is only available through the attackers. However, below we’ve listed three options you can use to try and recover your files.

Option 1: Search a decryption tool for this ransomware

The cybersecurity community is constantly working to create ransomware decryption tools, so you can try to search these sites for updates:

• https://id-ransomware.malwarehunterteam.com/
• https://decrypter.emsisoft.com/
• https://noransom.kaspersky.com/
• https://www.avast.com/ransomware-decryption-tools

Option 2: Use EaseUS Data Recovery Wizard Free to recover the encrypted files

EaseUS Data Recovery Wizard Free can restore files and repair corrupted files with simple clicks. Its powerful scanning algorithms can identify and retrieve huge file type library, including all of the popular video files, audio files, photos, and document formats.
While the free version only allows you to recover 2 GB of data, this can be helpful to see if the recovery is possible and restore back the most important files from your computer.

1. Download EaseUS Data Recovery Wizard Free.

   You can download EaseUS Data Recovery Wizard Free by clicking the link below.

   

   EASEUS DATA RECOVERY WIZARD FREE DOWNLOAD LINK
   (The above link will open a new page from where you can download EaseUS Data Recovery Wizard)

2. Double-click on the EaseUS Data Recovery Wizard Free setup file.

   When EaseUS Data Recovery Wizard Free has finished downloading, double-click on the setup file to install EaseUS Data Recovery Wizard on your computer. In most cases, downloaded files are saved to the Downloads folder.

   

   You may be presented with a User Account Control pop-up asking if you want to allow EaseUS to make changes to your device. If this happens, you should click “Yes” to continue with the EaseUS Data Recovery Wizard Free installation.

3. Follow the on-screen prompts to install EaseUS Data Recovery Wizard.

   When the EaseUS Data Recovery Wizard installation begins, click on the “Install Now” as seen in the image below.
   

   When your EaseUS Data Recovery Wizard installation completes, click the “Start Now” button to start the program.
   

4. Select a location to start recovering the encrypted files.

   Choose the drive or folder where you are the encrypted files that you want to recover and click “Scan“.
   

5. Wait for the EaseUS Data Recovery Wizard scan to complete.

   EaseUS Data Recovery Wizard will now scan your computer files that can be restored. This process can take a few minutes, so we suggest you do something else and periodically check on the status of the scan to see when it is finished.
   

6. Find the files you want to recover.

   When the EaseUS Data Recovery Wizard scan is finished scanning it will show a screen that displays the files that can be recovered. This tool can recover a lot of data, use the “Filter” button to quickly filter specific file types and find the files that you want to recover.
   

   Click the “Preview” button or double-click on a file for a full preview.
   

7. Select your files and click “Recover”.

   Finally, select the the files you want to recover and click “Recover“.
   
   Choose a safe location to save all the files.
   
   The free version only allow you to recover 2 GB of data, however, this will allow you to recover the most important files and see if EaseUS Data Recovery Wizard can correctly recover them.
   

Option 3: Try to restore your files with ShadowExplorer

This ransomware will attempt to delete all shadow copies when you first start any executable on your computer after becoming infected. Thankfully, the infection is not always able to remove the shadow copies, so you should continue to try restoring your files using this method.

1. Download ShadowExplorer.

   You can download ShadowExplorer from the below link.

   SHADOW EXPLORER DOWNLOAD LINK
   (This link will open a new web page from where you can download “ShadowExplorer”)

2. Install ShadowExplorer.

   Double-click on the ShadowExplorer-x.x-setup file to start the installation process, then follow the on-screen promts to install this program.
   

3. Select snapshot date.

   Open ShadowExplorer and then from the top bar select the drive where the files that you want to save are located, then select from the snapshot available one previous to this infection.

   

4. Export the files that you want to recover.

   Once you have found a copy of the original file or folder, right-click on it and the select “Export”. A window will prompt you where you want to save the file or folder.
   

Keep every .killback name exactly as it is. Do not bulk-rename 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback back to 1.jpg, or 2.png.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback back to 2.png, or 3.exe.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback back to 3.exe, or invoice-2026.pdf.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback back to a normal PDF, or taxes.xlsx.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback back to a spreadsheet, or vacation.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback back to a photo. Stripping the brace ID or the extra ending is not a repair. It can make a later trusted tool worse, not better. The original name is already sitting in front of {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. You do not need to edit it to know what you lost.

Keep README.TXT with the locked files. Keep one .killback sample next to it. Do not clean up every copy of the note. Do not fill in a wallet so the flyer looks complete. Do not fill in a Session ID or an onion so the flyer looks complete. The missing wallet is part of how you prove this was this strain and not a different locker that only sounds similar. Keep the contact marks as identifiers: killback@mailum.com, the 24-hour line, the weak-security sentence, the free test tease, and the Bitcoin line with no address. Those artifacts are evidence. They are not instructions you should carry out.

Do not mail that inbox. Do not ask a friend to write them just to see. Do not treat 24 hours as a clock you are late for. A calm Mailum address is still the people who locked the files. A printed fuse is still theater. If a later message quotes README.TXT and offers the real wallet, that is still the same shop. If a later message skips the original inbox and asks for Bitcoin over a different address, that is still the same shop, or a copycat standing in the same doorway.

Do not send a test file. The note’s free decrypt is bait. Do not attach 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback or any other locked sample to a stranger who recites {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}. If someone claims they can already decrypt KillBack files for a fee because they can recite weak security, 24 hours, and Bitcoin, they are selling a report, not a key. If they mention killback@mailum.com as proof they already called the crew, they read the same note you did. Do not answer. Do not complete their checkout. Do not help them fix the flyer.

Do not pay. Do not invent a price. Do not invent a wallet. If Windows still offers to pick an app for the .killback type, decline it. There is no player for that ending.

This is not the old KBK post. How To Remove {Killback@protonmail.com}KBK Ransomware is a different family and a different contact. KillBack, brace ID, .killback, README.TXT, killback@mailum.com is this locker. If a helper only knows the older title, they are not matching your folder.

The Bottom Line

KillBack ransomware is a locker with a readable filename, a README that shouts ENCRYPT, and a checkout that names a 24-hour Mailum inbox. It keeps the old name, appends a victim ID, then .killback. In the sample used here, 1.jpg becomes 1.jpg.{C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback. The note is README.TXT. It claims weak security, says only they can restore, points you at killback@mailum.com, offers one free test file, warns you off rename, third-party tools, and recovery companies, and says pay Bitcoin. There is no printed wallet. There is no printed $ amount. There is no Session ID. There is no onion. There is no public free decryptor known for this strain. This is not the older KBK protonmail locker in How To Remove {Killback@protonmail.com}KBK Ransomware.

Do not pay. Do not mail that inbox. Do not treat 24 hours as a real clock. Do not invent a wallet. Do not send a test file. Do not rename the pile in bulk. If you take one sentence with you, take this. A folder of names ending in {C5933A6B-53E5-1DE0-31E9-880B5ACAC94E}.killback, plus README.TXT with a weak-security shout, a 24-hour fuse, a free test tease, Bitcoin with no address, and killback@mailum.com, is ransomware on the first sighting. Treat it that way before anyone offers to fill a wallet the original note never printed, and before anyone hands you the old KBK guide as if the Mailum inbox did not matter.