Microsoft Exchange Servers Worldwide Hit by Stealthy New Backdoor

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Researchers have identified stealthy new malware that threat actors have been using for the past 15 months to backdoor Microsoft Exchange servers after they have been hacked.

Dubbed SessionManager, the malicious software poses as a legitimate module for Internet Information Services (IIS), the web server installed by default on Exchange servers. Organizations often deploy IIS modules to streamline specific processes on their web infrastructure. Researchers from security firm Kaspersky have identified 34 servers belonging to 24 organizations that have been infected with SessionManager since March 2021. As of earlier this month, Kaspersky said, 20 organizations remained infected.
Malicious IIS modules offer an ideal means to deploy powerful, persistent, and stealthy backdoors. Once installed, they will respond to specifically crafted HTTP requests sent by the operator instructing the server to collect emails, add further malicious access, or use the compromised servers for clandestine purposes. To the untrained eye, the HTTP requests look unremarkable, even though they give the operator complete control over the machine.

“Such malicious modules usually expect seemingly legitimate but specifically crafted HTTP requests from their operators, trigger actions based on the operators’ hidden instructions if any, then transparently pass the request to the server for it to be processed just like any other request,” Kaspersky researcher Pierre Delcher wrote. “As a result, such modules are not easily spotted by usual monitoring practices: they do not necessarily initiate suspicious communications to external servers, receive commands through HTTP requests to a server that is specifically exposed to such processes, and their files are often placed in overlooked locations that contain a lot of other legitimate files.”
Once SessionManager is deployed, operators use it to profile the infected environment further, gather passwords stored in memory, and install additional tools, including a PowerSploit-based reflective loader, Mimikat SSP, ProcDump, and a legitimate Avast memory dump tool. Kaspersky obtained multiple SessionManager variants that date back to at least March 2021. The samples show a steady evolution that has added more features with each new version.
 

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