An Autopsy on a Zombie In-the-Wild 0-day


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Apr 24, 2016
Whenever there’s a new in-the-wild 0-day disclosed, I’m very interested in understanding the root cause of the bug. This allows us to then understand if it was fully fixed, look for variants, and brainstorm new mitigations. This blog is the story of a “zombie” Safari 0-day and how it came back from the dead to be disclosed as exploited in-the-wild in 2022. CVE-2022-22620 was initially fixed in 2013, reintroduced in 2016, and then disclosed as exploited in-the-wild in 2022. If you’re interested in the full root cause analysis for CVE-2022-22620, we’ve published it here.

In the 2020 Year in Review of 0-days exploited in the wild, I wrote how 25% of all 0-days detected and disclosed as exploited in-the-wild in 2020 were variants of previously disclosed vulnerabilities. Almost halfway through 2022 and it seems like we’re seeing a similar trend. Attackers don’t need novel bugs to effectively exploit users with 0-days, but instead can use vulnerabilities closely related to previously disclosed ones. This blog focuses on just one example from this year because it’s a little bit different from other variants that we’ve discussed before. Most variants we’ve discussed previously exist due to incomplete patching. But in this case, the variant was completely patched when the vulnerability was initially reported in 2013. However, the variant was reintroduced 3 years later during large refactoring efforts. The vulnerability then continued to exist for 5 years until it was fixed as an in-the-wild 0-day in January 2022.
Usually when we talk about variants, they exist due to incomplete patches: the vendor doesn’t correctly and completely fix the reported vulnerability. However, for CVE-2022-22620 the vulnerability was correctly and completely fixed in 2013. Its fix was just regressed in 2016 during refactoring. We don’t know how long an attacker was exploiting this vulnerability in-the-wild, but we do know that the vulnerability existed (again) for 5 years: December 2016 until January 2022.

There’s no easy answer for what should have been done differently. The developers responding to the initial bug report in 2013 followed a lot of best-practices:
  • Patched all paths to trigger the vulnerability, not just the one in the proof-of-concept. This meant that they patched the variant that would become CVE-2022-22620.
  • Submitted a test case with the patch.
  • Detailed commit messages explaining the vulnerability and how they were fixing it.
  • Additional hardening measures during deserialization.
As an offensive security research team, we can make assumptions about what we believe to be the core challenges facing modern software development teams: legacy code, short reviewer turn-around expectations, refactoring and security efforts are generally under-appreciated and under-rewarded, and lack of memory safety mitigations. Developers and security teams need time to review patches, especially for security issues, and rewarding these efforts, will make a difference. It also will save the vendor resources in the long run. In this case, 9 years after a vulnerability was initially triaged, patched, tested, and released, the whole process had to be duplicated again, but this time under the pressure of in-the-wild exploitation.

While this case study was a 0-day in Safari/WebKit, this is not an issue unique to Safari. Already in 2022, we’ve seen in-the-wild 0-days that are variants of previously disclosed bugs targeting Chromium, Windows, Pixel, and iOS as well. It’s a good reminder that as defenders we all need to stay vigilant in reviewing and auditing code and patches.