Letting users block injected third-party DLLs in Firefox

[harlan4096]

In Firefox 110, users now have the ability to control which third-party DLLs are allowed to load into Firefox processes.

Let’s talk about what this means and when it might be useful.

What is third-party DLL injection?On Windows, third-party products have a variety of ways to inject their code into other running processes. This is done for a number of reasons; the most common is for antivirus software, but other uses include hardware drivers, screen readers, banking (in some countries) and, unfortunately, malware.

Having a DLL from a third-party product injected into a Firefox process is surprisingly common – according to our telemetry, over 70% of users on Windows have at least one such DLL! (to be clear, this means any DLL not digitally signed by Mozilla or part of the OS).

Most users are unaware when DLLs are injected into Firefox, as most of the time there’s no obvious indication this is happening, other than checking the about:third-party page.

Unfortunately, having DLLs injected into Firefox can lead to performance, security, or stability problems. This is for a number of reasons:

• DLLs will often hook into internal Firefox functions, which are subject to change from release to release. We make no special effort to maintain the behavior of internal functions (of which there are thousands), so the publisher of the third-party product has to be diligent about testing with new versions of Firefox to avoid stability problems.
  
• Firefox, being a web browser, loads and runs code from untrusted and potentially hostile websites. Knowing this, we go to a lot of effort to keep Firefox secure; see, for example, the Site Isolation Security Architecture and Improved Process Isolation. Third-party products may not have the same focus on security.
  
• We run an extensive number of tests on Firefox, and third-party products may not test to that extent since they’re probably not designed to work specifically with Firefox.
  

Indeed, our data shows that just over 2% of all Firefox crash reports on Windows are in third-party code. This is despite the fact that Firefox already blocks a number of specific third-party DLLs that are known to cause a crash (see below for details).

This also undercounts crashes that are caused indirectly by third-party DLLs, since our metrics only look for third-party DLLs directly in the call stack.

Additionally, third-party DLLs are a bit more likely to cause crashes at startup, which are much more serious for users.

Firefox has a third-party injection policy, and whenever possible we recommend third parties instead use extensions to integrate into Firefox, as this is officially supported and much more stable.
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