New audit: detecting potentially blasted caches (cache poisoning)

[ubiratansoares]

I was checking the roadmap yesterday and got to know about GHA cache poisoning attacks. Always learning, as usual 😄

I was wondering how could we add an initial support to this audit on zizmor. My first line of thought was simple :

• Start only with Github-provided actions with built-in caching(eg, the actions/cache/restore, actions/setup-java, etc)
• Detected dangerouns conditions attached to cache restoration (usually Workflow triggers, but eventually something else from the CI context)
• Assign a low confidence to anything we find
• Warn that caches are enabled by default and should be disabled when under those conditions

I got in touch with Adnan (from the blog post above), he drove a more generic recommendation to stay safe, but in the scope of zizmor, perphaps we can start simple and improve over time, eg by having a curated list of popular Actions built on top of actions/toolkit cache functionality.

@woodruffw Wdyt?

[woodruffw]

Thanks @ubiratansoares! Yeah, I think it would be great to begin experimenting here.

As you pointed out, a tricky thing with analyzing for cache poisoning is sensitivity: zizmor doesn't currently have enough context/detail in the analyses it does to say confidently that that a given workflow is vulnerable to cache poisoning, just potentially vulerable.

Given that, I think I'm OK with us having a pedantic-only audit for dangerous triggers + cache use for the time being, but I'd love it if we could more generally improve our analysis of dangerous-trigger workflows to be more confident that they're actually exploitable.

TL;DR: I'm good with adding this audit, but we should probably have it be pedantic-only for now. Alternatively, we could hold off on writing this audit and continue working on related audits that provide more context for dangerous triggers (like the GITHUB_ENV audit).

[ubiratansoares]

pedantic looks like the right fit, I totally agree with you!

I also agree that other audits can come first, happy to take another of them 🔥