Rust Team Warns Miri Cached Environment Variables Can Leak CI Secrets Through GitHub Actions

What happened

On 21 September 2026 the Rust Security Response Team published a notice about Miri, the interpreter used to check Rust code for undefined behaviour. To keep build-relevant settings between runs, Miri stored all environment variables into the target/ directory. When a CI workflow caches target/ and exposes that cache to pull requests, secrets present in the environment can be read by anyone who can trigger a PR build.

What we know

• The team says this is "not necessarily a vulnerability in and of itself", but combined with GitHub Actions caching it can expose secrets to PRs. A contributor whose first PR was approved can trigger later CI runs on every push and then hide the extraction commit by pushing over it.
• The short-term fix makes Miri keep only CARGO_* variables (excluding CARGO_*_TOKEN) and OUT_DIR. The post says the fix lands in the nightly dated 2026-09-22 and may not be on nightly at the time of writing.
• A scan of GitHub repositories found one repository with the issue and seven that did not appear vulnerable but should be cautious; maintainers were contacted. The team describes the scan as likely imperfect.
• You are affected if you run cargo miri in CI, the step has secrets in its environment, the workflow caches the target directory (for example with actions/cache or swatinem/rust-cache), and PRs can read that cache.
• Suggested quick fixes: disable the cache for that job, scope secrets to steps that do not call Miri, or temporarily disable Miri. Then clear the cache and consider rotating secrets that might have leaked.

What is not yet confirmed

The notice does not report any known exploitation. Whether the one affected repository found by the scan had secrets exposed is not stated.

Why it matters

The broader warning applies beyond Rust: many tools assume the whole environment can be written to disk, so any job that can write to a public cache should not have secrets in its environment. That is a CI hygiene rule worth checking in every repository that caches build output.