Arbitrary code execution when rerunning tests from a portable recording

Summary

## Summary

Nextest versions between 0.9.125 and 0.9.149 (inclusive) allow the `cargo nextest run --rerun` (or `-R`) command with a malicious [portable recording](https://nexte.st/docs/features/record-replay-rerun/portable-recordings/) to run arbitrary code during the build. The result is arbitrary code execution with the privileges of the user running `cargo nextest`.

This vulnerability requires that the experimental [run recording](https://nexte.st/docs/features/record-replay-rerun/) functionality is enabled. An attacker can plausibly trick a victim into enabling this feature.

## Details

Nextest's portable recordings contain [build scope arguments](https://nexte.st/docs/design/architecture/recording-runs/#build-scope-arguments). These arguments are deserialized verbatim from portable recordings, and are [not validated](https://github.com/nextest-rs/nextest/blob/70ba2393a05f29c54b72b4badb9635a86f162eb1/nextest-runner/src/record/format.rs#L421-L426). As a result, it is possible to construct an archive with a custom `--config` argument that causes Cargo to execute arbitrary code.

Nextest 0.9.150 records the build scope as structured data, and passes each value to Cargo joined to its flag (for example, `--package=<value>`), so a recording can no longer add other Cargo options.

## Impact

A malicious portable recording leads to arbitrary code execution as the user who runs `cargo nextest run --rerun`, during the build phase, before any test runs.

- The injected configuration takes effect during `cargo build`, so the payload is run even on a warm build cache, and even when no tests are ultimately executed. Inspecting the recorded tests (with e.g. `cargo nextest replay`) before running them does not help, since the code has already run by the time the tests would start. - Arbitrary code execution happens as the invoking user, without a prompt or trust warning. The only signal is a `rerun: inheriting build scope…` line. On CI, this includes access to whatever secrets are accessible to the job, including repository tokens and deployment credentials. - A malicious payload can invoke the real compiler afterwards, so the build and tests complete normally. The recording is otherwise a valid, ordinary recording. - The full Cargo `--config` surface is exposed. This includes `build.rustc-wrapper`, `build.rustc`, `target.*.linker`, `target.*.runner`, `env.*`, and `patch.*` and `source.*.replace-with` (redirecting dependencies to an attacker-controlled repository or registry). - The delivery mechanism matches the feature's intended use, since portable recordings are designed to be shared across machines. For example, an attacker could pose as a user asking a maintainer for help triaging their flaky test report. Because `--rerun` also accepts process substitution, `cargo nextest run --rerun <(curl …)` would fetch the recording directly from a URL.

Exploitation requires convincing the victim to run `cargo nextest run --rerun` against an attacker-supplied portable recording. The rerun feature is experimental and must be enabled, via e.g. `NEXTEST_EXPERIMENTAL_RECORD=1`.

## Remediations

- Upgrade cargo-nextest to version 0.9.150 or above. - In your repositories, set the [minimum required version](https://nexte.st/docs/configuration/minimum-versions/) to 0.9.150. In `.config/nextest.toml`: ```toml nextest-version = "0.9.150" ``` This way, anyone who interacts with your repository will be notified to upgrade their version of nextest. - If you are using an affected version, do not enable the experimental run recording feature.

## Credit

Reported by [inanna-malick](https://github.com/inanna-malick).