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Rand is unsound with a custom logger using rand::rng()GHSA-cq8v-f236-94qc

Fix: rust-random/rand#1763

GHSA-cq8v-f236-94qc is a security vulnerability in rand. A fix is available for rand — see the affected versions and patch details below.

Also known asRUSTSEC-2026-0097
Published
Updated
Affected
3 pkgs
Patched
3 / 3
Exploits
None indexed
Exploitation data as of Sep 10, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

3 pkgs affected
🦀rand🦀rand🦀rand

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.

Description

It has been reported (by @lopopolo) that the rand library is unsound (i.e. that safe code using the public API can cause Undefined Behaviour) when all the following conditions are met:

  • The log and thread_rng features are enabled
  • A custom logger is defined
  • The custom logger accesses rand::rng() (previously rand::thread_rng()) and calls any TryRng (previously RngCore) methods on ThreadRng
  • The ThreadRng (attempts to) reseed while called from the custom logger (this happens every 64 kB of generated data)
  • Trace-level logging is enabled or warn-level logging is enabled and the random source (the getrandom crate) is unable to provide a new seed

TryRng (previously RngCore) methods for ThreadRng use unsafe code to cast *mut BlockRng<ReseedingCore> to &mut BlockRng<ReseedingCore>. When all the above conditions are met this results in an aliased mutable reference, violating the Stacked Borrows rules. Miri is able to detect this violation in sample code. Since construction of aliased mutable references is Undefined Behaviour, the behaviour of optimized builds is hard to predict.

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iorand≥ 0.9.0&&< 0.9.30.9.3cargo update -p rand --precise 0.9.3
🦀crates.iorand≥ 0.10.0&&< 0.10.10.10.1cargo update -p rand --precise 0.10.1
🦀crates.iorand≥ 0.7.0&&< 0.8.60.8.6cargo update -p rand --precise 0.8.6

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rand, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update rand to 0.9.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-cq8v-f236-94qc is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

It has been reported (by @lopopolo) that the `rand` library is [unsound](https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#soundness-of-code--of-a-library) (i.e. that safe code using the public API can cause Undefined Behaviour) when all the following conditions are met: - The `log` and `thread_rng` features are enabled - A [custom logger](https://docs.rs/log/latest/log/#implementing-a-logger) is defined - The custom logger accesses `rand::rng()` (previously `rand::thread_rng()`) and calls any `TryRng` (previously `RngCore`) methods on `ThreadRng` - The `ThreadRng` (attempts to
O3 Security · Impact-Aware SCA

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Rand is unsound with a custom logger using rand::rng()