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CRITICAL severity

CVE-2020-25576 — rand_core

CRITICAL

CVE-2020-25576 is a critical-severity (CVSS 9.8) CWE-704 vulnerability in rand_core. A fix is available for rand_core — see the affected versions and patch details below.

Unaligned memory access in rand_core

Also known asGHSA-mmc9-pwm7-qj5wRUSTSEC-2019-0035
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2020-25576 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected
🦀rand_core🦀rand_core

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

Impact

Affected versions of this crate violated alignment when casting byte slices to integer slices, resulting in undefined behavior. rand_core::BlockRng::next_u64 and rand_core::BlockRng::fill_bytes are affected.

Patches

The flaw was corrected by Ralf Jung and Diggory Hardy for rand_core >= 0.4.2.

Workarounds

None.

References

See Rand's changelog.

For more information

If you have any questions or comments about this advisory, open an issue in the Rand repository.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iorand_core≥ 0.4.0&&< 0.4.20.4.2cargo update -p rand_core --precise 0.4.2
🦀crates.iorand_coreall versions0.3.1cargo update -p rand_core --precise 0.3.1

Affected Products

1 product · 1 configurations
Application
randrust-random
< 0.4.2
range

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_core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update rand_core to 0.4.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-25576 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

### Impact Affected versions of this crate violated alignment when casting byte slices to integer slices, resulting in undefined behavior. `rand_core::BlockRng::next_u64` and `rand_core::BlockRng::fill_bytes` are affected. ### Patches The flaw was corrected by Ralf Jung and Diggory Hardy for `rand_core >= 0.4.2`. ### Workarounds None. ### References See [Rand's changelog](https://github.com/rust-random/rand/blob/master/rand_core/CHANGELOG.md#050---2019-06-06). ### For more information If you have any questions or comments about this advisory, [open an issue in the Rand repository](https://
O3 Security · Impact-Aware SCA

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CVE-2020-25576: rand_core — Fixed in 0.4.2