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CRITICAL severity

CVE-2021-27378 — rand_core

CRITICALFix: rust-random/rand#1096

CVE-2021-27378 is a critical-severity (CVSS 9.8) CWE-131 vulnerability in rand_core. A fix is available for rand_core — see the affected versions and patch details below.

Incorrect check on buffer length in rand_core

Also known asGHSA-w7j2-35mf-95p7RUSTSEC-2021-0023
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.2%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs68th percentile — riskier than 68% of all scored CVEsHighest risk
0.00%0.58%1.16%1.74%0.5%1.2%Apr 26Aug 26Oct 26

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

How urgent is this, really

CVE-2021-27378 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

1 pkg affected
🦀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

An issue was discovered in the rand_core crate before 0.6.2 for Rust. Because read_u32_into and read_u64_into mishandle certain buffer-length checks, a random number generator may be seeded with too little data. The vulnerability was introduced in v0.6.0. The advisory doesn't apply to earlier minor version numbers.

Because read_u32_into and read_u64_into mishandle certain buffer-length checks, a random number generator may be seeded with too little data.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iorand_core≥ 0.6.0&&< 0.6.20.6.2cargo update -p rand_core --precise 0.6.2

Affected Products

1 product · 1 configurations
Application
rand corerand_core_project
≥ 0.6.0 && < 0.6.2
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 0.6.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-27378 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

An issue was discovered in the rand_core crate before 0.6.2 for Rust. Because `read_u32_into` and `read_u64_into` mishandle certain buffer-length checks, a random number generator may be seeded with too little data. The vulnerability was introduced in v0.6.0. The advisory doesn't apply to earlier minor version numbers. Because read_u32_into and read_u64_into mishandle certain buffer-length checks, a random number generator may be seeded with too little data.
O3 Security · Impact-Aware SCA

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CVE-2021-27378: rand_core — Fixed in 0.6.2