CVE-2021-29939 — stackvector
HIGHCVE-2021-29939 is a high-severity (CVSS 7.3) Out-of-bounds Write vulnerability in stackvector. 1 public exploit reference exists, so weaponization risk is real. A fix is available for stackvector — see the affected versions and patch details below.
Out of bounds write in stackvector
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-29939.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-29939 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
stackvectorReal-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
StackVec::extend used the lower and upper bounds from an Iterator's size_hint to determine how many items to push into the stack based vector. If the size_hint implementation returned a lower bound that was larger than the upper bound, StackVec would write out of bounds and overwrite memory on the stack. As mentioned by the size_hint documentation, size_hint is mainly for optimization and incorrect implementations should not lead to memory safety issues.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | stackvector | all versions | 1.0.9cargo update -p stackvector --precise 1.0.9 |
Affected Products
stackvectorstackvector_projectResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for stackvector, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update stackvector to 1.0.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29939 is resolved across your whole dependency graph.
Workarounds
Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.
Frequently Asked Questions
Is CVE-2021-29939 in your dependencies?
Find it across crates.io, including transitive dependencies.