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MEDIUM severity

CVE-2021-32559 — pywin32

MEDIUMFix: mhammond/pywin32#1701

CVE-2021-32559 is a medium-severity (CVSS 6.5) CWE-190 vulnerability in pywin32. A fix is available for pywin32 — see the affected versions and patch details below.

Integer overflow in pywin32

Also known asGHSA-hwfp-hg2m-9vr2PYSEC-2021-112
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.7%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs77th percentile — riskier than 77% of all scored CVEsHighest risk
0.00%0.74%1.49%2.23%0.2%1.7%Apr 26Aug 26Oct 26

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

How urgent is this, really

CVE-2021-32559 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
🐍pywin32

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

Description

An integer overflow exists in pywin32 prior to version b301 when adding an access control entry (ACE) to an access control list (ACL) that would cause the size to be greater than 65535 bytes. An attacker who successfully exploited this vulnerability could crash the vulnerable process.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIpywin32all versions301pip install --upgrade 'pywin32==301'

Affected Products

1 product · 1 configurations
Application
pywin32mhammond
< 301
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 pywin32, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update pywin32 to 301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-32559 is resolved across your whole dependency graph.

  3. 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

An integer overflow exists in pywin32 prior to version b301 when adding an access control entry (ACE) to an access control list (ACL) that would cause the size to be greater than 65535 bytes. An attacker who successfully exploited this vulnerability could crash the vulnerable process.
O3 Security · Impact-Aware SCA

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CVE-2021-32559: pywin32 — Fixed in 301