Integer overflow in pywin32

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

7.1 · CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2021-32559: 1.73% estimated exploitation activity in the next 30 days; percentile 77.61 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.

Published by the source

9 Aug 2021, 20:43 UTC

Last modified by the source

27 Mar 2025, 18:04 UTC

Source last checked

26 Oct 2026, 13:23 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-hwfp-hg2m-9vr2

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-hwfp-hg2m-9vr2

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# Patch Next advisory brief: CVE-2021-32559

Advisory: Integer overflow in pywin32
IDs: CVE-2021-32559, GHSA-hwfp-hg2m-9vr2
Record: https://patchnext.com/advisories/cve-2021-32559 (revision 3, source checked 26 Oct 2026, 13:23 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pywin32 301. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pywin32: affected < 301; fixed in 301

## Suggested inspection (read-only)
- `python -m pip show pywin32`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on pywin32.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-hwfp-hg2m-9vr2 (OSV record GHSA-hwfp-hg2m-9vr2 lists affected ranges: PyPI pywin32 >=0 <301 (fixed))
- Verified: https://github.com/advisories/GHSA-hwfp-hg2m-9vr2 (GitHub-reviewed advisory GHSA-hwfp-hg2m-9vr2 lists affected ranges: PyPI pywin32 >=0 <301)

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