PyMongo: PYTHON-5996 Heap out-of-bounds write via signed size overflow in BSON document encoding

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-96749: 0.19% estimated exploitation activity in the next 30 days; percentile 8.48 (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 in the BSON document encoding component of the MongoDB Python Driver's bundled native extension may occur when a single document is built from an unusually large amount of caller-supplied data. Size arithmetic is performed in a signed 32-bit type, and the guard meant to catch the overflow is written in a form whose behavior is not defined by the C language standard. A party with no privileges who can place a very large value into data that an application encodes may, depending on how the native extension was built, cause a write outside the bounds of an allocated buffer inside the application's own process.

Published by the source

5 Oct 2026, 23:47 UTC

Last modified by the source

15 Oct 2026, 14:56 UTC

Source last checked

26 Oct 2026, 13:55 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-v4x9-3549-crwv

2. Revision 3 · amended · 25 Oct 2026

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

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-v4x9-3549-crwv

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-v4x9-3549-crwv

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# Patch Next advisory brief: CVE-2026-96749

Advisory: PyMongo: PYTHON-5996 Heap out-of-bounds write via signed size overflow in BSON document encoding
IDs: CVE-2026-96749, GHSA-v4x9-3549-crwv
Record: https://patchnext.com/advisories/cve-2026-96749 (revision 4, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pymongo 4.18.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pymongo: affected < 4.18.2; fixed in 4.18.2

## Suggested inspection (read-only)
- `python -m pip show pymongo`: 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 pymongo.
- 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://github.com/advisories/GHSA-v4x9-3549-crwv (GitHub-reviewed advisory GHSA-v4x9-3549-crwv lists affected ranges: PyPI pymongo >=0 <4.18.2)
- Verified: https://osv.dev/vulnerability/GHSA-v4x9-3549-crwv (OSV record GHSA-v4x9-3549-crwv lists affected ranges: PyPI pymongo >=0 <4.18.2 (fixed))

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