MongoDB Python Driver: Out-of-bounds read in BSON decoding via a malformed Regex element

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-106435: 0.09% estimated exploitation activity in the next 30 days; percentile 0.56 (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.

Impact The C extension's BSON decoder scans a Regex element's pattern and flags strings with strlen before checking the document's remaining length, so a crafted truncated Regex element can read past the end of the caller's buffer. bson.decode and bson.decode_all accept mmap and array buffers, which carry no NUL sentinel; the read can cross into unmapped memory and kill the process with an uncatchable SIGSEGV. Any application decoding untrusted BSON from such buffers is affected — impact is availability only, since the out-of-bounds bytes are never returned as content. NUL-terminated buffers (bytes, bytearray) raise `InvalidBSON` at worst; the driver's own transport and pure-Python installs are not affected. All releases through 4.18.2 are affected when the C extension is loaded.

Patches Fixed in PyMongo 4.18.3. Malformed Regex elements now raise InvalidBSON on every accepted buffer type; valid documents decode unchanged.

Workarounds Decode untrusted BSON only from…

Published by the source

27 Oct 2026, 15:36 UTC

Last modified by the source

27 Oct 2026, 15:47 UTC

Source last checked

28 Oct 2026, 06:10 UTC

1. Revision 2 · amended · 28 Oct 2026

   Amended from osv record GHSA-98qm-wvgf-w4cf

2. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-98qm-wvgf-w4cf

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# Patch Next advisory brief: CVE-2026-106435

Advisory: MongoDB Python Driver: Out-of-bounds read in BSON decoding via a malformed Regex element
IDs: CVE-2026-106435, GHSA-98qm-wvgf-w4cf
Record: https://patchnext.com/advisories/cve-2026-106435 (revision 2, source checked 28 Oct 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pymongo 4.18.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pymongo: affected >= 3.9.0, < 4.18.3; fixed in 4.18.3

## 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://osv.dev/vulnerability/GHSA-98qm-wvgf-w4cf (OSV record GHSA-98qm-wvgf-w4cf lists affected ranges: PyPI pymongo >=3.9.0 <4.18.3 (fixed))
- Verified: https://github.com/advisories/GHSA-98qm-wvgf-w4cf (GitHub-reviewed advisory GHSA-98qm-wvgf-w4cf lists affected ranges: PyPI pymongo >=3.9.0 <=4.18.2)

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