What is CVE-2026-6691?

CVE-2026-6691 is a high-severity vulnerability in MongoDB C Driver, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 7.8/10. Published 2026-05-06.

How severe is CVE-2026-6691?

High severity. CVSS v3 base score is 7.8 out of 10.

Buffer overflow in MongoDB C Driver

CVE-2026-6691

The MongoDB C Driver's Cyrus SASL integration performs unsafe string copying during username canonicalization, enabling a heap buffer overflow before any authentication or network traffic. This may be triggered by passing untrusted input in the username of a MongoDB URI with authMechanism=GSSAPI.

Vulnerability class: Buffer Overflow

Published 2026-05-06 · last modified 2026-06-18.

CVSS v3 metric

CVSS v3 base score 7.8 (High). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.002 (7.5th percentile), scored 2026-10-20.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 8th percentile — 7.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.001 · newest: 0.002 · change: +0.001

Affected products

MongoDB C Driver
MongoDB Inc. C Driver — versions 2.1.0

Weakness classification (CWE)

References

jira.mongodb.org/browse/CDRIVER-6134 (Patch, Issue Tracking, Vendor Advisory)

Frequently asked questions

What is CVE-2026-6691?: CVE-2026-6691 is a high-severity vulnerability in MongoDB C Driver, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 7.8/10. Published 2026-05-06.
How severe is CVE-2026-6691?: High severity. CVSS v3 base score is 7.8 out of 10.