What is CVE-2026-6691?

CVE-2026-6691 is a high-severity vulnerability in Mongodb Inc. 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 Inc. 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 i…

Vulnerability class: Buffer Overflow

EPSS: 0.000 (4.4th percentile) — read the EPSS interpretation.

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.

Affected products

Mongodb Inc. C Driver — versions 2.1.0

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

References

cna@mongodb.com

Frequently asked questions

What is CVE-2026-6691?: CVE-2026-6691 is a high-severity vulnerability in Mongodb Inc. 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.