CVE-2026-6691: The MongoDB C Driver's Cyrus SASL integration performs unsafe string copying during username canonicalization,

Routine

HIGH 7.8UPGRADE

CVE-2026-6691 is a high-severity flaw (CVSS 7.8) affecting c driver. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	7.8 (High)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.1% (top 98%)
CISA exploited list	No
Weakness	CWE-120, CWE-787
Published	2026-05-06
Last modified	2026-10-26

Publisher (CNA): MongoDB, Inc. · Vendor: mongodb

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-06 16:16 UTCPublished CVE record published
2026-10-26 19:10 UTCUpdated Record last modified

mongodb c driver UPGRADE

Affected: >= 2.1.0 and < 2.1.2

Fixed in: 2.1.2

Safe to downgrade to: a release before 2.1.0

1. Check installed version

apt-cache policy c-driver || rpm -q c-driver || c-driver --version

2. UPGRADE to 2.1.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade c-driver

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade c-driver

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.1.0 (Debian / Ubuntu)

apt-cache madison c-driver sudo apt install c-driver=<VERSION_BEFORE_2.1.0> sudo apt-mark hold c-driver

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list c-driver sudo dnf downgrade c-driver-<VERSION_BEFORE_2.1.0> sudo dnf versionlock add c-driver

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-6691?

c driver: upgrade to 2.1.2. Exact commands for each OS are on this page.

Is CVE-2026-6691 being exploited?

CVE-2026-6691 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-6691 affect?

CVE-2026-6691 affects c driver. Severity is high (CVSS 7.8). 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

How urgent is CVE-2026-6691?

Priority 28/100. High severity

Software pages

MongoDB vulnerabilities

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References

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