# CVE-2026-6691: MongoDB C Driver Cyrus SASL Canonicalization Buffer Overflow

> Heap buffer overflow in MongoDB C Driver's SASL integration allows code execution via crafted GSSAPI URIs.

- **Severity:** High 8.6 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 May 2026
- **Updated:** 7 May 2026
- **Also known as:** DEBIAN-CVE-2026-6691, GHSA-587q-94wg-2pfp, OESA-2026-2690, OESA-2026-2691, OESA-2026-2692, UBUNTU-CVE-2026-6691

## What is CVE-2026-6691?

CVE-2026-6691 is a high-severity memory corruption vulnerability in MongoDB C Driver, published on 6 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-787 (Out-of-bounds Write), CWE-120 (Buffer Copy without Checking Size of Input)

## Which versions are affected by CVE-2026-6691?

It affects 1 product.


Products: Mongodb C Driver.

## How do I fix CVE-2026-6691?

Apply the vendor's patch.

## Is CVE-2026-6691 being exploited?

There is no known exploitation of CVE-2026-6691 in the wild and no public exploit code.

## Other vulnerabilities in MongoDB C Driver

- [CVE-2026-93393](https://insights.sbomify.com/CVE-2026-93393/): Heap overflow via oversized decrypted TLS record sequence in Windows Secure Channel stream (critical 9.2)
- [CVE-2026-84964](https://insights.sbomify.com/CVE-2026-84964/): Heap corruption via OCSP request double free from crafted multi-URL certificate in TLS client (high 8.2)
- [CVE-2024-7553](https://insights.sbomify.com/CVE-2024-7553/): Accessing Untrusted Directory May Allow Local Privilege Escalation (high 7.3)
- [CVE-2026-93395](https://insights.sbomify.com/CVE-2026-93395/): Integer Underflow → Heap Out-of-Bounds Read in `bson_new_from_buffer() (medium 6.9)
- [CVE-2025-12119](https://insights.sbomify.com/CVE-2025-12119/): Bulk write with options may read invalid memory (medium 6.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6691 and fix it if it is affected.

What it is: CVE-2026-6691 is a high-severity memory corruption vulnerability in MongoDB C Driver, published on 6 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-6691 in the wild and no public exploit code.

Affected product: MongoDB C Driver. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-6691/ (Markdown: https://insights.sbomify.com/CVE-2026-6691/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6691
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6691
- OSV: https://osv.dev/vulnerability/CVE-2026-6691

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6691/
Summaries are generated and may contain mistakes; the official description above is the record.