# CVE-2026-93394: libmongoc SCRAM client nonce-validation bypass

> A man-in-the-middle attacker can bypass SCRAM nonce validation in the MongoDB C Driver to capture client proofs for offline password cracking.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 18 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-93394, GHSA-cxv8-vf43-xcw5, UBUNTU-CVE-2026-93394

## What is CVE-2026-93394?

CVE-2026-93394 is a medium-severity vulnerability in MongoDB Inc. C Driver, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A flaw in libmongoc's SCRAM authentication implementation caused the client to continue the authentication handshake and transmit the client proof even when a nonce mismatch was detected in the server's first message. An unauthorized party with a man-in-the-middle position could exploit this by injecting a crafted server-first-message containing a controlled salt and low iteration count, then capturing the resulting client proof to perform offline password cracking. This vulnerability is mitigated by TLS, which is standard in production deployments.

Weakness: CWE-303 (Incorrect Implementation of Authentication Algorithm)

## Which versions are affected by CVE-2026-93394?

It affects 1 product.


Products: Mongodb C Driver.

## How do I fix CVE-2026-93394?

Apply the vendor's patch.

## Is CVE-2026-93394 being exploited?

There is no known exploitation of CVE-2026-93394 in the wild and no public exploit code.

## Other vulnerabilities in MongoDB Inc. 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-2026-6691](https://insights.sbomify.com/CVE-2026-6691/): MongoDB C Driver Cyrus SASL Canonicalization Buffer Overflow (high 8.6)
- [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)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-93394 and fix it if it is affected.

What it is: CVE-2026-93394 is a medium-severity vulnerability in MongoDB Inc. C Driver, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-93394 in the wild and no public exploit code.

Affected product: MongoDB Inc. 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-2026-93394/ (Markdown: https://insights.sbomify.com/CVE-2026-93394/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-93394
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-93394
- OSV: https://osv.dev/vulnerability/CVE-2026-93394

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-93394/
Summaries are generated and may contain mistakes; the official description above is the record.