# CVE-2026-84966: BSON element injection via NUL-embedded document keys in builder append

> An integer overflow in the MongoDB C++ Driver allows attackers to cause a denial of service via oversized field names.

- **Severity:** Medium 5.9 (`CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/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:** 3 Sep 2026
- **Updated:** 3 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-84966, UBUNTU-CVE-2026-84966

## What is CVE-2026-84966?

CVE-2026-84966 is a medium-severity vulnerability in MongoDB C++ Driver, published on 3 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

An incorrect numeric type conversion in the BSON document building component of the MongoDB C++ Driver may cause a length value to be interpreted incorrectly. When an application supplies an extremely large, non-terminated field name to the builder, the library may read memory outside the intended buffer and terminate the calling process. No authentication is required, but the calling application must pass the oversized name in a specific form.

Weakness: CWE-681 (Incorrect Conversion between Numeric Types)

## Which versions are affected by CVE-2026-84966?

It affects 1 product.


Products: Mongodb C\+\+ Driver.

## How do I fix CVE-2026-84966?

Upgrade to a fixed version.

## Is CVE-2026-84966 being exploited?

There is no known exploitation of CVE-2026-84966 in the wild and no public exploit code.

## Other vulnerabilities in MongoDB C++ Driver

- [CVE-2026-81522](https://insights.sbomify.com/CVE-2026-81522/): Cross-tenant database retargeting via dot/NUL injection in namespace strings in the C++ Driver (high 8.6)
- [CVE-2026-88034](https://insights.sbomify.com/CVE-2026-88034/): GridFS data disclosure and deletion via query-operator injection in file IDs in the MongoDB C++ Driver (medium 6.1)
- [CVE-2026-84970](https://insights.sbomify.com/CVE-2026-84970/): Heap over-read or silent misparse via 32-bit truncation of JSON length in BSON JSON parser (medium 5.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-84966 and fix it if it is affected.

What it is: CVE-2026-84966 is a medium-severity vulnerability in MongoDB C++ Driver, published on 3 Sep 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-2026-84966 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-2026-84966/ (Markdown: https://insights.sbomify.com/CVE-2026-84966/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-84966
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-84966
- OSV: https://osv.dev/vulnerability/CVE-2026-84966

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-84966/
Summaries are generated and may contain mistakes; the official description above is the record.