# CVE-2026-81521: Cross-database write retargeting via unvalidated dotted database name in Client.BulkWrite in the MongoDB Go Driver

> An attacker can redirect bulk write operations to unintended databases by supplying unvalidated dotted names, potentially modifying unauthorized data.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Aug 2026
- **Updated:** 6 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-81521, UBUNTU-CVE-2026-81521

## What is CVE-2026-81521?

CVE-2026-81521 is a high-severity vulnerability in MongoDB GO Driver, published on 27 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

The MongoDB Go Driver's client-level bulk write operation may accept a caller-supplied database name containing a reserved separator character without escaping it before the name is used to build the target namespace for the operation. An application that passes untrusted input as a database name could therefore have the write directed at a database and collection other than the ones it intended. Only the Client.BulkWrite API is affected.

Weakness: CWE-99 (Improper Control of Resource Identifiers)

## Which versions are affected by CVE-2026-81521?

It affects 1 product.


Products: Mongodb Go Driver.

## How do I fix CVE-2026-81521?

Upgrade to a fixed version.

## Is CVE-2026-81521 being exploited?

There is no known exploitation of CVE-2026-81521 in the wild and no public exploit code.

## Other vulnerabilities in MongoDB GO Driver

- [CVE-2026-107324](https://insights.sbomify.com/CVE-2026-107324/): Application denial of service via integer overflow in BSON value-length validation in MongoDB Go Driver (high 8.2)
- [CVE-2026-107325](https://insights.sbomify.com/CVE-2026-107325/): Application denial of service via missing BSON array length validation in MongoDB Go Driver (high 8.2)
- [CVE-2021-20329](https://insights.sbomify.com/CVE-2021-20329/): Specific cstrings input may not be properly validated in the Go Driver (medium 6.8)
- [CVE-2026-88031](https://insights.sbomify.com/CVE-2026-88031/): GridFS data deletion via query-operator injection in file IDs in the MongoDB Go Driver (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81521 and fix it if it is affected.

What it is: CVE-2026-81521 is a high-severity vulnerability in MongoDB GO Driver, published on 27 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-81521 in the wild and no public exploit code.

Affected product: MongoDB GO 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-2026-81521/ (Markdown: https://insights.sbomify.com/CVE-2026-81521/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81521
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81521
- OSV: https://osv.dev/vulnerability/CVE-2026-81521

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81521/
Summaries are generated and may contain mistakes; the official description above is the record.