# CVE-2026-84971: Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path

> An attacker can cause a denial of service by triggering a crash in the MongoDB client encryption library via malformed ciphertext.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/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:** 3 Sep 2026
- **Updated:** 3 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-84971, GHSA-pxxc-vwf4-r3mr, UBUNTU-CVE-2026-84971

## What is CVE-2026-84971?

CVE-2026-84971 is a high-severity denial of service vulnerability in MongoDB libmongocrypt, published on 3 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Improper handling of an unexpected value size in the decryption path of a client-side encryption library can cause a failed internal check that terminates the process using the library. A party able to place a suitably formed encrypted value where an application will decrypt it, or able to control the responses the application receives, may cause that application to stop running.

Weakness: CWE-617 (Reachable Assertion)

## Which versions are affected by CVE-2026-84971?

It affects 1 product.


Products: Mongodb Libmongocrypt.

## How do I fix CVE-2026-84971?

Upgrade to a fixed version.

## Is CVE-2026-84971 being exploited?

There is no known exploitation of CVE-2026-84971 in the wild and no public exploit code.

## Other vulnerabilities in MongoDB libmongocrypt

- [CVE-2026-106433](https://insights.sbomify.com/CVE-2026-106433/): Heap corruption via duplicate masterKey fields in MongoDB libmongocrypt (high 8.7)
- [CVE-2026-84962](https://insights.sbomify.com/CVE-2026-84962/): Authenticated KMS request forgery via CRLF injection in GCP key identifier strings (medium 5.7)
- [CVE-2026-81523](https://insights.sbomify.com/CVE-2026-81523/): Cross-tenant database retargeting via dot/NUL injection in namespace strings in libmongocrypt (low 2.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-84971 and fix it if it is affected.

What it is: CVE-2026-84971 is a high-severity denial of service vulnerability in MongoDB libmongocrypt, published on 3 Sep 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-2026-84971 in the wild and no public exploit code.

Affected product: MongoDB libmongocrypt. 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-2026-84971/ (Markdown: https://insights.sbomify.com/CVE-2026-84971/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-84971
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-84971
- OSV: https://osv.dev/vulnerability/CVE-2026-84971

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-84971/
Summaries are generated and may contain mistakes; the official description above is the record.