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.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-84971?
It affects 1 product.
Products include Mongodb Libmongocrypt.
How do I fix CVE-2026-84971?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
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
Sources
Also known as DEBIAN-CVE-2026-84971, GHSA-pxxc-vwf4-r3mr, UBUNTU-CVE-2026-84971.