Vulnerability · Published Sep 3, 2026 · Assigned by mongodb
CVE-2026-84971Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path
Medium 6.5
In plain English
A medium-severity security flaw in libmongocrypt from MongoDB. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 18% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | libmongocrypt | no fix listed | DEBIAN-CVE-2026-84971 |
| Debian:13 | libmongocrypt | no fix listed | DEBIAN-CVE-2026-84971 |
| Debian:14 | libmongocrypt | 1.20.4-1 | DEBIAN-CVE-2026-84971 |
| Ubuntu:22.04:LTS | libmongocrypt | no fix listed | UBUNTU-CVE-2026-84971 |
| Ubuntu:24.04:LTS | libmongocrypt | no fix listed | UBUNTU-CVE-2026-84971 |
| Ubuntu:26.04:LTS | libmongocrypt | no fix listed | UBUNTU-CVE-2026-84971 |
| Ubuntu:26.10 | libmongocrypt | no fix listed | UBUNTU-CVE-2026-84971 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| MongoDB | libmongocrypt | 1.7.0 – before 1.20.4 |
Weakness: CWE-617 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H