CVE-2021-4209Public exploitNetwork attack vector

Authenticated client crash via zero-length hash update

Published Aug 24, 2022 · Updated Aug 3, 2024

NULL pointer dereference in GnuTLS before 3.7.3 allows remote authenticated users to crash TLS clients during a crafted PSK handshake. The wrap_nettle_hash_fast function passes a null data pointer with zero length to Nettle's hash update routine, which calls memcpy. The affected path requires a Guile-disabled build and a PSK-authenticated TLS 1.3 handshake; the demonstrated result is a client-process crash.

CVSS severity6.5
Medium
EPSS probability1.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

NULL pointer dereference in GnuTLS before 3.7.3 allows remote authenticated users to crash TLS clients during a crafted PSK handshake. The wrap_nettle_hash_fast function passes a null data pointer with zero length to Nettle's hash update routine, which calls memcpy. The affected path requires a Guile-disabled build and a PSK-authenticated TLS 1.3 handshake; the demonstrated result is a client-process crash.

The record

CVE
CVE-2021-4209
Published
Aug 24, 2022
Updated
Aug 3, 2024
Vendor
GnuTLS Project
Product
GnuTLS
Classifications
CWE-476, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Aug 24, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Fixed in gnutls v3.7.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

GnuTLS Project · GnuTLSversion=Fixed in gnutls v3.7.3

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-476NULL Pointer Dereference
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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