CVE-2021-20231Network attack vector

Client memory corruption via stale key-share pointer

Published Mar 12, 2021 · Updated Aug 3, 2024

A use-after-free in GnuTLS before 3.7.1 allows remote attackers to corrupt client memory during a TLS 1.3 handshake. The key_share extension sender saves a length pointer before buffer growth can reallocate its storage, then writes through that stale pointer. Reachability requires a large Client Hello, such as an HRR during resumption after large FFDHE parameters, and exploitation depends on allocator behavior.

CVSS severity9.8
Critical
EPSS probability3.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A use-after-free in GnuTLS before 3.7.1 allows remote attackers to corrupt client memory during a TLS 1.3 handshake. The key_share extension sender saves a length pointer before buffer growth can reallocate its storage, then writes through that stale pointer. Reachability requires a large Client Hello, such as an HRR during resumption after large FFDHE parameters, and exploitation depends on allocator behavior.

The record

CVE
CVE-2021-20231
Published
Mar 12, 2021
Updated
Aug 3, 2024
Vendor
GnuTLS Project
Product
GnuTLS
Classifications
T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 12, 2021CVE 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=gnutls 3.7.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

GnuTLS Project · GnuTLSversion=gnutls 3.7.1

Published CVSS scores

9.8Oracle LinuxCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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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