CVE-2021-4044Network attack vector

TLS client interruption via misclassified verification errors

Published Dec 14, 2021 · Updated Sep 17, 2024

Improper error handling in OpenSSL 3.0.0 libssl allows remote attackers to interrupt TLS clients via a server certificate chain. ssl_verify_cert_chain passes a negative X509_verify_cert result through as retry-required, causing SSL_get_error to emit SSL_ERROR_WANT_RETRY_VERIFY without an application verification callback. A server-supplied chain lacking a Subject Alternative Name under CA name constraints can trigger the internal error; the client application then crashes, loops indefinitely, or otherwise mishandles the connection.

CVSS severity7.5
High
EPSS probability50.10%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper error handling in OpenSSL 3.0.0 libssl allows remote attackers to interrupt TLS clients via a server certificate chain. ssl_verify_cert_chain passes a negative X509_verify_cert result through as retry-required, causing SSL_get_error to emit SSL_ERROR_WANT_RETRY_VERIFY without an application verification callback. A server-supplied chain lacking a Subject Alternative Name under CA name constraints can trigger the internal error; the client application then crashes, loops indefinitely, or otherwise mishandles the connection.

The record

CVE
CVE-2021-4044
Published
Dec 14, 2021
Updated
Sep 17, 2024
Vendor
OpenSSL Project
Product
OpenSSL
Classifications
CWE-835, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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 OpenSSL 3.0.1 (Affected 3.0.0)

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

OpenSSL Project · OpenSSLversion=Fixed in OpenSSL 3.0.1 (Affected 3.0.0)

Published CVSS scores

7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

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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