OpenSSL Security Advisory

Matt Caswell matt at
Tue Dec 14 17:02:01 UTC 2021

Hash: SHA256

OpenSSL Security Advisory [14 December 2021]

Invalid handling of X509_verify_cert() internal errors in libssl (CVE-2021-4044)

Severity: Moderate

Internally libssl in OpenSSL calls X509_verify_cert() on the client side to
verify a certificate supplied by a server. That function may return a negative
return value to indicate an internal error (for example out of memory). Such a
negative return value is mishandled by OpenSSL and will cause an IO function
(such as SSL_connect() or SSL_do_handshake()) to not indicate success and a
subsequent call to SSL_get_error() to return the value
SSL_ERROR_WANT_RETRY_VERIFY. This return value is only supposed to be returned
by OpenSSL if the application has previously called
SSL_CTX_set_cert_verify_callback(). Since most applications do not do this the
SSL_ERROR_WANT_RETRY_VERIFY return value from SSL_get_error() will be totally
unexpected and applications may not behave correctly as a result. The exact
behaviour will depend on the application but it could result in crashes,
infinite loops or other similar incorrect responses.

This issue is made more serious in combination with a separate bug in OpenSSL
3.0 that will cause X509_verify_cert() to indicate an internal error when
processing a certificate chain. This will occur where a certificate does not
include the Subject Alternative Name extension but where a Certificate Authority
has enforced name constraints. This issue can occur even with valid chains.

By combining the two issues an attacker could induce incorrect, application
dependent behaviour.

OpenSSL 3.0.0 SSL/TLS clients are affected by this issue. Users of this version
should upgrade to OpenSSL 3.0.1.

OpenSSL 1.1.1 and 1.0.2 are not affected by this issue.

This issue was reported to OpenSSL on 29th November 2021 by Tobias Nießen. The
fix was developed by Matt Caswell and Tobias Nießen.


OpenSSL 1.0.2 is out of support and no longer receiving public updates. Extended
support is available for premium support customers:

OpenSSL 1.1.0 is out of support and no longer receiving updates of any kind.
The impact of these issues on OpenSSL 1.1.0 has not been analysed.

Users of these versions should upgrade to OpenSSL 3.0 or 1.1.1.


URL for this Security Advisory:

Note: the online version of the advisory may be updated with additional details
over time.

For details of OpenSSL severity classifications please see:


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