CVE-2021-4044

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. Fixed in OpenSSL 3.0.1 (Affected 3.0.0).