CVE-2020-1971: 
vulnerability analysis and mitigation

The X.509 GeneralName type is a generic type for representing different types of names. One of those name types is known as EDIPartyName. OpenSSL provides a function GENERALNAMEcmp which compares different instances of a GENERALNAME to see if they are equal or not. This function behaves incorrectly when both GENERALNAMEs contain an EDIPARTYNAME. A NULL pointer dereference and a crash may occur leading to a possible denial of service attack. All OpenSSL 1.1.1 and 1.0.2 versions are affected by this issue (OpenSSL Advisory).

OpenSSL uses the GENERALNAMEcmp function for two purposes: 1) Comparing CRL distribution point names between an available CRL and a CRL distribution point embedded in an X509 certificate, and 2) When verifying that a timestamp response token signer matches the timestamp authority name (exposed via the API functions TSRESPverifyresponse and TSRESPverifytoken). An unrelated bug means that affected versions of OpenSSL cannot parse or construct correct encodings of EDIPARTYNAME. However, it is possible to construct a malformed EDIPARTYNAME that OpenSSL's parser will accept and hence trigger this attack (OpenSSL Advisory).

If an attacker can control both items being compared then that attacker could trigger a crash leading to a denial of service condition. For example, if the attacker can trick a client or server into checking a malicious certificate against a malicious CRL then this may occur. Note that some applications automatically download CRLs based on a URL embedded in a certificate. This checking happens prior to the signatures on the certificate and CRL being verified (OpenSSL Advisory).

OpenSSL 1.1.1 users should upgrade to 1.1.1i. OpenSSL 1.0.2 users should upgrade to 1.0.2x if they have premium support, otherwise they should upgrade to OpenSSL 1.1.1i as version 1.0.2 is out of support and no longer receiving public updates (OpenSSL Advisory).