OpenSSL Security Advisory [08 December 2020]
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EDIPARTYNAME NULL pointer de-reference (CVE-2020-1971)
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Severity: High

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 GENERAL_NAME_cmp which compares different instances of a GENERAL_NAME
to see if they are equal or not. This function behaves incorrectly when both
GENERAL_NAMEs contain an EDIPARTYNAME. A NULL pointer dereference and a crash
may occur leading to a possible denial of service attack.

OpenSSL itself uses the GENERAL_NAME_cmp function for two purposes:
1) Comparing CRL distribution point names between an available CRL and a CRL
   distribution point embedded in an X509 certificate
2) When verifying that a timestamp response token signer matches the timestamp
   authority name (exposed via the API functions TS_RESP_verify_response and
   TS_RESP_verify_token)

If an attacker can control both items being compared then that attacker could
trigger a crash. 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's s_server, s_client and verify
tools have support for the "-crl_download" option which implements automatic
CRL downloading and this attack has been demonstrated to work against those
tools.

Note that 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.

All OpenSSL 1.1.1 and 1.0.2 versions are affected by this issue. Other OpenSSL
releases are out of support and have not been checked.

OpenSSL 1.1.1 users should upgrade to 1.1.1i.

OpenSSL 1.0.2 is out of support and no longer receiving public updates. Premium
support customers of OpenSSL 1.0.2 should upgrade to 1.0.2x. Other users should
upgrade to OpenSSL 1.1.1i.

This issue was reported to OpenSSL on 9th November 2020 by David Benjamin
(Google). Initial analysis was performed by David Benjamin with additional
analysis by Matt Caswell (OpenSSL). The fix was developed by Matt Caswell.

Note
====

OpenSSL 1.0.2 is out of support and no longer receiving public updates. Extended
support is available for premium support customers:
https://www.openssl.org/support/contracts.html

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

Users of these versions should upgrade to OpenSSL 1.1.1.

References
==========

URL for this Security Advisory:
https://www.openssl.org/news/secadv/20201208.txt

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

For details of OpenSSL severity classifications please see:
https://www.openssl.org/policies/secpolicy.html