OpenSSL ACCF Vulnerability (CVE-2015-1793)

A new vulnerability has been recently patched in OpenSSL:

During certificate verification, OpenSSL (starting from version 1.0.1n and 1.0.2b) will attempt to find an alternative certificate chain if the first attempt to build such a chain fails. An error in the implementation of this logic can mean that an attacker could cause certain checks on untrusted certificates to be bypassed, such as the CA flag, enabling them to use a valid leaf certificate to act as a CA and “issue” an invalid certificate.

This issue will impact any application that verifies certificates including SSL/TLS/DTLS clients and SSL/TLS/DTLS servers using client authentication.

This issue affects OpenSSL versions 1.0.2c, 1.0.2b, 1.0.1n and 1.0.1o.

OpenSSL 1.0.2b/1.0.2c users should upgrade to 1.0.2d
OpenSSL 1.0.1n/1.0.1o users should upgrade to 1.0.1p

This issue was reported to OpenSSL on 24th June 2015 by Adam Langley/David Benjamin (Google/BoringSSL). The fix was developed by the BoringSSL project.

The vulnerability description and its lack of a cool name (Heartbleed, POODLE, etc) makes it feel like this vulnerability is not that critical as it was believed to be.

The circumstances that are required here and the outcome, are a bit weak at the moment – though as more details come to light, the severity could be better justified.