OpenSSL Alternative Chains Certificate Forgery

August 3, 2015

A few weeks ago, OpenSSL released patches that fix sereral vulnerabilities. Among the vulnerabilities, the "Alternative Chains Certificate Forgery" can lead to man-in-the-middle (MITM) attacks.

This MITM vulnerability affects OpenSSL versions 1.0.2c, 1.0.2b, 1.0.1n and 1.0.1o. During certificate verification, OpenSSL 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, enabling them to use a valid leaf certificate to act as a CA and "issue" an invalid certificate.

The vulnerability is referred by CVE as CVE-2015-1793.

Dell SonicWALL has released an IPS signature to detect and block exploitation attempts targeting this vulnerability. The signature is listed below:

  • 11041 OpenSSL X509_verify_cert Function Security Bypass

Dell SonicWALL has observed several attack attempts in the past week:

Targeted IPs affected by the vulnerability: