Conference:  Defcon 31

Authors: Bill Demirkapi Microsoft Security Response Center

2023-08-01

Digital signatures are fundamental for verifying the authenticity and integrity of untrusted data in the digital world. They ensure that software, firmware, and other digital content are not tampered with during transmission or at rest. Code signing certificates are significantly more challenging to obtain when compared to alternatives like SSL or S/MIME certificates. The latter only has a single criterion- proof of control over a domain, while the former requires significant validation of the publisher itself. This project uncovered a systemic vulnerability present in numerous signature validation implementations, enabling attackers to exploit valid certificates in an unintended manner. Vulnerable implementations mistakenly perceive files signed with incompatible certificates as legitimate, violating their respective specifications and allowing threat actors to sign untrusted code at little to no cost. In this talk, we will explore the problem at all levels, ranging from the fundamental theory to its application across multiple formats and real-world situations.