Stuxnet-style code signing is more widespread than anyone thought

Enlarge / The two legitimate signing certificates Stuxnet used to bypass Windows protections.

One of the breakthroughs of the Stuxnet worm that targeted Iran’s nuclear program was its use of legitimate digital certificates, which cryptographically vouched for the trustworthiness of the software’s publisher. Following its discovery in 2010, researchers went on to find the technique was used in a handful of other malware samples both with ties to nation-sponsored hackers and, later on, with ties to for-profit criminal enterprises.

Now, researchers have presented proof that digitally signed malware is much more common than previously believed. What’s more, it predated Stuxnet, with the first known instance occurring in 2003. The researchers said they found 189 malware samples bearing valid digital signatures that were created using compromised certificates issued by recognized certificate authorities and used to sign legitimate software. In total, 109 of those abused certificates remain valid. The researchers, who presented their findings Wednesday at the ACM Conference on Computer and Communications Security, found another 136 malware samples signed by legitimate CA-issued certificates, although the signatures were malformed.

The results are significant because digitally signed software is often able to bypass User Account Control and other Windows measures designed to prevent malicious code from being installed. Forged signatures also represent a significant breach of trust because certificates provide what’s supposed to be an unassailable assurance to end users that the software was developed by the company named in the certificate and hasn’t been modified by anyone else. The forgeries also allow malware to evade antivirus protections. Surprisingly, weaknesses in the majority of available AV programs prevented them from detecting known malware that was digitally signed even though the signatures weren’t valid.