Stuxnet-style code signing is more widespread than anyone thought

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.