The large number of recent incidents involving malicious or potentially unwanted applications (PUAs) signed with digital certificates should make users and system administrators aware of the fact that digital signatures must not be blindly trusted, Kaspersky has warned.
The number of certificates issued by certificate authorities (CAs) to companies and independent software developers over the past years has increased considerably.
Compared to 2008, when the security firm identified roughly 1,500 certificates that had been used to sign malware and suspicious software, in 2014, after a steady year-over-year increase, the number reached well over 6,000.
Using valid or forged digital certificates to sign malicious files is a common technique that has been seen in many advanced persistent threat (APT) operations, including Stuxnet, Winnti and Darkhotel.
Threat actors and companies with a shady agenda can acquire certificates by stealing them, or by purchasing them legally. The problem is that, in some cases, CAs don’t investigate the activities of a company before issuing a digital certificate.
“It is companies manufacturing potentially unwanted software that most often purchase certificates. On the one hand these companies do not manufacture malware programs, so they can legally purchase a digital certificate to sign their software,” Andrey Ladikov, head of strategic research at Kaspersky Lab, wrote in a blog post. “On the other hand, they produce software annoys users. In fact, they get their software signed with digital certificates precisely to encourage users to trust them.”
In addition to legally purchasing certificates, cybercriminals can use several other techniques to sign malware. A rare, but not unheard of, method involves compiling a piece of malware using a company’s build server. Software compiled on these build servers is automatically signed with a digital certificate. These types of attacks are uncommon because major software companies have proper security measures in place to protect their servers.
Stealing private keys, which is often done with specialized malware, can also be lucrative because software signed by a reputable developer doesn’t attract suspicion. However, major companies store private keys on dedicated hardware modules, forcing cybercrooks to focus their attention on smaller companies whose networks might not be very secure, Kaspersky said.
Another method involves modifying signed Web installers so that they download and install malware instead of the legitimate application. This is done by exploiting weaknesses in the algorithms that check executable file signatures.
When CAs discovered that one of their certificates has been misused, they recall it. However, in some cases, especially when the certificate has been used to sign PUAs, the CA doesn’t recall it. The untrusted certificates remain valid in this case and they can be used to sign malicious software. According to Kaspersky, just over half of untrusted certificates are used to sign PUAs, while the rest are used to sign malware.
“Given the growing number of threats associated with malicious files signed with digital certificates, users and administrator can no longer risk placing blind faith in signed files and just allow them to be launched simply because they have a digital certificate,” Ladikov said.