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Attackers compromised an Adobe server and hijacked a code-signing certificate -- a mechanism that is supposed to validate that computer programs are from trusted sources -- and manipulated it to give the green light for malware to enter computer systems, the company said.

When software is signed with a certificate, this tells antivirus software that it isn’t malicious, allowing it to bypass protective filters and slip into a system. One of the viruses signed extracts password hashes from the Windows operating system, effectively undoing the encryption that protects users’ passwords. The other most likely affects commands for Web server applications, Adobe said.  

Analysts said that the effort taken to hijack a valid Adobe code-signing certificate to plant the malware into a system points to a high-value target. Adobe senior director of product security and privacy Brad Arkin indicated in a blog post the malware was likely used by “sophisticated threat actors” for a “highly targeted” attack and added that the majority of users were not at risk. The breach is still under investigation. 

Adobe plans to revoke the code-signing certificate effective Oct. 4. The firm didn’t disclose when the breach happened, but said it had put in place a method to re-sign components validated with the hijacked key after July 10.

Criticism of whether certificate issuers have the infrastructure to guard against breaches has been mounting in antivirus circles. The use of phony signatures and the hijacking of signatures by attackers undermine the credibility of the antivirus tools, which rely heavily on signatures to detect malware.

Dutch certificate authority Diginotar went bankrupt last year after it was hit by a security breach that resulted in more than 500 fraudulent certificates being issued. Most recently, the Stuxnet worm used digital security certificates stolen from Taiwanese firms.