Adobe is still investigating what looks like illegal use of its code-signing certificate for Windows. In order to maintain trust in genuine Adobe software , the company is revoking the affected certificate on Oct. 4 for all software code signed after July 10, 2012, and issuing updates signed using a new digital certificate.

Code-signing certificates are used to digitally sign software programs. Many software developers digitally sign the programs they create in order to assure customers that the programs are legitimate and have not been modified. Digital signatures use public key cryptography technology to secure and authenticate code.

"We have strong reason to believe that this issue does not present a general security risk," Adobe said in an FAQ list about the issue. "The evidence we have seen has been limited to a single isolated discovery of two malicious utilities signed using the certificate and indicates that the certificate was not used to sign widespread malware."

Another APT Attack

Paul Zimski, vice president of solution marketing at Lumension, said it looks like Adobe was hit with an advanced persistent threat attack and, in turn, was used to refine a second APT against another target. He likened it to an "APT inception."

"Another, unnamed company that uses Adobe software discovered the suspect files signed with valid Adobe certificates while investigating an incident," Zimski said. "They shared them with Adobe, who then investigated their build servers and found that they had been compromised and had malware installed on the servers."

Zimski isn't sure why the build server wasn't locked down and admits that it may have indeed been secured only to be breached by a group of sophisticated hackers. He suspects the hackers went after a laptop or a desktop inside of Adobe to laterally attack the build server and access the signing certificates.

Tip of the Iceberg?

"This is what we've seen as a recurring theme in APTs. You don't necessarily try to attack where defenses are strongest," Zimski said. "You probe your target extensively to find the weakness -- usually some low-hanging fruit or lower-valued asset -- to gain a foothold. Then you start laterally attacking until you get into your final destination."

Zimski pointed to the recent Flame attack as one example, along with an incident earlier this summer where researchers suspected Iran accessed some Google certificates. Adobe marks the third such attack -- attacks that he says have far-reaching implications for trust.

"At the end of the day, no matter what protection mechanisms and technology you have in place, there is no single answer to defeating APTs. There is no silver bullet, no matter how much we wish there was one," Zimski said. "The best approach is to layer your defenses and practices sound systems management . Ensure that you aren't trusting any revoked certificates and align your protective technologies, your systems management and your employee training in a way that is disruptive to the APT lifecycle."