"We received the malicious utilities in the late evening of Sept. 12 from a single, isolated (unnamed) source," Wiebke Lips, senior manager of corporate communications at Adobe, said Thursday via email. "As soon as the validity of the signatures was confirmed, we immediately initiated steps to deactivate and revoke the certificate used to generate the signatures."

One of the malicious utilities was a digitally signed copy of Pwdump7 version 7.1, a publicly available Windows account password extraction tool that also included a signed copy of the libeay32.dll OpenSSL library.

The second utility was an ISAPI filter called myGeeksmail.dll. ISAPI filters can be installed in IIS or Apache for Windows Web servers in order to intercept and modify HTTP streams.

The two rogue tools could be used on a machine after it was compromised and would likely pass a scan by security software since their digital signatures would appear legitimate coming from Adobe.

"Some antivirus solutions don't scan files signed with valid digital certificates coming from trustworthy software makers such as Microsoft or Adobe," said Bogdan Botezatu, a senior e-threat analyst at antivirus vendor BitDefender. "This would give the attackers a huge advantage: Even if these files were heuristically detected by the locally installed AV, they would be skipped by default from scanning, which dramatically enhances the attackers' chance of exploiting the system."