U.S. CERT on Thursday warned that new attack kits for exploiting vulnerabilities in Adobe's PDF-reading software are circulating on the Internet.

Secure Computing's Anti-Malware Research Labs has published a screenshot of one such Web-based attack kit, called "PDF Xploit Pack."

"This new toolkit targets only PDFs, no other exploits are used to leverage vulnerabilities," a Secure Computing blog post explains. "Typical functions like caching the already infected users are deployed by this toolkit on the sever-side. Whenever a malicious PDF exploit is successfully delivered, the victim’s IP address is remembered for a certain period of time. During this 'ban time' the exploit is not delivered to that IP again, which is another burden for incident handling."

Adobe has patched 17 security vulnerabilities in the Windows version of its Reader 8 software so far this year, in four separate patches. The Macintosh version of Reader 8 has been patched 16 times in three patches to date this year.

Ian Amit, director of security research at Aladdin Knowledge Systems, attributes the rise in PDF exploits to the revival of the Neosploit toolkit, according to a report in Computerworld.

Secure Computing says that other exploit toolkits, like "El Fiesta," have also been enhanced to take advantage of PDF vulnerabilities.

Another security company, Finjan, released a report earlier this week that examined the rising sophistication of obfuscated malicious code -- code written to be difficult to read and detect. The report notes that PDF files make an appealing delivery mechanism for malicious code because they allow JavaScript code to be embedded for the purpose of PDF file customization and manipulation.

"During our research, we noticed an increase in the amount of PDF files containing malicious code," the Finjan report explains. "Obviously, crimeware authors became aware of this new capability for distributing malicious code and took the necessary steps to protect their 'bread- and- butter' from being detected by security vendors."

U.S. CERT offers the standard precautionary advice: Don't open files from sources you don't trust, and keep your antivirus software and patches up to date.

Keeping one's antivirus software up to date, while advisable, may not be particularly effective. The Finjan report says that company researchers submitted a PDF with obfuscated malicious code to Virus Total, a service that analyzes whether submitted malware is detectable by various antivirus engines. It found that only 10% of the antivirus engines in its test suite recognized the file as malicious.