Security organisations are warning users to be vigilant following the discovery of a new crop of tools for exploited PDF flaws.

The US Computer Emergency Response Team (US-Cert) and Secure Computing issued alerts after several exploit tools were found to be making the rounds in the cyber-crime underworld.

The toolkits are especially dangerous due to their ability to avoid security systems, according to Secure Computing.

The latest attacks catalogue infected users and refrain from sending attacks more than once, making it more difficult for security vendors to track down the source of the attacks and prevent further infections.

In addition to new tools, researchers have also found PDF exploit components being added on to existing attack systems.

The risk is further compounded by the fact that PDF files are commonly found among the home and enterprise worlds and are not always considered to be as risky as other formats by many users.

"The Portable Document Format is one of the file formats of choice commonly used in today's enterprises, since it is widely deployed across different operating systems," the company noted.

"But on a down-side this format has known vulnerabilities which are exploited in the wild."

US-Cert recommends that users protect against the attacks by avoiding untrusted or unsolicited downloads and email attachments.

The organisation also recommends that users keep antivirus and system software up-to-date with the latest patches.