Independent security researcher Alex Wheeler discovered the flaw and published an advisory (download PDF) yesterday. In it, he details how the Symantec Antivirus Library can be overwhelmed by "heap overflows" while decompressing an RAR file, a commonly used file format for storing large video or audio files.

To create a heap overflow, a malicious hacker sends large amounts of data that overwhelm a buffer, an area used for temporary data storage. This attack, similar to a buffer overflow, lets attackers overwrite portions of a system's memory in order to run their own malicious code.

Symantec users are vulnerable to the attack when their antivirus software scans the RAR files for viruses or worms, Wheeler wrote. The attack can be launched via e-mail without the user having to open the message or click on an attachment, he said.

Wheeler recommends that users turn off the scanning of RAR files until Symantec fixes the code in the library.

A Symantec spokeswoman provided a statement confirming the vulnerability and the existence of a patch that will detect exploits using this vulnerability. Symantec users should update their antivirus software to obtain the patch, which protects desktop, server and gateway antivirus software, the statement said.

The company plans to update the Antivirus Software Library to fix the vulnerability. Details about that update were posted to Symantec's Web site, the spokeswoman said. No exploits using the vulnerability have been reported to Symantec as of midday, she said.