AOL is advising users of its ICQ IM (instant messaging) service to update to the latest version of the IM software, following the discovery of a bug in an older version of the product.

Security researchers at Core Security yesterday reported that they had discovered the flaw in ICQ Pro 2003b, a version of the ICQ client that AOL still offers for download, billing it as a 'veteran' version of the product for users who prefer the earlier look and feel.

Although the bug doesn't affect more recent ICQ software such as ICQ 5.1, it could mean serious problems for ICQ Pro 2003b users, according to Max Caceres, director of product management at Core, a vendor of penetration testing software.

Core researchers have developed proof-of-concept code that causes ICQ Pro 2003b to crash, and they believe this vulnerability could eventually be exploited to run unauthorised software on a user's PC. More information on this flaw can be found here.

Hackers would attack a PC by sending a maliciously encoded IM to any ICQ Pro 2003b user connected to the service. Victims don't have to do anything, Caceres claimed. "Just by being there, someone can send them a message and they can be compromised," he said.

Core has also discovered less-critical issues in AOL's ICQ Toolbar 1.3 for Internet Explorer. These flaws could allow attackers to change the toolbar's configuration settings or possibly even run scripting code by sending victims maliciously encoded RSS feeds. More information on these bugs can be found here.

AOL said it was working to fix the bugs, but the company classifies them as "minor and low-risk", according to spokesman Andrew Weinstein. "Any users who are concerned can simply upgrade to the latest version of ICQ or not load suspicious RSS feeds," he said via IM.