A security hole in widely used versions of AOL's instant-messaging program could let a crook grab control of a victim's computer, according to a security firm that says AOL's steps to repair the problem don't go far enough.

Core Security Technologies says it notified AOL LLC, part of Time Warner Inc., about the programming flaw in late August, and AOL contends the problem has been fixed. But Core Security's chief technology officer, Ivan Arce, said the solution should be considered temporary because of the underlying design of AOL's market-leading Instant Messenger service, better known as AIM.

''I would say this is critical, this is very serious,'' Arce said. It's unclear whether the hole has been exploited.

The flaw exists in the most recent versions of AIM 6.1, and in 6.2, which is still in beta-test mode. Core Security also found it in the business-focused AIM Pro and in AIM Lite. The problem does not crop up in AIM 5.9, an older edition that many users still have, or in version 6.5, which also is in beta mode.

The security hole arose because of the way the vulnerable versions of AIM let instant-messaging chatters augment their conversations with various fonts and pictographic ''emoticons.'' The flawed versions of AIM do this by using Microsoft Corp.'s Internet Explorer program to render images.

The problem, Core Security contends, is that AIM was enabling full access to all of Internet Explorer's functions, including its ability to carry out programming commands and direct a computer to Web sites. So by embedding certain commands in an IM session, an attacker could direct a victim's computer to do lots of regrettable things, such as visiting a malicious Web site where even more bad code would be installed.

AOL spokeswoman Erin Gifford said her company had resolved the issues Core Security raised. As a result, she said, AIM users should consider themselves ''completely safe.''

Gifford would not detail the steps AOL had taken, other than saying that AOL was blocking malicious programming code and other suspicious content from being transmitted in IM traffic.

Arce responded that an attacker has a decent chance of getting around the blockade.

''That filtering mechanism, it doesn't remove the bug from the IM client. It just prevents people from exploiting it. If someone finds a way to bypass the filter, the problem still exists,'' he said. The filtering approach also wouldn't save AIM users who ''direct connect'' with other IMers to share files, a process that skips AOL servers.

His advice: Switch to the Web-based AIM Express, AIM 5.9 or AIM 6.5, though corporate technology staffs often consider beta versions off-limits.

AIM users should also consider rejecting incoming messages from anyone not on their ''buddy lists,'' though Arce advises that is an uncertain protection, since a friend's computer might have been corrupted through this hole. In that scenario, an attacker could conceivably instruct the buddy's computer to send malicious code to his friends.

The security firm also said other programs with built-in access to Internet Explorer could fall prey to the same vulnerability. Core Security says it does not believe that flaw exists in the second- and third-most popular IM programs, from Yahoo Inc. and Microsoft, though Core Security has not fully tested that idea.