After warning on Tuesday that hackers were exploiting an unpatched bug in Adobe Systems' Flash Player software, Symantec has backtracked from this claim, saying the flaw is "very similar" to another vulnerability that was patched last month.

Symantec's initial warning described a disturbing threat -- a previously unknown and unpatched flaw that was being exploited on tens of thousands of Web pages. The flaw allowed attackers to install unauthorized software on a victim's machine and was being used to install botnet programs and password-logging software, Symantec said.

Now Symantec believes that the bug was previously known and patched by Adobe on April 8, said Ben Greenbaum, a senior research manager with Symantec Security Response. However, the Linux version of Adobe's stand-alone Flash Player, version 9.0.124, is vulnerable to the attack.

On Tuesday Symantec researchers saw that the attack worked on Linux and that it caused Flash Player to crash on Windows XP, so they reasoned that they had a new bug that was just not working properly on the Windows platform, possibly due to a programming error by the hackers. "We thought it was a problem with the exploit," he said.

Now Symantec believes that the vulnerability was simply not properly patched in this one version of Adobe's software, Greenbaum said.

That means that Windows and Mac OS X users with the latest updates are not vulnerable, and even Linux users who are running the latest Flash Player plugin inside their browser, rather than as stand-alone software, are safe. However, Windows XP users running the older Flash Player, version 9.0.115, are vulnerable to the attack, Greenbaum said.

This kind of missed security assessment is rare, but it does happen from time to time, said Matt Richard, director of VeriSign's iDefense Rapid Response Team.

"It looks like they just jumped the gun and put it out a little bit too early without doing all the homework," he said of Symantec. "When we did our testing in the lab, the latest version completely fixes the issue: No crashes, no exploits, no nothing."

IBM's Internet Security Systems (ISS), which is credited with discovering the Flash Player bug, echoed Richard's analysis. "Several reports have stated that a zero-day Flash vulnerability is being exploited through several Chinese hacker websites," ISS wrote in its advisory on the flaw. "All of the samples X-Force has seen target the vulnerability disclosed in this Advisory."

In a note on its Web site, Symantec said that it was working with Adobe to figure things out.

An Adobe spokesman said Wednesday that his company was "still trying to get to the bottom of this," but expected to have an update by around noon Pacific time on Wednesday.