Windows clients running the popular WinZip application are at risk from a number of critical security flaws, according to WinZip Computing and security researchers. The compression/decompression tool is one of the most widely used pieces of software on the Windows platform.

WinZip versions 3.x, 6.x, 7.x, 8.x and 9.x contain vulnerabilities that could allow an attacker to execute malicious code on a Windows PC, the vendor warned. In an advisory on Thursday, Danish security firm Secunia gave the bugs a "highly critical" rating, the fourth-highest out of its five severity levels.

While no exploits are known to be circulating, the wide deployment of WinZip makes the vulnerabilities important to patch immediately, WinZip said. Users of older WinZip versions must upgrade to version 9.x in order to get the fix, which is contained in WinZip 9.0 Service Release 1 (SR1). "WinZip Computing recommends that all WinZip users upgrade to WinZip 9.0 SR1 to avoid the possibility of future exploitation of these vulnerabilities," the company said. The update can be found on WinZip's site.

The company disclosed few details about the vulnerabilities, which it said were detected in the course of internal review and testing. Previous versions of WinZip contain potential buffer overflows which could allow an attacker to execute malicious code, according to WinZip and Secunia. In addition, the update fixes a security hole reported to WinZip by an undisclosed user, which could allow an attacker to take over a system by sending a specially-crafted invalid input at the WinZip command line. The command line bug could probably only be exploited on a system whose security had already been compromised in some other way, the company said.

"As of the release of WinZip 9.0 SR1, WinZip Computing was not aware that any of these vulnerabilities had been publicly described or exploited," the company said.

Taking its cue from Microsoft's security-oriented Windows XP Service Pack 2, the WinZip update adds warning messages in situations where the user could be in danger of launching a virus, such as when a user double-clicks on a .exe file compressed within a Zip file.

Late last month, security experts warned of a serious flaw in WinAmp, another popular Windows client application. In that case, however, the bug was discovered only after attackers began using it to execute malicious code on users' desktops.