In an alert on its Web site, Cisco said all VPN Client software programs earlier than version 3.6 and earlier than version 3.5.4 are affected.
The problem occurs on all supported operating systems including Windows and Red Hat Version 6.2 Linux (Intel), or compatible distributions based on kernel Version 2.2.12 or later, Solaris UltraSPARC running a 32-bit kernel OS Version 2.6 or later and Mac OS X Version 10.1.0 or later.
The software contains a number of buffer overflow bugs.
Cisco added that the VPN Client software locks up a PC if receives a data packet containing no information. Cisco assured users that the bugs posed no risk to the confidentiality and integrity of users' data. It said a hacker would only be able to prevent the Cisco VPN Client software program from functioning correctly.
A fix for Cisco VPN Client version 3.6 or later is available for download from the Cisco Web site.
Cisco said users of VPN Client version 3.5.4 would have to wait until then end of next month for the required updated.
Cisco said the flaw does not affect users of its VPN 5000 Client software.