Anti-Virus Products Miss Malware in Modified Zip Files

An anonymous researcher has reported through several sources that anti-virus products from six vendors fail to detect malware when it is contained in a corrupted Zip file.The modification to the Zip file prevents the anti-virus programs from detecting files in it, but it doesn’t prevent users from accessing those files, according to an advisory from Internet security research firm iDefense. The vulnerable products are from McAfee Inc., Computer Associates International Inc., Kaspersky Labs, Sophos plc., Eset Software and RAV, according to the advisory.

The problem has to do with the products’ handling of an error condition in Zip files, which store information about compressed files stored within them in two locations. There is a local header preceding each file in the archive and a global header at the end of the archive. When the uncompressed size of the file within both archives is set to zero, the affected programs fail to detect malware in the files.

According to their advisory, iDefense notified the affected vendors of the problem on September 16. Some responded in time for Monday’s advisory. McAfee provided a detailed explanation, fixes for their products, and noted that there are no known exploits of this technique. Computer Associates and Eset also responded and provided fixes, according to iDefense.

Kaspersky indicated that the problem would be fixed in their next release. Neither RAV nor Sophos responded, according to iDefense.

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