Problem
When IWSS is used as the HTTP proxy, access to websites is blocked and the Corrupted_Zip_File error appears even if compressed files are not being downloaded.
This error occurs only if IWSS is in HTTP proxy and ICAP modes, as well as when the browser used is configured to use HTTP 1.1. It affects only HTTP 1.1 because the web server of the visited web site returns the site content in GZIP format when the request is from HTTP 1.1. In HTTP, the web server displays the site content in plain text.
Our Scan Engine component returns the Corrupted_Zip_File error when it attempts to read the header field of a GZIP data block or decompresses using the GZIP tool. Also, the Scan Engine’s inability to identify the real corrupted ZIP file is due to the GZIP's incomplete header in the last segment, so there is not enough information to extract with confidence.
On the other hand, WinZip uses the brute force method of decompression. This does not use much parity check, thus all the site contents are shown on the web site. However, this decompression method compromises security and makes users vulnerable to viruses.
http://esupport.trendmicro.com/support/viewprint.do?ContentID=en-124474&View=p