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The Netsky.P worm was first discovered in March 2004, and two years later it continues to maintain a presence on antivirus prevalence charts. Netsky.P exploits an old (and patched) vulnerability, using a malformed MIME header to trick Internet Explorer into launching its email attachment. Microsoft provided a patch for the vulnerability, MS01-020, in March 2001. So what keeps an old worm circulating? The most likely reason Netsky.P continues to dominate may be the fact that it copies itself to P2P shared folders, using a wide variety of provocative filenames.