Winzapper is a freeware utility / hacking tool used to delete events from the Microsoft Windows NT 4.0 and Windows 2000 Security Log. It was developed by Arne Vidstrom as a proof-of-concept tool, demonstrating that once the Administrator account has been compromised, event logs are no longer reliable.[1] According to Hacking Exposed: Windows Server 2003, Winzapper works with Windows NT/2000/2003.[2]

Prior to Winzapper's creation, Administrators already had the ability to clear the Security log either through the Event Viewer or through third-party tools such as Clearlogs.[3] However, Windows lacked any built-in method of selectively deleting events from the Security Log. An unexpected clearing of the log would likely be a red flag to system administrators that an intrusion had occurred. Winzapper would allow a hacker to hide the intrusion by deleting only those log events relevant to the attack. Winzapper, as publicly released, lacked the ability to be run remotely without the use of a tool such as Terminal Services. However, according to Arne Vidstrom, it could easily be modified for remote operation.[4]

There is also an unrelated trojan horse by the same name.[5]


Winzapper creates a backup security log, "dummy.dat," at %systemroot%\system32\config. This file may be undeleted after an attack to recover the original log.[6] Conceivably, however, a savvy user might copy a sufficiently large file over the dummy.dat file and thus irretrievably overwrite it. Winzapper causes the Event Viewer to become unusable until after a reboot, so an unexpected reboot may be a clue that Winzapper has recently been used.[7] Another potential clue to a Winzapper-based attempt would be corruption of the Security Log (requiring it to be cleared), since there is always a small risk that Winzapper will do this.

According to, "One way to prevent rogue admins from using this tool on your servers is to implement a Software Restriction Policy using Group Policy that prevents the WinZapper executable from running".[8]


  1. Winzapper FAQ, NTSecurity.
  2. Joel Scambray, Stuart McClure (October 27, 2006). Hacking Exposed Windows Server 2003. McGraw-Hill Osborne Media, 1 edition. p. 228.
  3. "Hacktool.Clearlogs".
  4. Vidstrom, Arne (September 6, 2000). "Announcing WinZapper - erase individual event records in the security log of Windows NT 4.0 / 2000".
  5. "Winzapper Trojan".
  6. "Forensic Footprint of Winzapper".
  7. Seifried, Kurt. "Microsoft Security Whitepaper - Windows NT".
  8. "Gaps in Security Log".
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