Microsoft investigates security hole in IIS

Microsoft late yesterday issued a security advisory (971492) about a publicly reported vulnerability in IIS versions 5, 5.1 and 6.0 that could allow an attacker to elevate privileges. The vulnerability is an IIS authentication bypass but it currently requires a narrow configuration, the company says. Microsoft is currently investigating the vulnerability to see if other configurations could be successfully targeted.

The hole can currently only work if your web server meets all of the following criteria:

  • IF an IIS 5, 5.1, or 6.0 webserver is running with WebDAV enabled;
  • AND the IIS server is using IIS permissions to restrict a subfolder of content to authenticated users;
  • AND file system access is granted for the restricted content to the IUSR_[MachineName] account;
  • AND a parent folder of the private subfolder allows anonymous access;

    THEN an anonymous remote user may be able to leverage this vulnerability to access files that normally would only be served to authenticated webserver users.

Microsoft says it has not seen exploits of the vulnerability in the wild. It has not issued a patch, but has spelled out a number of workarounds, most of which involve modifying one or more of the configuration settings in the above list.

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