Excessive Escape Chars Can Slow IIS

Excessive Escape Chars Can Slow IIS
Reported April 12, 2000 by
Vanja Hrustic

  • Microsoft Internet Information Server 4.0
Microsoft Internet Information Server 5.0


According to Microsoft"s report on the matter, "Special characters can be embedded in URLs by use of so-called escaped character sequences. By providing a specially-malformed URL with an extremely large number of escaped characters, a malicious user could arbitrarily increase the work factor associated with parsing the escaped characters, thereby consuming much or all of the CPU availability on the server and preventing useful work from being done.

"The vulnerability does not provide any capability to cause the server to fail, or to add, change or delete data on it. The slowdown would only last until the URL had been processed, at which point service would return to normal."


Microsoft has issued a patch for IIS 4.0 and IIS 5.0, a FAQ, and Support Online article Q254142

Discovered and reported by
Vanja Hrustic

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