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Mozilla Foundation Security Advisory 2009-05

Title: XMLHttpRequest allows reading HTTPOnly cookies
Impact: Low
Announced: February 3, 2009
Reporter: Wladimir Palant
Products: Firefox, SeaMonkey

Fixed in: Firefox 3.0.6
  SeaMonkey 1.1.15


Developer and Mozilla community member Wladimir Palant reported that cookies marked HTTPOnly were readable by JavaScript via the XMLHttpRequest.getResponseHeader and XMLHttpRequest.getAllResponseHeaders APIs. This vulnerability bypasses the security mechanism provided by the HTTPOnly flag which intends to restrict JavaScript access to document.cookie.

The fix prevents the XMLHttpRequest feature from accessing the Set-Cookie and Set-Cookie2 headers of any response whether or not the HTTPOnly flag was set for those cookies.