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Mozilla Foundation Security Advisory 2011-39

Title: Defense against multiple Location headers due to CRLF Injection
Impact: Moderate
Announced: September 27, 2011
Reporter: Ian Graham
Products: Firefox, Thunderbird, SeaMonkey

Fixed in: Firefox 7.0
  Firefox 3.6.23
  Thunderbird 7.0
  Thunderbird 3.1.15
  SeaMonkey 2.4


Ian Graham of Citrix Online reported that when multiple Location headers were present in a redirect response Mozilla behavior differed from other browsers: Mozilla would use the second Location header while Chrome and Internet Explorer would use the first. Two copies of this header with different values could be a symptom of a CRLF injection attack against a vulnerable server. Most commonly it is the Location header itself that is vulnerable to the response splitting and therefore the copy preferred by Mozilla is more likely to be the malicious one. It is possible, however, that the first copy was the injected one depending on the nature of the server vulnerability.

The Mozilla browser engine has been changed to treat two copies of this header with different values as an error condition. The same has been done with the headers Content-Length and Content-Disposition