Mozilla Foundation Security Advisory 2015-127

CORS preflight is bypassed when non-standard Content-Type headers are received

November 3, 2015
Shinto K Anto
Firefox, Firefox ESR, Firefox OS, Thunderbird
Fixed in
  • Firefox 42
  • Firefox ESR 38.4
  • Firefox OS 2.5
  • Thunderbird 38.4


Security researcher Shinto K Anto reported an issue with cross-origin resource sharing (CORS) "preflight" requests when receiving certain Content-Type headers. This is due to an error in implementation resulting in trying to process multiple media types when they are returned in the Content-Type headers from a server. This is disallowed in the CORS specification and results in a simple instead of a "preflight" request, leading to potential same-origin policy violation.

In general this flaw cannot be exploited through email in the Thunderbird product because scripting is disabled, but is potentially a risk in browser or browser-like contexts.