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Mozilla Foundation Security Advisory 2014-14

Script execution in HTML mail replies

February 6, 2014
Fabián Cuchietti, Ateeq ur Rehman Khan
SeaMonkey, Thunderbird
Fixed in
  • SeaMonkey 2.20
  • Thunderbird 23


Security researcher Fabián Cuchietti discovered that it was possible to bypass the restriction on JavaScript execution in mail by embedding an <iframe> with a data: URL within a message. If the victim replied or forwarded the mail after receiving it, quoting it "in-line" using Thunderbird's HTML mail editor, it would run the attached script. The running script would be restricted to the mail composition window where it could observe and potentially modify the content of the mail before it was sent. Scripts were not executed if the recipient merely viewed the mail, only if it was edited as HTML. Turning off HTML composition prevented the vulnerability and forwarding the mail "as attachment" prevented the forwarding variant.

Ateeq ur Rehman Khan of Vulnerability Labs reported additional variants of this attack involving the use of the <object> tag and which could be used to attach object data types such as images, audio, or video.

This affected the Thunderbird 17 branch. It was fixed in all versions based on Gecko 23 or later. Thunderbird 24 and later are not affected by this vulnerability.