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Mozilla Foundation Security Advisory 2006-21

JavaScript execution in mail when forwarding in-line

April 21, 2006
Georgi Guninski
Mozilla Suite, SeaMonkey, Thunderbird
Fixed in
  • Mozilla Suite 1.7.13
  • SeaMonkey 1.0.1
  • Thunderbird 1.0.8
  • Thunderbird


Georgi Guninski reports that forwarding mail in-line while using the default HTML "rich mail" editor will execute JavaScript embedded in the e-mail message. Forwarding mail in-line is not the default setting but it is easily accessed through the "Forward As" menu item.

This JavaScript runs with the full privileges of the client and could be used to install malware or send spam.

In Thunderbird 1.0.7 and below and Mozilla Suite 1.7.12 and below JavaScript is also executed when you reply to a mail, though limited to the sandbox of the message. This script could spy on or alter the message you are composing.


Switch to "plain text" mail composition, this vulnerability only affects HTML mail composition (the default). On the "Composition and Addressing" tab of Thunderbird's Account Settings dialog uncheck the "Compose messages in HTML format" option to compose messages in plain text.