Software Engineering Institute

Vulnerability Lab has reported a vulnerability in the way Mozilla Thunderbird handles HTML elements in email content. Mozilla Thunderbird blocks the creation of certain HTML elements, such as script, when displaying email messages. Traditionally, a script element is created through the use of a <script> HTML tag. HTML elements, including script, can also be created through the use of an <object> tag that specifies a Data URI scheme (RFC 2397). The Data URI can specify a text/html mime type and encode the script in base64. In such cases, Thunderbird will execute the script contained in the email message when it is forwarded or replied to and the outgoing message is in HTML format. Simply displaying the email message does not appear to cause the script to execute.

See Mozilla Bug Bounty #5 - WireTap Remote Web Vulnerability for more details.

Testing indicates that Thunderbird 17.0.{6,7,8} are vulnerable. Earlier versions may also be vulnerable.

By creating a specially-crafted email message, an attacker can cause arbitrary script to execute in Thunderbird when that message is forwarded or replied to.

Apply an update

Limited testing has shown that Thunderbird versions 24.0 and later are not affected by this vulnerability.

Compose email in plain text format

Disabling the setting to "Compose messages in HTML format" for each email account will help protect against attacks. This will cause outgoing messages to be constructed in plain text, which does not contain HTML elements.