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Flaw in Thunderbird bypasses Firefox 'Torified' security and privacy defenses

After waiting more than two years for a flaw to be fixed, a researcher publicly disclosed a security bug in the Thunderbird email client.

Do you use the free email client Thunderbird? Do you also use Tor? If so, then you should know there's been a security flaw awaiting a fix from Mozilla for over two years; now the bug has been publicly disclosed. Mike Cardwell, a developer, IT consultant, sysadmin and security researcher in the UK, informed the Tor-talk mailing list about a security issue in the Thunderbird app.

Normally, when you click on a link in email, the link opens in your default web browser. Hopefully, you've all but weaponized your browser with extensions and addons to better protect your privacy and security. If you are using Tor, then you're going to a bit more trouble to protect yourself, and you don't want your defenses bypassed. However, when blogging more details about the security leak in Thunderbird, Cardwell explained: "I've discovered a way of crafting a link such that when you're using Thunderbird and you click on that link, it opens the website in a new Thunderbird tab instead of in the external web browser."

In Cardwell's case, his "browser of choice is Firefox." He wrote:

I have made various configuration changes and installed various addons in Firefox to enhance my security and privacy. Amongst other things, I use RequestPolicy, NoScript, RefControlAdBlock, CipherFoxHTTPS-Everywhere, I have proxy settings and sometimes I use Tor. If a link opens in a Thunderbird tab instead of a Firefox tab, all of those defenses are bypassed.

Secondly, when the external website opens in a Thunderbird tab, there is no identifying chrome around the page which would allow the user to differentiate between a tab containing any other part of the Thunderbird interface and a malicious site which is spoofing part of the Thunderbird interface.

Then Cardwell laid out the details:

The email must contain a text/html part. That text/html part must contain an anchor embedded in an inline SVG [Scalable Vector Graphics]. That anchor tag must have either the target attribute set to "_blank", or the "xlink:show attribute set to "new". Example:

With normal anchor tags you can right click on a link and then select "Copy Link Location" from the context menu and paste it into the web browser. However, that option is not available in the context menu when right clicking SVG anchors.

Cardwell added that he reported the security flaw "to Mozilla in November 2011 (26 months ago) and it was promptly acknowledged as a 'moderate' security problem by them. It has not been fixed yet." If you try to check security bug 700979, you currently see "access denied," but Cardwell included a link for when the bug details are unlocked.

Since finding the flaw and waiting for the vulnerability to be patched, Cardwell has moved away from using Thunderbird. Instead, he suggested using the desktop email client Evolution. "It has built in PGP and Calendar support, without needing to use third-party addons, has a much faster UI than Thunderbird, and unlike Thunderbird is currently under heavy development. Oh, and it also has write support for LDAP-based address books (unlike Thunderbird)."

If you are wondering why you should take Cardwell's security advice about migrating away from Thunderbird to Evolution, then consider that, among other things, he's discovered numerous security and privacy flaws, as well as contributing rulesets to HTTPS Everywhere.

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