872469 - X-Forwarded-Message-ID/References headers expose information about S/MIME message

Status: UNCONFIRMED

(Thunderbird :: Security, defect)

Description

[René Puls]

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:21.0) Gecko/20100101 Firefox/21.0
Build ID: 20130511120803

Steps to reproduce:

There is an information leak when forwarding e-mails via S/MIME. It should be a minor issue in most cases, but could have serious consequences in others.

Scenario:

1. I receive an unencrypted e-mail.
2. I want to forward this e-mail as an encrypted S/MIME message to someone else.
3. I don't want a third party to know that I have forwarded the first message, so I change the subject (removing "Fwd: ...").


Actual results:

Thunderbird adds "References" and "X-Forwarded-Message-ID" headers to my encrypted message, containing the ID of the original message. The headers themselves are not encrypted.

First of all, a third party can clearly see which message I have forwarded (the original message was unencrypted, so its Message-ID is known). This may or may not have consequences for the person sending the message (think about a "whistleblower" scenario, for example).

Second, the third party now knows at least part of the content (the forwarded part) of my encrypted e-mail. They cannot read the text I have added, but this information alone might help in further attacks.

A similar problem exists when simply replying to an encrypted e-mail: The "References" header allows a third party to track which e-mail I have replied to.


Expected results:

From a secure-by-default point of view, Thunderbird should probably strip all "References" and "X-Forwarded-Message-ID" headers from outgoing encrypted e-mails. Other solutions would be to warn the user about the possible information leak, or to provide an advanced option to deactivate these headers for encrypted e-mails. (The optimal solution would be to somehow move this information inside the encrypted message part, but I'm not sure if that is technically possible.)

Comment 1

[Wayne Mery (:wsmwk)]

More generally, what you really need is the recipient to think that you are the original author of the message being forwarded.  bug 485593 would address that