Post Syndicated from Robert Graham original https://blog.erratasec.com/2018/05/some-notes-on-efail.html
I’ve been busy trying to replicate the “eFail” PGP/SMIME bug. I thought I’d write up some notes.
PGP and S/MIME encrypt emails, so that eavesdroppers can’t read them. The bugs potentially allow eavesdroppers to take the encrypted emails they’ve captured and resend them to you, reformatted in a way that allows them to decrypt the messages.
Disable remote/external content in email
The most important defense is to disable “external” or “remote” content from being automatically loaded. This is when HTML-formatted emails attempt to load images from remote websites. This happens legitimately when they want to display images, but not fill up the email with them. But most of the time this is illegitimate, they hide images on the webpage in order to track you with unique IDs and cookies. For example, this is the code at the end of an email from politician Bernie Sanders to his supporters. Notice the long random number assigned to track me, and the width/height of this image is set to one pixel, so you don’t even see it:
Such trackers are so pernicious they are disabled by default in most email clients. This is an example of the settings in Thunderbird:
The problem is that as you read email messages, you often get frustrated by the fact the error messages and missing content, so you keep adding exceptions:
The correct defense against this eFail bug is to make sure such remote content is disabled and that you have no exceptions, or at least, no HTTP exceptions. HTTPS exceptions (those using SSL) are okay as long as they aren’t to a website the attacker controls. Unencrypted exceptions, though, the hacker can eavesdrop on, so it doesn’t matter if they control the website the requests go to. If the attacker can eavesdrop on your emails, they can probably eavesdrop on your HTTP sessions as well.
Some have recommended disabling PGP and S/MIME completely. That’s probably overkill. As long as the attacker can’t use the “remote content” in emails, you are fine. Likewise, some have recommend disabling HTML completely. That’s not even an option in any email client I’ve used — you can disable sending HTML emails, but not receiving them. It’s sufficient to just disable grabbing remote content, not the rest of HTML email rendering.
I couldn’t replicate the direct exfiltration
There rare two related bugs. One allows direct exfiltration, which appends the decrypted PGP email onto the end of an IMG tag (like one of those tracking tags), allowing the entire message to be decrypted.
An example of this is the following email. This is a standard HTML email message consisting of multiple parts. The trick is that the IMG tag in the first part starts the URL (blog.robertgraham.com/…) but doesn’t end it. It has the starting quotes in front of the URL but no ending quotes. The ending will in the next chunk.
The next chunk isn’t HTML, though, it’s PGP. The PGP extension (in my case, Enignmail) will detect this and automatically decrypt it. In this case, it’s some previous email message I’ve received the attacker captured by eavesdropping, who then pastes the contents into this email message in order to get it decrypted.
What should happen at this point is that Thunderbird will generate a request (if “remote content” is enabled) to the blog.robertgraham.com server with the decrypted contents of the PGP email appended to it. But that’s not what happens. Instead, I get this:
I am indeed getting weird stuff in the URL (the bit after the GET /), but it’s not the PGP decrypted message. Instead what’s going on is that when Thunderbird puts together a “multipart/mixed” message, it adds it’s own HTML tags consisting of lines between each part. In the email client it looks like this:
The HTML code it adds looks like:
That’s what you see in the above URL, all this code up to the first quotes. Those quotes terminate the quotes in the URL from the first multipart section, causing the rest of the content to be ignored (as far as being sent as part of the URL).
So at least for the latest version of Thunderbird, you are accidentally safe, even if you have “remote content” enabled. Though, this is only according to my tests, there may be a work around to this that hackers could exploit.