Some thoughts on efail  - sky - Despite Efail, the sky is not falling

By now, everyone and their dog will have probably heard about the so-called “Efail” attack, and read the researchers’ description that was released early (because although they had managed to create a logo and a domain name, they didn’t manage to get an embargo to stick.)

Some folks may even have read the researchers’ technical paper detailing Efail.

Here’s the issue summarised by the researchers themselves:

In a nutshell, abuses active content of HTML emails, for example externally loaded images or styles, to exfiltrate plaintext through requested URLs.

The attacker changes an encrypted email in a particular way and sends this changed encrypted email to the victim. The victim’s email client decrypts the email and loads any external content, thus exfiltrating the plaintext to the attacker.

Efail 123  - efail 123 - Despite Efail, the sky is not falling

Sneaky. However, in order to do this an attacker needs to have access to past encrypted emails you have received, as the researchers acknowledge:

To create these exfiltration channels, the attacker first needs access to the encrypted emails, for example, by eavesdropping on network traffic, compromising email accounts, email servers, backup systems or client computers. The emails could even have been collected years ago.

That makes me raise an eyebrow about just how much of a realistic risk is posed by the Efail attack. If a malicious already has access to your email servers, networks, and such like, there’s probably all manner of worse and less convoluted things they could be doing to make your life a misery, steal secrets, and destroy your .

Here are some of my other thoughts:

  • Yes, it’s a sneaky attack method. But it’s not reliant on any inherent weakness in the PGP/GPG encryption being used. Instead it exploits users who haven’t told their email clients to stop remote or external content from being automatically rendered.
  • It’s not strictly a brand new problem either. The root problem of mail clients attempting to display corrupted S/MIME messages has been known about since 2000 or 2001. A better-behaving email client might notice that, to use the researchers’ example pictured above, the HTML image tag does not get closed properly until after the encrypted parts of the email message.
  • Who would be interested in going to all this effort to read your messages? Intelligence agencies and state-sponsored hackers are the most likely culprits. And yet, because the attack relies upon past encrypted emails being sent to the target, this is a very visible and obvious attack method. Typically state-sponsored spies don’t want their victims to know that they are being targeted.
  • Is this kind of attack hard to spot? No. It wouldn’t be tricky I suspect to write a script that scanned incoming email for malformed IMG tags, for instance.
  • Is Efail a good reason for folks who use PGP/GPG to disable it entirely? I don’t think so. You’re probably putting yourself at greater risk if you have something sensitive to communicate by reverting to unencrypted email. Of course, there are other end-to-end encrypted messaging solutions out there which don’t face the same clunky challenges as encrypted email.

In :

The is not falling, stop freaking out.

Keep your email clients updated with the latest patches as they become available.

Consider disabling rendering of remote content until the issue is resolved (this will have all manner of other privacy advantages), and maybe even embrace good old fashioned non-HTML email!

You may also wish to prevent automatic decryption of email messages, and require users to manually request decryption instead to reduce the chances of leakage via active content.

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About the author, Graham Cluley

Graham Cluley is a veteran of the anti-virus industry having worked for a number of security companies since the early 1990s when he wrote the first ever version of Dr Solomon’s Anti-Virus Toolkit for Windows. Now an independent security analyst, he regularly makes media appearances and is an international public speaker on the topic of computer security, hackers, and online privacy.

Follow him on Twitter at @gcluley, or drop him an email.

Follow @gcluley





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