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The EFAIL Hoax

In 2018 a security issue dubbed EFAIL1) was all over the technical media and even leaked into the regular media. Many of those articles were misleading or incorrect. Some gave dangerous advice. Here is an example of a particularly hyperbolic headline:

To be completely clear… The word “Hoax” in the title of this article refers exclusively to the media attempts to make it seem that EFAIL represented some deficiency in PGP. It also can be considered a satire of needlessly provocative headlines. EFAIL represents real issues. They were just misrepresented.

EFAIL was a list of different ways to cause inherently insecure message content (HTML email) to leak decrypted messages. Such data leakage was a known issue and was under routine exploitation at the time. This fact alone should be enough to convince most people EFAIL had nothing to do with either PGP (or S/MIME). When you have a hole big enough to drive a truck through there is no extra value in discussing the size and shape of the truck. Unfortunately in the case of EFAIL we need to spend time discussing truck dimensions.

The EFAIL effect is triggered by creating a message so that when it is decrypted and then interpreted as HTML the interpretation process will leak the decrypted message. Something like <img src=“ placed immediately before the desired text would create a URL that would leak the message to a web server controlled by the attacker. The details depend on the mail client and there are probably a lot of different methods available, limited only by the attacker's cleverness and imagination. The EFAIL paper listed several.

One of these methods (dubbed “CFB gadgets”) involved modifying the encrypted HTML message so that when it was decrypted and interpreted it would leak the message by the EFAIL effect. The suggestion was made that the OpenPGP specification was flawed for allowing such attacks in the form of a CVE2).

The traditional solution to this particular modification is an extra check on top of the encryption called authenticated encryption. The suggested fix in the EFAIL paper was the addition of authenticated encryption to the OpenPGP specification. There is a fundamental problem with this line of thinking.

The OpenPGP specification already has a simple integrity check called the MDC that is entirely adequate to detect this particular attack. The addition of authenticated encryption to PGP would make no difference at all for any of the demonstrated EFAIL leaks. It is not possible to suggest authenticated encryption here without also acknowledging that the OpenPGP specification is not susceptible to EFAIL. The CVE was most likely disputed on this basis.

This might be a good place to point out how wildly impractical the CFB gadget method is compared to the other methods listed in the EFAIL paper. To make a CFB gadget attack work you need to know the first 11 bytes/characters of the unencrypted message. The EFAIL paper suggested that the use of the multipart/mixed MIME attribute could allow as many as 500 guesses at those 11 bytes/characters in the same message. Unfortunately the paper did not give any examples of clients where this might work. Even if there were email clients out there that will allow hundreds of attempts at decoding anonymous encrypted messages in the same message while ignoring hundreds of modification errors the attack was still quite impractical. The attack email is seen by the user and would normally inform them of the fact they were under some sort of attack. Then they would likely have reported it and the email client(s) would have fixed their bug(s). This sort of thing is typical for encrypted email where the recipient always gets to see the attack message.

It is sometimes claimed that OpenPGP implementations (GnuPG in particular) are at fault for EFAIL because they “release unauthenticated plaintext”. After all, if the OpenPGP implementation had detected that there was a potential modification and had blown up with an error before releasing any of that modified text, then none of this would of happened.

Any data leak could be prevented by detecting the potential problem ahead of time and then refusing to release data. We are adding a known insecure mode (HTML email) to a system and then blaming the cryptography code for not anticipating the potential problem when things inevitably go wrong? This seems a bit random. At any rate, the “just blow up” approach is not practical for the sort of offline applications that OpenPGP is normally used for. An email shows up with a possible modification. Then what? You can't just show the user a blank screen. It would be a perfectly reasonable approach to warn the user and then show the message in a very safe environment. If the user is under attack they need to be allowed to try to figure out how. Chances are it will just be some sort of transmission error. The same sort of thing applies to encrypted files. The user might want to try to recover the corrupted data.

The spin required to make this a PGP problem was unfortunate in that it distracted from the actual issue of inherently insecure message content in encrypted messaging. The good work done by the security researchers ended up being in a sense wasted. Inherently insecure message content is still a serious issue today (2020).

A more fundamental problem with all this is that when used for messaging, PGP uses a combined identity/integrity check in the form of a cryptgraphic signature. In PGP messaging both the MDC and hypothetical authenticated encryption are less important compared to the question of allowing the handling of anonymous (invalid or missing signature) messages in a very insecure way. So this ends up being a kind of a straw man. This would of been much better if it was discussed in terms of how PGP actually works and how these vulnerabilities affect the clients in that context.

The OpenPGP standard and implementations of that standard have suffered from security weaknesses of greater significance than EFAIL with no media coverage at all. That is why the media blowup over EFAIL is so odd. I have no idea why this ended up so wrong. What ever the reasons, this serves as yet another indication of the poor quality of the technical press.

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pgpfan/efail.txt · Last modified: 2022/05/16 21:17 by b.walzer