oss-sec mailing list archives
Re: PGP/MIME and S/MIME mail clients vulnerabilities
From: Brian May <brian () linuxpenguins xyz>
Date: Tue, 15 May 2018 17:40:04 +1000
Yves-Alexis Perez <corsac () debian org> writes:
So, as far as I can tell, in that attack scenario (where the attacker has read/write access to encrypted mails): - S/MIME is completely broken at the protocol level since it has no way to defend against blind modification. Only mitigation for the clients are to prevent HTML mails and/or prevent loading of external resources. There might be other avenues to exploit the vulnerability in the future though. - PGP/MIME is a bit safer because the OpenPGP format compresses plaintext before encryption (which makes it harder for the attacker) and has some kind of authenticated (symmetric) encryption (the MDC), which helps gnupg detects modifications to the cyphertext. Most mail clients properly handle gnupg hints when something went wrong but the external interface is a bit fragile (gnupg will still output the cleartext, for example). One exception is apparently Thunderbird with enigmail before 2.0.0, but this is now fixed (I didn't find the proper commit yet). Again, not displaying HTML mails and not allowing remote content loading can help, but other “backchannels” might be found in the future.
Have a look at some official statements on this: * https://lists.gnupg.org/pipermail/gnupg-users/2018-May/060334.html * https://protonmail.com/blog/pgp-vulnerability-efail/ For the case of PGP it sounds like the only problems occur when mail clients ignore the GPG hints. For S/MIME, it does sound like the standard is broken and needs fixing. If I understand this correctly, the "Direct Exfiltration" is an attack that doesn't require modifying the encrypted data - so presumably the MDC in PGP won't help. To me this sounds like a email client problem (allowing mixing encrypted and encrypted data in the one HTML document seems like a very bad idea), but the https://efail.de/ page says the standards need to be updated to fix this. -- Brian May <brian () linuxpenguins xyz> https://linuxpenguins.xyz/brian/
Current thread:
- PGP/MIME and S/MIME mail clients vulnerabilities Yves-Alexis Perez (May 14)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Jakub Wilk (May 14)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Christian Brabandt (May 14)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Yves-Alexis Perez (May 14)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Brian May (May 15)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Yves-Alexis Perez (May 15)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Leo Gaspard (May 15)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Brian May (May 16)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Brian May (May 16)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Yves-Alexis Perez (May 14)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Yves-Alexis Perez (May 16)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Matthew Fernandez (May 16)
- Re: PGP/MIME and S/MIME mail clients vulnerabilities Florian Weimer (May 22)