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By: Joeri Jungschlager
May 14, 2018
The PGP key Panic: Safety 101

By: Joeri Jungschlager
May 14, 2018

The topic of that paper is that HTML is used as a back channel to create an oracle for modified encrypted mails. It is long known that HTML mails and in particular external links like <img href="tla.org/TAG"/> are evil if the MUA actually honors them (which many meanwhile seem to do again; see all these newsletters). Due to broken MIME parsers a bunch of MUAs seem to concatenate decrypted HTML mime parts which makes it easy to plant such HTML snippets.There are two ways to mitigate this attack:What do now?The latter is actually easy for OpenPGP because we started to use authenticated encryption (AE) since 2000 or 2001. Our AE is called MDC (Modification detection code) and was back then introduced for a very similar attack. Unfortunately, some OpenPGP implementations were late to introduce MDC and thus GPG could not fail hard on receiving a mail without an MDC. However, an error is returned during decrypting and no MDC is used:"
- Don't use HTML mails. Or if you really need to read them use a proper MIME parser and disallow any access to external links.
- Use authenticated encryption.
- Disable the usage of HTML in e-mails.
- Use authenticated encryption.
- Look for what clients and plugins are being used in your company.
- Thunderbird with Enigmail
- Apple Mail with GPGTools
- Outlook with Gpg4win
- https://www.eff.org/deeplinks/2018/05/attention-pgp-users-new-vulnerabilities-require-you-take-action-now
- https://lists.gnupg.org/pipermail/gnupg-users/2018-May/060315.html
- https://lists.gnupg.org/pipermail/gnupg-users/2018-May/060317.html