Researchers from Ruhr University Bochum and Münster University of Applied Sciences tested 25 popular e-mail clients from various operating systems including Windows, Linux™, macOS, iOS, and Android as well as web-based clients to see how they fared against signature spoofing attacks. The team used five attack classes with the goal of the attacker being able to “create and send an email with arbitrary content to Bob whose email client falsely indicates that the email has been digitally signed by Alice” where Bob and Alice are legitimate communicators who have securely exchanged cryptographic keys/certificates.
These classes are:
• Exploiting flaws due to mishandling of Cryptographic Message Syntax (CMS).
• Performing GnuPG API injection attacks.
• MIME attacks against handling of partially signed messages.
• Displaying a valid ID on the e-mail header with a false signature.
• Using HTML and CSS to mimic valid signatures in the user interface.
The testing revealed that 14 of 20 OpenPGP clients and 15 of 22 S/MIME clients were at least partially vulnerable to these attacks. Many were able to be tricked with spoofed signatures on all UI levels, with all of the subset being able to spoof a signature even with limitations that could still go unnoticed by users. The only client to show no vulnerabilities on the OpenPGP or S/MIME tests was the web client Horde/IMP. This testing shows that just because certain standards and methods may be in wide use doesn’t necessarily mean they are secure by default. For a full list of tested clients and detailed testing methods and results, please refer to the “johnny-fired” PDF from the researchers linked below.