“Johnny, you are fired!” – Spoofing OpenPGP and S/MIME Signatures in Emails


Jens Müller and Marcus Brinkmann, Ruhr University Bochum; Damian Poddebniak, Münster University of Applied Sciences; Hanno Böck, unaffiliated; Sebastian Schinzel, Münster University of Applied Sciences; Juraj Somorovsky and Jörg Schwenk, Ruhr University Bochum


OpenPGP and S/MIME are the two major standards to encrypt and digitally sign emails. Digital signatures are supposed to guarantee authenticity and integrity of messages. In this work we show practical forgery attacks against various implementations of OpenPGP and S/MIME email signature verification in five attack classes: (1) We analyze edge cases in S/MIME's container format. (2) We exploit in-band signaling in the GnuPG API, the most widely used OpenPGP implementation. (3) We apply MIME wrapping attacks that abuse the email clients' handling of partially signed messages. (4) We analyze weaknesses in the binding of signed messages to the sender identity. (5) We systematically test email clients for UI redressing attacks. Our attacks allow the spoofing of digital signatures for arbitrary messages in 14 out of 20 tested OpenPGP-capable email clients and 15 out of 22 email clients supporting S/MIME signatures. While the attacks do not target the underlying cryptographic primitives of digital signatures, they raise concerns about the actual security of OpenPGP and S/MIME email applications. Finally, we propose mitigation strategies to counter these attacks.

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@inproceedings {235491,
author = {Jens M{\"u}ller and Marcus Brinkmann and Damian Poddebniak and Hanno B{\"o}ck and Sebastian Schinzel and Juraj Somorovsky and J{\"o}rg Schwenk},
title = {{{\textquotedblleft}Johnny}, you are {fired!{\textquotedblright}} {\textendash} Spoofing {OpenPGP} and {S/MIME} Signatures in Emails},
booktitle = {28th USENIX Security Symposium (USENIX Security 19)},
year = {2019},
isbn = {978-1-939133-06-9},
address = {Santa Clara, CA},
pages = {1011--1028},
url = {https://www.usenix.org/conference/usenixsecurity19/presentation/muller},
publisher = {USENIX Association},
month = aug,

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