"I don’t know why I check this...'' - Investigating Expert Users' Strategies to Detect Email Signature Spoofing Attacks


Peter Mayer, SECUSO - Security, Usability, Society, Karlsruhe Institute of Technology; Damian Poddebniak, Münster University of Applied Sciences; Konstantin Fischer and Marcus Brinkmann, Ruhr University Bochum; Juraj Somorovsky, Paderborn University; Angela Sasse, Ruhr University Bochum; Sebastian Schinzel, Münster University of Applied Sciences; Melanie Volkamer, SECUSO - Security, Usability, Society, Karlsruhe Institute of Technology


OpenPGP is one of the two major standards for end-to-end email security. Several studies showed that serious usability issues exist with tools implementing this standard. However, a widespread assumption is that expert users can handle these tools and detect signature spoofing attacks. We present a user study investigating expert users' strategies to detect signature spoofing attacks in Thunderbird. We observed 25 expert users while they classified eight emails as either having a legitimate signature or not. Studying expert users explicitly gives us an upper bound of attack detection rates of all users dealing with PGP signatures. 52% of participants fell for at least one out of four signature spoofing attacks. Overall, participants did not have an established strategy for evaluating email signature legitimacy. We observed our participants apply 23 different types of checks when inspecting signed emails, but only 8 of these checks tended to be useful in identifying the spoofed or invalid signatures. In performing their checks, participants were frequently startled, confused, or annoyed with the user interface, which they found supported them little. All these results paint a clear picture: Even expert users struggle to verify email signatures, usability issues in email security are not limited to novice users, and developers may need proper guidance on implementing email signature GUIs correctly.

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@inproceedings {281268,
author = {Peter Mayer and Damian Poddebniak and Konstantin Fischer and Marcus Brinkmann and Juraj Somorovsky and Angela Sasse and Sebastian Schinzel and Melanie Volkamer},
title = {"I {don{\textquoteright}t} know why I check this...{\textquoteright}{\textquoteright} - Investigating Expert Users{\textquoteright} Strategies to Detect Email Signature Spoofing Attacks},
booktitle = {Eighteenth Symposium on Usable Privacy and Security (SOUPS 2022)},
year = {2022},
isbn = {978-1-939133-30-4},
address = {Boston, MA},
pages = {77--96},
url = {https://www.usenix.org/conference/soups2022/presentation/mayer},
publisher = {USENIX Association},
month = aug

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