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Matthias Fassl

I am an interdisciplinary HCI researcher working on security, privacy, and safety. My current work focuses on technology-mediated stalking and intimate-image-based abuse in end-to-end-encrypted messengers, as part of Adam Aviv's group at GWU.

🎉 Incoming Collaborateur Scientifique at EPFL.

I hold a Dr.-Ing. from Saarland University and bring a background in computer engineering, cultural & social anthropology, privacy advocacy, and EU cybercrime policy.

Research

Online Safety & Stalking

How technology enables intimate partner surveillance and stalking — from stalkerware detection to AirTag-facilitated tracking — and how we can better protect at-risk people.

Intimate-Image-Based Abuse

Understanding and countering the non-consensual sharing of intimate images within end-to-end-encrypted messaging platforms, where moderation is architecturally constrained.

Authentication & Security UX

Why security features like authentication ceremonies and consent mechanisms fail users in practice — and how to design them to be genuinely usable rather than theatrical.

Methods in Usable Security

Developing and evaluating rigorous empirical methods — including qualitative analysis, autoethnography, and systematic reviews — for usable privacy and security research.

Publications

2025
Towards Anti-Imperialist Security, Privacy, and Safety Research in HCI
Miranda Wei, Matthias Fassl
NSPW '25
The Hidden Cost of Location Tracking
Dañiel Gerhardt, Matthias Fassl
USENIX ;login: Online · July 2025
AirTag-Facilitated Stalking Protection: Evaluating Unwanted Tracking Notifications and Tracker Locating Features
Dañiel Gerhardt, Matthias Fassl, Carolyn Guthoff, Adrian Dabrowski, Katharina Krombholz
USENIX Security 2025
2024
Investigating Security Folklore: A Case Study on the Tor over VPN Phenomenon
Matthias Fassl, Alexander Ponticello, Adrian Dabrowski, Katharina Krombholz
SOUPS 2024 · Poster
Will our Colleagues Detect the AirTag? Let's Check (Consensually)
Dañiel Gerhardt, Matthias Fassl, Katharina Krombholz
SOUPS 2024 · Poster ★ Best Poster Runner-Up
Averting Security Theater: Methods to Investigate and Integrate Secure Experience in a User-Centered Security Design Process
Matthias Fassl
Doctoral Dissertation · Saarland University
2023
Investigating Security Folklore: A Case Study on the Tor over VPN Phenomenon
Matthias Fassl, Alexander Ponticello, Adrian Dabrowski, Katharina Krombholz
ACM CSCW 2023 ★ Honorable Mention ★ Methods Recognition
Different Researchers, Different Results? Analyzing the Influence of Researcher Experience and Data Type During Qualitative Analysis
Anna-Marie Ortloff, Matthias Fassl, Alexander Ponticello, Florin Martius, Anne Mertens, Katharina Krombholz, Matthew Smith
ACM CHI 2023
Why I Can't Authenticate – Understanding the Low Adoption of Authentication Ceremonies with Autoethnography
Matthias Fassl, Katharina Krombholz
ACM CHI 2023
Empirical Research Methods in Usable Privacy and Security
Verena Distler, Matthias Fassl, Hana Habib, Katharina Krombholz, Gabriele Lenzini, Carine Lallemand, Vincent Koenig, Lorrie Faith Cranor
Book Chapter · Springer
2022
Comparing User Perceptions of Anti-Stalkerware Apps with the Technical Reality
Matthias Fassl, Simon Anell, Sabine Houy, Martina Lindorfer, Katharina Krombholz
SOUPS 2022
Exploring User-Centered Security Design for Usable Authentication Ceremonies
Matthias Fassl, Lea Gröber, Katharina Krombholz
SOUPS 2022 · Poster
Investigating Car Drivers' Information Demand after Safety and Security Critical Incidents
Lea Gröber, Matthias Fassl, Abhilash Gupta, Katharina Krombholz
SOUPS 2022 · Poster
2021
A Systematic Literature Review of Empirical Methods and Risk Representation in Usable Privacy and Security Research
Verena Distler, Matthias Fassl, Hana Habib, Katharina Krombholz, Gabriele Lenzini, Carine Lallemand, Vincent Koenig, Lorrie Faith Cranor
ACM TOCHI · Vol. 28, Issue 6
Exploring Authentication for Security-Sensitive Tasks on Smart Home Voice Assistants
Alexander Ponticello, Matthias Fassl, Katharina Krombholz
SOUPS 2021
Transferring Update Behavior from Smartphones to Smart Consumer Devices
Matthias Fassl, Michaela Neumayr, Oliver Schedler, Katharina Krombholz
ESORICS 2021 Workshop
Stop the Consent Theater
Matthias Fassl, Lea Gröber, Katharina Krombholz
alt.CHI 2021
Exploring User-Centered Security Design for Usable Authentication Ceremonies
Matthias Fassl, Lea Gröber, Katharina Krombholz
ACM CHI 2021
Investigating Car Drivers' Information Demand after Safety and Security Critical Incidents
Lea Gröber, Matthias Fassl, Abhilash Gupta, Katharina Krombholz
ACM CHI 2021
2018
Usable Authentication Ceremonies in Secure Instant Messaging
Matthias Fassl
Master Thesis · TU Wien
Wenn der Algorithmus zum Unterdrücker wird (When the Algorithm becomes the Oppressor)
Sabrina Burtscher, Matthias Fassl, Gabriel Grill
Progress Magazine · Vienna

Awards

Distinguished Reviewer Award
USENIX Security  ·  August 2025
Best Poster Runner-Up
SOUPS 2024  ·  August 2024
For "Will our Colleagues Detect the AirTag? Let's Check (Consensually)"
Google Trust & Safety Research Award
Google  ·  November 2023  ·  CISPA Helmholtz Center for Information Security
Best Paper Honorable Mention Award
ACM CSCW 2023  ·  October 2023  ·  CISPA Helmholtz Center for Information Security
Awarded to the top 3% of submissions  ·  For "Investigating Security Folklore: A Case Study on the Tor over VPN Phenomenon"

Press

Radio
Deutschlandfunk Warum Messenger-Kontakte nur selten verifiziert werdende EN ↗
Why messenger contacts are rarely verified
Nov 2023
Deutschlandfunk Is Tor still secure? Timing attacks on the anonymization networkde EN ↗
Expert commentary on the Boys Town case and the limits of Tor's anonymity guarantees
Oct 2024

Contact

Matthias Fassl
Postdoctoral Researcher, George Washington University
School of Engineering and Applied Science
Room 4390, 800 22nd Street NW, Washington, DC 20052

matthias.fassl@gwu.edu