Andre Büttner

• Büttner, Andre & Gruschka, Nils (2025). Device-Bound vs. Synced Credentials: A Comparative Evaluation of Passkey Authentication. ICISSP. 2. doi: 10.5220/0013380600003899. Full text in Research Archive Show summary

  With passkeys, the FIDO Alliance introduces the ability to sync FIDO2 credentials across a user’s devices through passkey providers. This aims to mitigate user concerns about losing their devices and promotes the shift toward password-less authentication. As a consequence, many major online services have adopted passkeys. However, credential syncing has also created a debate among experts about their security guarantees. In this paper, we categorize the different access levels of passkeys to show how syncing credentials impacts their security and availability. Moreover, we use the established framework from Bonneau et al.’s Quest to Replace Passwords and apply it to different types of device-bound and synced passkeys. By this, we reveal relevant differences, particularly in their usability and security, and show that the security of synced passkeys is mainly concentrated in the passkey provider. We further provide practical recommendations for end users, passkey providers, and relying parties.

• Büttner, Andre; Gruschka, Nils; Broen, Sverre Stafsengen & Pöhn, Daniela (2025). Authentication Inconsistencies Across Online Services: A Multi-Scenario Security Analysis. In Coppens, Bart; Volckaert, Bruno; Naessens, Vincent & Sutter, Bjorn De (Ed.), Availability, Reliability and Security: ARES 2025 International Workshops, Ghent, Belgium, August 11–14, 2025, Proceedings, Part IV. Springer. ISSN 9783032006394. doi: 10.1007/978-3-032-00639-4_10. Full text in Research Archive Show summary

  Online services are integral to modern life, supporting activities such as communication, commerce, and travel. These services typically require user authentication, traditionally relying on user ID and password combinations. However, this approach is increasingly vulnerable to attacks such as phishing. Many services have adopted stronger authentication mechanisms, including multi-factor authentication, risk-based authentication, and passkeys. Despite extensive research on login procedures, limited attention has been given to these post-login authentication processes. This paper presents a first study investigating the interplay between multi-factor authentication and context-specific authentication for ten popular online services. The results indicate that various authentication methods and behaviors can be observed across different scenarios and services.

• Büttner, Andre & Gruschka, Nils (2024). Evaluating the Influence of Multi-Factor Authentication and Recovery Settings on the Security and Accessibility of User Accounts. ICISSP. doi: 10.5220/0012319000003648. Full text in Research Archive Show summary

  Nowadays, most online services offer different authentication methods that users can set up for multi-factor authentication but also as a recovery method. This configuration must be done thoroughly to prevent an adversary’s access while ensuring the legitimate user does not lose access to their account. This is particularly important for fundamental everyday services, where either failure would have severe consequences. Nevertheless, little research has been done on the authentication of actual users regarding security and the risk of being locked out of their accounts. To foster research in this direction, this paper presents a study on the account settings of Google and Apple users. Considering the multi-factor authentication configuration and recovery options, we analyzed the account security and lock-out risks. Our results provide insights into the usage of multi-factor authentication in practice, show significant security differences between Google and Apple accounts, and reveal (More)

• Büttner, Andre; Pedersen, Andreas Thue; Wiefling, Stephan; Gruschka, Nils & Iacono, Luigi Lo (2024). Is It Really You Who Forgot the Password? When Account Recovery Meets Risk-Based Authentication, Ubiquitous Security. Springer Nature. ISSN 9789819712748. doi: 10.1007/978-981-97-1274-8_26. Full text in Research Archive Show summary

  Risk-based authentication (RBA) is used in online services to protect user accounts from unauthorized takeover. RBA commonly uses contextual features that indicate a suspicious login attempt when the characteristic attributes of the login context deviate from known and thus expected values. Previous research on RBA and anomaly detection in authentication has mainly focused on the login process. However, recent attacks have revealed vulnerabilities in other parts of the authentication process, specifically in the account recovery function. Consequently, to ensure comprehensive authentication security, the use of anomaly detection in the context of account recovery must also be investigated. This paper presents the first study to investigate risk-based account recovery (RBAR) in the wild. We analyzed the adoption of RBAR by five prominent online services (that are known to use RBA). Our findings confirm the use of RBAR at Google, LinkedIn, and Amazon. Furthermore, we provide insights into the different RBAR mechanisms of these services and explore the impact of multi-factor authentication on them. Based on our findings, we create a first maturity model for RBAR challenges. The goal of our work is to help developers, administrators, and policy-makers gain an initial understanding of RBAR and to encourage further research in this direction.

• Hansen, Malte & Büttner, Andre (2024). Secure and Privacy-Preserving Authentication for Data Subject Rights Enforcement, Privacy and Identity Management. Sharing in a Digital World. Springer Nature. ISSN 9783031579783. p. 175–191. doi: 10.1007/978-3-031-57978-3_12. Full text in Research Archive Show summary

  In light of the GDPR, data controllers (DC) need to allow data subjects (DS) to exercise certain data subject rights. A key requirement here is that DCs can reliably authenticate a DS. Due to a lack of clear technical specifications, this has been realized in different ways, such as by requesting copies of ID documents or by email address verification. However, previous research has shown that this is associated with various security and privacy risks and that identifying DSs can be a non-trivial task. In this paper, we review different authentication schemes and propose an architecture that enables DCs to authenticate DSs with the help of independent Identity Providers in a secure and privacy-preserving manner by utilizing attribute-based credentials and eIDs. Our work contributes to a more standardized and privacy-preserving way of authenticating DSs, which will benefit both DCs and DSs.

• Pöhn, Daniela; Gruschka, Nils; Ziegler, Leonhard & Büttner, Andre (2023). A framework for analyzing authentication risks in account networks. Computers & Security. ISSN 0167-4048. 135. doi: 10.1016/j.cose.2023.103515. Full text in Research Archive
• Büttner, Andre & Gruschka, Nils (2023). Protecting FIDO Extensions Against Man-in-the-Middle Attacks, Emerging Technologies for Authorization and Authentication, 5th International Workshop, ETAA 2022, Copenhagen, Denmark, September 30, 2022, Revised Selected Papers. Springer. ISSN 9783031254673. p. 70–87. doi: 10.1007/978-3-031-25467-3_5. Full text in Research Archive Show summary

  FIDO authentication has many advantages over password-based authentication, since it relies on proof of possession of a security key. It eliminates the need to remember long passwords and, in particular, is resistant to phishing attacks. Beyond that, the FIDO protocols consider protocol extensions for more advanced use cases such as online transactions. FIDO extensions, however, are not well protected from Man-in-the-Middle (MitM) attacks. This is because the specifications require a secure transport between client and server, but there exists no end-to-end protection between server and authenticator. In this paper, we discuss MitM scenarios in which FIDO extensions may be intercepted. We further propose an application-layer security protocol based on the CBOR Object Signing and Encryption (COSE) standard to mitigate these threats. This protocol was verified in a formal security evaluation using ProVerif and, finally, implemented in a proof-of-concept.

• Sæbø, Johan Ivar; Büttner, Andre; Gruschka, Nils; Jolliffe, Bob & McGee, Austin (2022). Where There is No CISO. In Zheng, Y.; Abbott, P. & Robles-Flores, J. A. (Ed.), Freedom and Social Inclusion in a Connected World : 17th IFIP WG 9.4 International Conference on Implications of Information and Digital Technologies for Development. Springer. ISSN 9783031194290. doi: 10.1007/978-3-031-19429-0_12. Full text in Research Archive
• Büttner, Andre & Gruschka, Nils (2021). Enhancing FIDO Transaction Confirmation with Structured Data Formats. NIKT: Norsk IKT-konferanse for forskning og utdanning. ISSN 1892-0713. Full text in Research Archive Show summary

  FIDO Transaction Confirmation is an extension for the FIDO authentication protocols to enable the verification and signing of digital transactions, e.g., for online banking. The standard currently considers only to include a transaction message text in the assertion which is signed by the user’s authenticator. However, this is not useful for more complex transactions and leaves room for ambiguities that might lead to security vulnerabilities. Therefore, we propose to include the transaction information to the FIDO protocols in a structured data format with a strictly defined schema to validate and sign transactions more reliably and securely.

• Büttner, Andre; Nguyen, Hoai Viet; Gruschka, Nils & Iacono, Luigi Lo (2021). Less is Often More: Header Whitelisting as Semantic Gap Mitigation in HTTP-Based Software Systems. In Jøsang, Audun; Futcher, Lynn & Hagen, Janne Merete (Ed.), ICT Systems Security and Privacy Protection. Springer Nature. ISSN 9783030781200. p. 332–347. doi: 10.1007/978-3-030-78120-0_22. Full text in Research Archive Show summary

  The web is the most wide-spread digital system in the world and is used for many crucial applications. This makes web application security extremely important and, although there are already many security measures, new vulnerabilities are constantly being discovered. One reason for some of the recent discoveries lies in the presence of intermediate systems—e.g. caches, message routers, and load balancers—on the way between a client and a web application server. The implementations of such intermediaries may interpret HTTP messages differently, which leads to a semantically different understanding of the same message. This so-called semantic gap can cause weaknesses in the entire HTTP message processing chain. In this paper we introduce the header whitelisting (HWL) approach to address the semantic gap in HTTP message processing pipelines. The basic idea is to normalize and reduce an HTTP request header to the minimum required fields using a whitelist before processing it in an intermediary or on the server, and then restore the original request for the next hop. Our results show that HWL can avoid misinterpretations of HTTP messages in the different components and thus prevent many attacks rooted in a semantic gap including request smuggling, cache poisoning, and authentication bypass.

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• Büttner, Andre; Gruschka, Nils & Sæbø, Johan Ivar (2024). Security of Evolving Authentication Technologies. Universitetet i Oslo. Full text in Research Archive Show summary

  In today’s digital age, people are burdened with a plethora of online profiles, e.g., for social media, banking, and governmental services. Traditionally, passwords have been the primary method to sign in to those accounts. They, however, are increasingly recognized as insecure. Thus, service providers implement new technologies to protect their users. This thesis explores the security of modern authentication technologies, focusing on multi-factor authentication, passwordless authentication, and self-sovereign identity (SSI). The research reveals that many users configure their accounts poorly, entailing the risk of account takeovers or account lockouts. The thesis also presents the first systematic study on risk-based account recovery, a hitherto overlooked concept that impacts the users' security and accessibility. Moreover, FIDO2, a recent innovation aimed at replacing passwords, is found to have vulnerabilities in certain conditions. A protocol is therefore suggested that can prevent such vulnerabilities effectively. Finally, a review of authentication in the context of data subject rights uncovers questionable practices, which inspired the proposal of an SSI-based concept to mitigate associated security and privacy risks.

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