Clemens Hlauschek

I am a researcher at the ESSE group. I am always interested in exceptionally motivated students to work with me on challenging and highly technical topics. If you feel addressed and feel the intersection of our interests would build a good foundation, feel free to drop me an email to discuss possible topics for your thesis or practical coursework project. I have extensive industrial experience in systems security, security analysis, security engineering, and various forms of penetration testing. My mind is trained to work well on low level, close-to-the-hardware issues (binary reversing and exploitation, OS security), as well as on more abstract and theoretical problems (theoretical cryptography and complexity theory, machine learning and machine intelligence). In my research, I try to close the gap that exists between practitioners and theorists. Maybe the best way to get to know me personally is by joining our CTF team defragmented.brains and playing together with me and a bunch of smart, skilled, and rather swag fellows with various backgrounds at the next competition. Again, feel free to drop me an email and I will hook you up.


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Office Location:
    Wiedner Hauptstraße 76/2/2
    1040 Vienna
Office Hours: by appointment: please write an e-mail.

Research Interests


I am involved in the following courses/curriculum:

Professional Service

External Reviewer for ACM CCS, Crypto, etc.


For an updated list, check my Google Scholar profile


Qian Guo, Clemens Hlauschek, Thomas Johansson, Norman Lahr, Alexander Nilsson and Robin Leander Schröder. Don’t reject this: Key-recovery timing attacks due to rejection-sampling in HQC and BIKE. In Transactions on cryptographic hardware and embedded systems 2022 issue 3 (2022). [ Download: ]

Daniel Marth, Clemens Hlauschek, Christian Schanes and Thomas Grechenig. Abusing trust: Mobile kernel subversion via trustzone rootkits. In 16th ieee workshop on offensive technologies (2022). [ Download: ]
The Arm TrustZone is the de facto standard for hardware-backed Trusted Execution Environments (TEEs) on mobile devices, providing isolation for secure computations to be shielded from the normal world, and thus from the rest of the system. Most real-world TEEs are proprietary, difficult- to-inspect, and notoriously insecure: In the past years, it has been demonstrated over and over again that TEEs of millions of devices worldwide, and the Trusted Applications (TAs) they harbor, are often vulnerable to attacks such as control flow hijacking. Not only do we have to trust these TEEs to provide a secure environment for TAs such as keystore and Digital Rights Management (DRM), code running in the secure world provided by the Arm TrustZone also has full access to the memory of the regular operating system (OS). Since Thomas Roth first proposed a TrustZone-based rootkit in 2013, progress regarding such rootkits seems to have stalled in the offensive research community. The biggest challenge for TrustZone rootkits is that no interpretation of normal world memory is available in the secure world. Automated reverse engineering of kernel data structures at runtime is one way to implement rootkit functions. We present mechanisms to engineer the interpretation of Linux kernel memory for malicious subversion and the circumvention of basic protection mechanisms from the secure world. We provide a fully working proof-of-concept rootkit located in the Arm TrustZone to demonstrate the proposed mechanisms. We evaluate and show compatibility of the rootkit across different versions of the Linux kernel despite changing data structures. Our results highlight the feasibility of TrustZone rootkits that potentially survive kernel updates and raise awareness about the real danger of having to put trust into unvetted proprietary vendor code, which, as we show, can easily be abused.

Clemens Hlauschek, Norman Lahr and Robin Leander Schröder. On the timing leakage of the deterministic re-encryption in HQC KEM (2021). [ Download: ]
Well before large-scale quantum computers will be available, traditional cryptosystems must be transitioned to post-quantum secure schemes. The NIST PQC competition aims to standardize suitable cryptographic schemes. Candidates are evaluated not only on their formal security strengths, but are also judged based on the security of the optimized implementation, for example, with regard to resistance against side-channel attacks.HQC is a promising code-based key encapsulation scheme and selected as an alternate candidate in the third round of the competition, which puts it on track for getting standardized separately to the finalists, in a fourth round.Despite having already received heavy scrutiny with regard to side channel attacks, in this paper, we show a novel timing vulnerability in the optimized implementations of HQC, leading to a full secret key recovery. The attack is both practical, requiring only approx. 866,000 idealized decapsulation timing oracle queries in the 128-bit security setting, and structurally different from previously identified attacks on HQC: Previously, exploitable side-channel leakages have been identified in the BCH decoder of a previously submitted version, in the ciphertext check as well as in the PRF of the Fujisaki-Okamoto (FO) transformation employed by several NIST PQC KEM candidates. In contrast, our attack uses the fact that the rejection sampling routine invoked during the deterministic re-encryption of the KEM decapsulation leaks secret-dependent timing information. These timing leaks can be efficiently exploited to recover the secret key when HQC is instantiated with the (now constant-time) BCH decoder, as well as with the RMRS decoder of the current submission. Besides a detailed analysis of the new attack, we discuss possible countermeasures and their limits.

Clemens Hlauschek, Markus Gruber, Florian Fankhauser and Christian Schanes. Prying open pandoras box: KCI attacks against TLS. In 9th usenix workshop on offensive technologies (woot 15) (August 2015). Washington, D.C.: USENIX Association. [ Download: ]
Protection of Internet communication is becoming more common in many products, as the demand for privacy in an age of state-level adversaries and crime syndicates is steadily increasing. The industry standard for doing this is TLS. The TLS protocol supports a multitude of key agreement and authentication options which provide various different security guarantees. Recent attacks showed that this plethora of cryptographic options in TLS (including long forgotten government backdoors, which have been cunningly inserted via export restriction laws) is a Pandoras box, waiting to be pried open by heinous computer whizzes. Novel attacks lay hidden in plain sight. Parts of TLS are so old that their foul smell of rot cannot be easily distinguished from the flowery smell of “strong” cryptography and water-tight security mechanisms. With an arcane (but well-known among some theoretical cryptographers) tool, we put new cracks into Pandoras box, achieving a full break of TLS security. This time, the tool of choice is KCI, or Key Compromise Impersonation. The TLS protocol includes a class of key agreement and authenticationmethods that are vulnerable to KCI attacks: non-ephemeralDiffie-Hellman key exchange with fixed Diffie-Hellman client authentication – both on elliptic curve groups, as well as on classical integer groups modulo a prime. We show that TLS clients that support these weak handshakes pose serious security concerns in modern systems, opening the supposedly securely encrypted communication to full-blown Man-in-the-Middle (MitM) attacks. This paper discusses and analyzes KCI attacks in regard to the TLS protocol. We present an evaluation of the TLS software landscape regarding this threat, including a successful MitM attack against the Safari Web Browser on Mac OS X. We conclude that the insecure TLS options that enable KCI attacks should be immediately disabled in TLS clients and removed from future versions and implementations of the protocol: their utility is extremely limited, their raison d’etre is practically nil, and the existence of these insecure key agreement options only adds to the arsenal of attack vectors against cryptographically secured communication on the Internet.

Clemens Hlauschek, John Black, Giovanni Vigna and Christopher Kruegel. Limited-linkable group signatures with distributed-trust traceability (2012). Vienna University of Technology. [ DOI: ]
Groupsignatures allow a group member to sign anonymously on behalf of a group. In the dynamic case, a group manager can add and revoke group members. An opening manager can revoke the anonymity of a signature and trace it back to the original group member. We introduce limited-linkable group signatures: two signatures on identical messages by the same group member can be efficiently linked. Furthermore, we show how to distribute the opening manager, so that no trusted third party is required to guarantee anonymity. Our system generates short and efficient signatures, and is provably secure in the random oracle model.

Stefan Taber, Christian Schanes, Clemens Hlauschek, Florian Fankhauser and Thomas Grechenig. Automated security test approach for sip-based voip softphones. In The second international conference on advances in system testing and validation lifecycle, august 2010, nice, france (August 2010). IEEE Computer Society Press. [ DOI: ]
Voice over Internet Protocol based systems become more and more part of business critical IT infrastructures. To increase the robustness of voice applications, automated security testing is required to detect security vulnerabilities in an efficient way. In this paper we present a fuzzer framework to detect security vulnerabilities in Voice over Internet Protocol Softphones, which implement Session Initiation Protocol. The presented approach automates the Graphical User Interface interaction for softphones during fuzzing and also observes the behavior of the softphone Graphical User Interfaces to automatically detect application errors. Results of testing two open source softphones by using our fuzzer showed that various unknown vulnerabilities could be identified with the implemented fuzzer and some vulnerabilities were found that are only detectable by using Graphical User Interface observation.

Keywords: ESSE, Software testing; Computer network security; Graphical user interfaces; Internet telephony; Fuzzing

Ulrich Bayer, Paolo Milani Comparetti, Clemens Hlauschek, Christopher Kruegel and Engin Kirda. Scalable, Behavior-Based Malware Clustering. In 16th symposium on network and distributed system security (ndss) (2009). [ Download: ]
Anti-malware companies receive thousands of malware samples every day. To process this large quantity, a number of automated analysis tools were developed. These tools execute a malicious program in a controlled environment and produce reports that summarize the program’s actions. Of course, the problem of analyzing the reports still re- mains. Recently, researchers have started to explore au- tomated clustering techniques that help to identify samples that exhibit similar behavior. This allows an analyst to dis- card reports of samples that have been seen before, while focusing on novel, interesting threats. Unfortunately, pre- vious techniques do not scale well and frequently fail to generalize the observed activity well enough to recognize related malware. In this paper, we propose a scalable clustering approach to identify and group malware samples that exhibit simi- lar behavior. For this, we first perform dynamic analysis to obtain the execution traces of malware programs. These execution traces are then generalized into behavioral pro- files, which characterize the activity of a program in more abstract terms. The profiles serve as input to an efficient clustering algorithm that allows us to handle sample sets that are an order of magnitude larger than previous ap- proaches. We have applied our system to real-world mal- ware collections. The results demonstrate that our tech- nique is able to recognize and group malware programs that behave similarly, achieving a better precision than previous approaches. To underline the scalability of the system, we clustered a set of more than 75 thousand samples in less than three hours.

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