Advisory X41-2017-04: Multiple Vulnerabilities in tnef

Eric Sesterhenn of X41 found multiple vulnerabilities in the tnef tool which is used to extract winmail.dat files.
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Advisory X41-2017-02: Multiple Vulnerabilities in ytnef

Eric Sesterhenn of X41 found multiple vulnerabilities in the ytnef TNEF library which is used to extract winmail.dat files.
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Wire Cryptography Audit (with Kudelski Security)

X41 D-Sec's Markus Vervier and Kudelski Security's JP Aumasson were hired to audit Wire's cryptography core, the proteus library.
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Vulnerabilities in Signal Private Messenger

Two vulnerabilities in signal have been uncovered by Markus Vervier and Jean-Philippe Aumasson, which allow attackers to modify encrypted attachments and crash the Signal app permanently.
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Advisory X41-2016-001: Memory Corruption Vulnerability in "libotr"

X41 found a memory corruption vulnerability in the widely used libotr library.
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Our Services

About X41

We do IT-Security

  • Our mission.


    We are experts in Application Security Testing. We enable you to make your products safe and sound.

  • Secure by Design

    Secure from the ground up.

    Great design is the base of secure products and systems. Rely on our expertise in security design.

  • Start-Up!

    Emerging companies have special needs.

    Believing in start-up-culture we care for them! We help you gaining an advantage.

  • What


What we do.




We have the expertise in code reviewing, red team penetration testing and security design reviews of a wide variety of products. Finding defects will improve the quality of your products.

Vulnerability Management


We help you handle vulnerabilities in products you use or develop. We take care of vendor contacts and work out the technical details with security researchers and developers to do vulnerability reporting in a professional way. Years of experience in uncovering vulnerabilities created an expert process in getting vulnerabilities fixed.


  • Application Security (APPSEC)
  • Mobile Security
  • Embedded Security
  • Network Security
  • Cloud Security

X41 - Security Research Lab

Serious research requires serious expertise.

Vulnerability Research


There is no defense without understanding offense. Security Research is essential to keep up with emerging threats. From uncovering new vulnerabilities to creating better mitigations we are a strongly research driven company.

Mitigation and Hardening


No application is bug-free. A good security concept will include measures to harden and mitigate yet unknown vulnerabilities making it harder for an adversary to exploit them. We develop new techniques to mitigate exploitation and contain threats.

If you are interested in custom research contact us.


Is security your passion? Do you like being creative? Do you want to face new challenges to expand your skills even further? Contact us, we're hiring!

Contact Us

Office Locations:

Aachen Frankfurt