Leveraging Anthropic’s Mythos for Cryptographic Weakness Exploitation in Rejetto HFS

In a recent attack analysis by Horizon3 researchers, the discovery and exploitation of cryptographic vulnerabilities in Rejetto HFS (HTTP File Server) showcased how advanced tools like Anthropic’s Mythos can be leveraged for harmful purposes. This case study focuses on the use of Mythos to uncover predictable signing keys, leading to the successful forging of an admin session and subsequent remote code execution (RCE) in Rejetto HFS. The implications of this method in cybersecurity highlight both the risks of outdated cryptographic implementations and the potential for sophisticated exploits in the wild. More details can be found in the original research documentation from Horizon3.

Campaign or TTP Overview

In a coordinated research initiative, Horizon3 capitalized on vulnerabilities in Rejetto HFS primarily due to flawed cryptographic implementations. Rejetto HFS, a widely used lightweight file server program, presented an attractive target due to its prevalent use in environments lacking regular patch management or cryptographic updates. By utilizing Anthropic’s Mythos toolkit, the researchers were able to decode the weak cryptographic residue employed by HFS, leading to full session hijacking.

The campaign, conducted in mid-2023, aimed specifically at HFS instances across networks that had not been updated to mitigate known flaws. The methodology was ingeniously simple yet devastatingly effective: it focused on recovering predictable signing keys exploited to gain unauthorized administrative access. This exercise demonstrated not just a proof of concept but a real-world viable attack vector when leveraging sophisticated cryptographic analysis tools.

How It Was Built

Leveraging Anthropic’s Mythos, the researchers executed a multi-stage attack starting with reconnaissance to identify vulnerable HFS servers. The delivery mechanism involved a series of HTTP requests crafted to interact with the server’s signing mechanism. A carefully constructed payload embedded in these requests capitalized on the predictable nature of the HFS signing key, exploiting its cryptographic weaknesses.


GET /?search=exploit HTTP/1.1

Host: vulnerable-target.com

Cookie: sessionid=ABCD12345; signature=9876FACB

User-Agent: MythosClient/1.0

This specific crafted request illustrated above cleverly masquerades as legitimate, yet it bypasses security checks by exploiting the now-predictable signing key structure. The next phase involved command injection through URL parameters, granting an attacker full administrative privileges.

Why It Worked

The success of this attack lay in a few critical choices. First, the use of Mythos allowed the precise recovery of cryptographic weaknesses inherent in Rejetto HFS. This tool’s capability to analyze and exploit weak cryptographic implementations was key. Moreover, targeting overlooked servers with outdated software increased the attack’s reach and efficacy, as many organizations do not promptly upgrade systems due to operational dependencies.

The attacker’s choice of an innocuous HTTP request as a delivery mechanism facilitated stealth; such requests are commonplace and can easily blend into routine traffic, minimizing detection chances. Additionally, the false sense of security many users have in their system’s cryptographic capabilities made the choice of attack vector both unexpected and effective.

A predictable signing key is a critical vulnerability — it allows an attacker to mimic legitimate users effortlessly, bypassing authentication barriers.

Operator Takeaways

Red team operators can glean several insights from this case study. Utilizing advanced cryptographic analysis tools, such as Mythos, allows for the identification and exploitation of weaknesses that are typically invisible to standard scanning tools. This highlights the importance of focusing not only on exploiting known vulnerabilities but probing systems for weak cryptographic practices that can be easily overlooked.

Additionally, operators should consider targeting environments where upgrades are difficult to manage due to service continuity requirements. These instances often have latent vulnerabilities ripe for exploitation under the guise of regular traffic.

Good / Better / Best

Good: Identify and exploit known vulnerabilities using traditional reconnaissance and scanning tools for surface exploitation.
Better: Deploy cryptographic tools to carry out deeper analysis on cryptographic operations, identifying exploitable patterns or weaknesses.
Best: Integrate both surface and deep cryptographic analysis with real-world delivery techniques for seamless exploitation without raising alarms.