Math.random

Critical CVE-2026-61500 in Rejetto HFS Allows Admin Session Forgery Leading to Remote Code Execution

A critical vulnerability tracked as CVE-2026-61500 is being actively exploited in Rejetto HTTP File Server (HFS), enabling unauthenticated attackers to forge administrator sessions and achieve remote code execution. The flaw impacts versions 3.0.0 through 3.2.0 and was addressed in release 3.2.1, making immediate updates essential for any internet-exposed instances. The root cause lies in the use of JavaScript Math.random() to generate the session cookie signing key instead of a cryptographically secure random number generator. Attackers can reconstruct the internal state of this weak PRNG from login responses, allowing them to create valid admin cookies. Once authenticated as an administrator, the attacker can abuse the server_code functionality to execute arbitrary JavaScript on the server. Exploitation activity was first observed on October 1, 2026, targeting U.S. systems and attributed to an unidentified actor based in China, following the public release of a Python proof-of-concept in late September.