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CVE-2026-61500 is a critical authentication-bypass and remote code execution vulnerability discovered in Rejetto HTTP File Server (HFS) versions 3.0.0 through 3.2.0.
The underlying security flaw originates from the application’s reliance on JavaScript’s built-in Math.random function to generate signing secrets.
Because Math.random is a non-cryptographic pseudo-random number generator, its internal state can be predicted or mathematically reconstructed.
Rejetto HFS passes this predictable value to the Koa web framework via keygrip to cryptographically sign user session cookies.
An unauthenticated attacker can interact with the server login process to leak output values generated by the weak PRNG.
By observing these exposed outputs, the attacker reconstructs the internal state of the random number generator without needing valid credentials.
Once the state is recovered, the attacker can mathematically determine and forge the session-cookie signing key used by the backend framework.
Armed with the valid signing key, the attacker constructs a forged administrator session cookie for the targeted instance.
Submitting this forged cookie grants unauthenticated remote administrative access directly to the application control dashboard.
Since Rejetto HFS includes built-in capabilities for executing custom server-side JavaScript scripts, administrative access immediately facilitates remote code execution.
Executing arbitrary code shifts the security boundary past the web application level down to the underlying host operating system.
Attackers can then access local files, sensitive environment variables, system credentials, and network resources.
The vulnerability was initially uncovered with the assistance of artificial intelligence models capable of linking complex cryptographic missteps.
Technical details and proof-of-concept workflows were published by security researchers on September 30, 2026.
Immediate weaponization and automated scanning activity were detected by threat intelligence platforms as early as October 1, 2026.
Because the flaw attacks the session management and signing state rather than traditional passwords, changing admin passwords offers no remediation.
Remediation requires upgrading the software codebase to version 3.2.1 or later where secure cryptographic random bytes are properly implemented.
The incident highlights the shrinking window between complex vulnerability disclosure and widespread automated exploitation in the wild.
DailyCVE Form:
Platform: Rejetto HFS Server
Version: Versions 3.0-3.2
Vulnerability: Authentication Bypass RCE
Severity: Critical CVSS 9.3
date: September 30 2026
Prediction: Already Patched 2026
What Undercode Say:
Bash Commands and Codes
python3 -m pip install requests curl -s https://target-hfs-server/api/login | grep -i "session" python3 exploit_cve_2026_61500.py --target http://target:8080
Exploit: (Educational Purposes!)
The exploit works by harvesting sequential outputs from the Math.random implementation in the HFS login endpoint. Scripts analyze these leaked values to reverse-engineer the PRNG seed state. Once the internal state is known, the script computes the exact keygrip signing secret used by Koa. A malicious administrative cookie is then crafted using this forged key and sent in an HTTP header to bypass authentication checks entirely, followed by invoking the server-side script execution feature to spawn a reverse shell.
Protection: from this CVE
Upgrade the Rejetto HFS installation immediately to version 3.2.1 or higher. The patch replaces JavaScript Math.random() with Node.js crypto.randomBytes() to ensure cryptographic unpredictability for session keys. Additionally, restrict public internet access to administrative endpoints, monitor server logs for unauthorized login attempts or abnormal process executions, and review network security boundaries.
Impact:
Successful exploitation grants unauthenticated attackers full administrative privileges over the affected server instance. From there, execution of arbitrary code leads to complete system compromise, potential data exfiltration, deployment of malicious payloads, and lateral movement across connected internal enterprise networks.
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Sources:
Reported By: github.com
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