Exploitation Hits Rejetto HFS Vulnerability Discovered by AI

Threat actors are exploiting a critical vulnerability in Rejetto HTTP File Server (HFS) to bypass authentication and gain remote code execution (RCE), VulnCheck warns.

Tracked as CVE-2026-61500 (CVSS score of 9.3), the flaw exists because the open source file server discloses outputs of its non-cryptographic session cookie generator to unauthenticated clients during login. It also derives the session-cookie signing key from the same generator.

The sensitive information leak allows an attacker to reconstruct the generator’s state and recover the signing key using a small set of collected login responses.

Using the recovered key, the attacker can then forge valid administrator session cookies to gain elevated access to the server and RCE via the server_code configuration feature.

The issue was that Rejetto HFS’s generator, Math.random(), was using the xorshift128+ algorithm to generate the ‘random’ value that was then passed to the server’s Node.js web framework Koa for signing session cookies.

Because the algorithm’s outputs are reversible, an attacker able to collect other numbers generated by Math.random() could determine other generated numbers and forge the authentication cookies, cybersecurity firm Horizon3 explained in a technical report .

Horizon3.ai researchers uncovered the flaw using Anthropic’s Mythos AI model, which used advanced mathematical reasoning to recognize that Math.random() PRNG outputs could be reversed to reconstruct the secret session-cookie signing key.

The security firm discovered the weakness in June, and Rejetto HFS version 3.2.1 was released on July 13 with the necessary patches.

“Multiple security vulnerabilities have been found in all versions, potentially allowing an attacker to gain administrative access to HFS,” Rejetto noted in its advisory .

On October 2, VulnCheck warned that hackers have begun targeting CVE-2026-61500 as part of small-scale reconnaissance originating from a China Telecom IP. The attempts hit canaries in Japan and the US.

Related: Exploitation of Citrix NetScaler Zero-Day Hits Appliances Patched Days Earlier

Related: Fortra Patches Critical Vulnerabilities in BoKS

Related: Warlock Expands SharePoint Exploitation in Critical Infrastructure Attacks

Related: Exploited Fortinet FortiMail Zero-Day Calls for Urgent Action

Exploitation of Citrix NetScaler Zero-Day Hits Appliances Patched Days Earlier

Citrix has confirmed that a new zero-day vulnerability, CVE-2026-88779, emerged just days after two other exploited flaws were patched.

Fortra Patches Critical Vulnerabilities in BoKS

The bugs could lead to authentication bypass, shell command execution, and memory corruption.

Warlock Expands SharePoint Exploitation in Critical Infrastructure Attacks

The China-based hacking group has been exploiting SharePoint vulnerabilities since July 2025.

Exploited Fortinet FortiMail Zero-Day Calls for Urgent Action

CVE-2026-104286 is a critical-severity path traversal vulnerability that could allow attackers to write arbitrary files to the system.

Zimbra Vulnerability Exploited in the Wild Prior to Public Disclosure

Under certain conditions, CVE-2026-73570 can be exploited via specially crafted emails without user interaction.

Artificial Intelligence

Zammad Zero-Days Exploited in AI-Powered DIVD Hack

The flaws were chained to hijack sessions, achieve remote code execution, and elevate privileges to root.

Cisco Patches Exploited Catalyst SD-WAN Zero-Day Vulnerability

The flaw could allow remote, unauthenticated attackers to access vulnerable appliances with administrative privileges.

Artificial Intelligence

Google: AI Is Changing the Pace and Profile of Vulnerability Discovery

Google’s analysis found that AI-discovered vulnerabilities are more likely to enable remote code execution.