Attackers have begun exploiting a critical authentication bypass in Rejetto HTTP File Server that was found with Anthropic's Mythos vulnerability-hunting model, The Register reported Saturday. The flaw, CVE-2026-61500, lets an attacker forge session cookies, take over the admin account and run code on the server, and it is fixed in HFS 3.2.1 and later.

What Happened

Zach Hanley, a researcher at penetration-testing company Horizon3.ai, found the bug using Mythos. Horizon3 joined Project Glasswing, Anthropic's limited-access program for security firms, in July.

HFS, an open-source web file server, generated the keys that sign its session cookies with JavaScript's Math.random() function. In the V8 engine, that function relies on the xorshift128+ algorithm, whose output can be reversed if an attacker sees enough of it. HFS also exposed raw Math.random() values through a separate code path. Mythos connected the two problems and determined that Z3, Microsoft's SMT solver, could recover the generator's internal state from the leaked values and with it the signing key.

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With the key, an attacker can mint valid session cookies, gain administrator access and achieve remote code execution. Hanley said the researchers could not recall seeing an SMT solver used that way against a cryptographic weakness in a real application.

Hanley disclosed the flaw Wednesday, publishing a write-up and a video that walks through exploitation. By Thursday evening, VulnCheck researcher Patrick Garrity reported that the company's canary systems had caught an actor in China targeting vulnerable hosts in the U.S. and Japan. On Friday, VulnCheck recorded four more attempts from two U.S. IP addresses in the same subnet that appeared to belong to proxy services.

Why It Matters

The case shows AI-assisted research finding bugs that require combining several observations, not just flagging a known bad function. A weak random number generator alone is a common code-review finding; turning it into a working exploit required spotting the leak and choosing the right mathematical tool to invert the generator.

It also shows how short the window can be. Exploitation began roughly a day after a detailed write-up and video became public, leaving HFS operators little time to upgrade.

Context

Anthropic unveiled Mythos and Project Glasswing in April, giving selected security companies access to a model designed to find vulnerabilities. As of Friday, the program had produced 286 CVEs, according to The Register, and this is only the second of them known to have been exploited in the wild. The Register did not name the first, and its article included no comment from Anthropic.

HFS is popular for quick file sharing from Windows machines, and older releases have drawn attackers before. The unsupported 2.x line was hit by CVE-2024-23692, a template injection flaw exploited in 2024.

What to Watch

HFS operators should upgrade to version 3.2.1 or later and check for unexpected admin sessions. Watch for a listing in CISA's Known Exploited Vulnerabilities catalog and for whether other Glasswing-found bugs are weaponized as quickly once their details are published.