Anthropic Mythos Autonomously Chains Rejetto HFS PRNG Flaw to RCE, Exploited Within a Day
Date: 2026-10-04
Tags: apt
Executive Summary
Anthropic's Mythos model autonomously found and chained a critical authentication bypass in Rejetto HTTP File Server tracked as CVE-2026-61500, and VulnCheck canaries recorded a China based IP hitting real vulnerable HFS servers in the US and Japan within one day of the October 1 disclosure. The flaw sits in how HFS derives the Koa session cookie signing key from Math.random(), which uses the reversible xorshift128+ algorithm. Operators running internet facing HFS should patch immediately and hunt for forged admin session activity.
Campaign Summary
| Field | Detail |
|---|---|
| Campaign / Malware | Rejetto HFS CVE-2026-61500 exploitation |
| Attribution | Unknown, China based IP observed (confidence: low) |
| Target | Internet facing Rejetto HTTP File Server installations in the US and Japan |
| Vector | Forged Koa session cookie via reversed PRNG state leading to admin session and remote code execution |
| Status | active |
| First Observed | 2026-10-01 |
Detailed Findings
According to aiweekly.co, Anthropic's Mythos model found a critical authentication bypass bug in Rejetto HTTP File Server. The flaw, tracked as CVE-2026-61500, sits in how HFS derives the signing key for its Koa session cookies from Math.random(), and V8's implementation of Math.random() runs on the xorshift128+ algorithm, which Mythos determined was fully reversible once the application leaked outputs through a separate code path. Horizon3's Zach Hanley said Mythos did not just flag the insecure PRNG in isolation but simultaneously identified that the application leaked raw Math.random() outputs through a separate code path and recognized the two facts as a chain. Horizon3 confirmed Mythos used the Z3 SMT solver to reverse xorshift128+ state, making a theoretically known PRNG attack practically executable without additional prompting. VulnCheck's Patrick Garrity said the firm's canaries detected an actor in China targeting real vulnerable hosts in the US, reporting four hits from two US proxy IPs on the day after the October 1 disclosure.
MITRE ATT&CK Mapping
| Technique | ID | Context |
|---|---|---|
| Exploit Public-Facing Application | T1190 | Attacker exploitation of the HFS authentication bypass on internet facing servers |
| Valid Accounts | T1078 | Use of a forged admin session cookie to authenticate as an administrator |
IOCs
Domains
_IP addresses reported by aiweekly.co citing VulnCheck canary hits from two US proxy IPs._
Full URL Paths
_IP addresses reported by aiweekly.co citing VulnCheck canary hits from two US proxy IPs._
Splunk Format
_No IOCs available for Splunk query_
Affected Platforms
Rejetto HTTP File Server
CVE-2026-61500Detection Recommendations
Patch Rejetto HTTP File Server to a version that no longer derives the Koa session signing key from Math.random(). Hunt for admin session cookies in web logs that do not correspond to a legitimate authentication event, and alert on requests using proxy IPs 173.239.211.248 and 173.239.211.249. Monitor for post authentication file writes or process execution consistent with remote code execution following session forgery.
References
- [aiweekly.co] Anthropic's Mythos finds Rejetto HFS flaw, exploited in a day (2026-10-04) — https://aiweekly.co/alerts/anthropics-mythos-finds-rejetto-hfs-flaw-exploited-in-a-day