Rejetto HFS CVE-2026-61500: AI-Found RCE Now Under Attack

A flaw that Horizon3.ai found with Anthropic’s Mythos model lets anyone who can reach a Rejetto HFS 3.x server forge an administrator login and run code on it. The full research went public on September 30. Within days, honeypots recorded the first probes. The patch has been sitting on GitHub since July.

DataWater Threat Intelligence Desk | Published October 6, 2026 | 7-minute read

Threat level: High

What: CVE-2026-61500, a session-forgery flaw in Rejetto HTTP File Server (HFS) 3.0.0 through 3.2.0 caused by a weak random number generator (CWE-338). Rated CVSS 4.0 9.3 and CVSS 3.1 9.8 by VulnCheck, the assigning CNA.

Status: Exploitation attempts observed in the wild by VulnCheck, described as small-scale reconnaissance so far. No confirmed successful compromises have been reported.

Deadline: None set. As of October 6, 2026, CVE-2026-61500 is not listed in CISA’s Known Exploited Vulnerabilities catalog, so there is no federal due date. Do not wait for one.

Fix: Upgrade to HFS 3.2.1 (released July 13, 2026) or later; BleepingComputer reports 3.3.4 as the current stable release. If you cannot upgrade today, take HFS off the internet.

Executive Summary

The window between “research published” and “attackers knocking” is now measured in days, even for a bug that was patched three months ago.

Rejetto HFS is a free, lightweight web file server, popular for quick file sharing and often stood up outside normal IT change control. CVE-2026-61500 affects the 3.x line. The server derived the secret it uses to sign session cookies from Math.random(), a generator built for speed rather than secrecy. Worse, the login process handed unauthenticated visitors raw outputs from that same generator. Collect enough of those outputs and you can rebuild the generator’s internal state, work backward to the signing key, and mint yourself an administrator cookie.

The maintainer shipped a fix in HFS 3.2.1 on July 13, 2026, and VulnCheck published the CVE the same day. Horizon3.ai released its full technical write-up on September 30. VulnCheck’s canary network then logged exploitation attempts: The Hacker News dates the first activity to October 1, while SecurityWeek and Security Affairs cite October 2.

The bottom line: any HFS 3.x server below 3.2.1 that faces the internet should be upgraded or pulled offline today, and checked for unexpected admin activity.

Why This Matters to CISOs and Security Leaders

HFS is shadow IT by design. It is a single-binary tool people run to share a folder fast. That means it often lives on a workstation or a forgotten VM, outside your asset inventory and vulnerability scanner. You may have it and not know it.

Admin means code execution. HFS lets administrators define custom server-side JavaScript. Once an attacker holds a forged admin session, that feature becomes a remote shell running with the privileges of the HFS process.

AI just compressed the research cycle. This is a concrete, public example of a frontier model finding a real, exploitable chain in production software, followed by real-world probing. DataWater has tracked how quickly AI offensive capability is scaling and how attacker speed is outpacing enterprise defenses. CVE-2026-61500 is what that looks like at the level of a single CVE.

The Timeline

The patch is three months old. The exploit recipe is one week old. That gap is where attackers live.

Vulnerability & Exploit Analysis

What CVE-2026-61500 is

Three weaknesses combine. First, HFS 3.x builds its cookie-signing key from consecutive Math.random() calls at startup. Second, V8’s implementation of Math.random() uses the xorshift128+ algorithm, which is mathematically reversible once you have enough outputs. Third, an unauthenticated login endpoint exposes raw values from that same generator. Horizon3.ai reports that Mythos recognized the chain, suggested the Z3 constraint solver to recover the generator state, and produced working proof-of-concept code. According to Horizon3.ai’s write-up (single-source detail), roughly a dozen login requests are enough, and the published chain also uses a separate information leak to identify a valid admin username first.

With the signing key recovered, the attacker forges an admin cookie, then uses HFS’s custom server-code feature to execute commands. The CVE record describes the same end state: session forgery leading to remote code execution.

Affected and fixed versions

Exposure

None of the sources reviewed for this briefing published an internet-wide count of exposed HFS 3.x servers. The scale of exposure is unknown. What matters is whether you run HFS anywhere reachable from the internet.

Threat Intelligence Breakdown

Attribution: Unknown. VulnCheck observed reconnaissance from one IP address on China Telecom’s network, targeting canaries in Japan and the United States. One source IP on a large ISP is not attribution, and no threat actor has been named.

Scope: Reporting describes the activity as small-scale and reconnaissance-only. No successful compromises or post-exploitation activity have been publicly confirmed.

Indicators of compromise: No source reviewed for this briefing published IP addresses, hashes or other IoCs for CVE-2026-61500 activity. Focus on behavioral signs instead: bursts of unauthenticated login requests, admin sessions from unfamiliar addresses, and unexpected changes to HFS server-code settings.

Enterprise Impact: What’s Actually at Risk

The files themselves: HFS exists to serve files. An attacker with admin rights can read, replace or delete everything it shares, and swap legitimate downloads for malicious ones.

The host: Code execution runs with the HFS process’s privileges. On a workstation where a user launched it, that may be the user’s full account.

The network behind it: A compromised file server inside the perimeter is a launch point for lateral movement, with no EDR alert tied to the initial entry.

Governance: If the server was unmanaged shadow IT, you may also have a policy problem: an internet-facing service nobody approved, holding data nobody classified.

What Security Leaders Should Do Next

Today

1. Find every HFS instance. Search asset inventories, EDR software lists and external attack-surface scans for Rejetto HFS, including test and personal machines.
2. Upgrade to HFS 3.2.1 or later, preferably the current stable release, and confirm the running version.
3. If you cannot upgrade immediately, remove internet access to the HFS port or shut the service down.

Within 48 hours

1. Review HFS logs for repeated unauthenticated login attempts and admin sessions since September 30.
2. Inspect HFS configuration for custom server-side code or new accounts you did not create.
3. If anything is unexplained, treat the host as compromised: isolate it, rebuild it, and rotate any credentials stored on or used from it.

Long-term

1. Do not let KEV be your only trigger. Track vendor fixes plus public exploit releases for internet-facing tools.
2. Bring ad-hoc file sharing under policy. Offer a sanctioned alternative so staff stop standing up their own servers.
3. Plan for AI-speed research. Assume a public write-up means exploitation within days, and shorten patch windows for exposed services accordingly.

Winners and Losers

Final Executive Takeaway

CVE-2026-61500 is a small product with a big lesson.

An AI model found a real exploit chain, the fix shipped quietly, and attackers started probing within days of the details going public. The defense is not complicated: know where HFS runs, upgrade it, and check what changed while it was exposed.

Find it. Patch it. Check it. Then fix the process that let it go unnoticed.

Frequently Asked Questions

Sources

Primary sources: Horizon3.ai technical disclosure; Rejetto HFS release history; CVE Program record for CVE-2026-61500; and the CISA Known Exploited Vulnerabilities catalog, checked October 6, 2026. Additional reporting reviewed: VulnCheck, The Hacker News, BleepingComputer, SecurityWeek and Security Affairs.