Which versions of Rejetto HFS are affected by CVE-2026-61500?

Rejetto HFS versions 3.0.0-3.2.0 contain a critical vulnerability enabling attackers without credentials to impersonate administrators and execute arbitrary code on affected servers.. A patch is available.

How to fix or mitigate CVE-2026-61500?

Within 24 hours, identify all Rejetto HFS instances on your network and determine which are running versions 3.0.0-3.2.0; immediately isolate or disable external network access to affected servers. Within 7 days, update all affected instances to HFS v3.2.1 or later and validate connectivity post-deployment. Within 30 days, complete security baseline testing and conduct a forensic review of server logs from the past 90 days for evidence of unauthorized administrative access, file downloads, or configuration changes.

Rejetto HFS CVE-2026-61500

Q: What is the CVSS score of CVE-2026-61500?

CVE-2026-61500 has a CVSS 4.0 base score of 9.3 (Critical). CVSS vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X. EPSS exploitation probability: 0.7%.

CRITICAL

Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) (CWE-338)

2026-07-13 VulnCheck

RCE Hfs Rejetto

9.3

CVSS 4.0 · Vendor: VulnCheck

Severity by source

Vendor (VulnCheck) PRIMARY

9.3 CRITICAL

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

vuln.today AI

8.1 HIGH

Network, unauthenticated (PR:N/UI:N) path to admin RCE gives C/I/A High; AC:H reflects the sampling and PRNG-state reconstruction needed to recover the key.

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

4.0 AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Primary rating from Vendor (VulnCheck).

CVSS VectorVendor: VulnCheck

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Lifecycle Timeline

Source Code Evidence Fetched

Jul 13, 2026 - 19:26 vuln.today

Analysis Generated

Jul 13, 2026 - 19:26 vuln.today

CVE Published

Jul 13, 2026 - 17:21 cve.org

CRITICAL 9.3

DescriptionCVE.org

Rejetto HFS 3.0.0 through 3.2.0 derives its session-cookie signing key from the non-cryptographic Math.random() generator and discloses outputs of the same generator to unauthenticated clients during login. A remote attacker can collect a small number of login responses, reconstruct the generator's state, recover the signing key, and forge a valid administrator session cookie, leading to full administrative access and remote code execution via the server_code configuration feature.

AnalysisAI

Administrator session forgery in Rejetto HFS (HTTP File Server) versions 3.0.0 through 3.2.0 lets a remote unauthenticated attacker derive the server's session-cookie signing key. Because HFS seeds that key from JavaScript's non-cryptographic Math.random() and simultaneously leaks outputs of the same generator to clients during login, an attacker can sample a few login responses, reconstruct the PRNG state, and mint a valid admin cookie - yielding full administrative control and remote code execution through the server_code configuration feature. No public exploit is identified at time of analysis, but the flaw was reported by VulnCheck/Horizon3.ai and a vendor patch (v3.2.1) exists.

Technical ContextAI

HFS is a lightweight, self-contained HTTP file server; the modern 3.x line is built on Node.js, where the vulnerable design relies on Math.random(). Math.random() is a fast, non-cryptographic PRNG (typically an xorshift128+ variant in V8) whose internal state can be recovered from a handful of observed outputs. This maps directly to CWE-338 (Use of Cryptographically Weak Pseudo-Random Number Generator): the same weak generator both derives the secret HMAC/signing key for session cookies and produces values exposed to unauthenticated clients during the login flow. Knowing the generator's outputs allows an attacker to solve for its state and thereby regenerate the signing key deterministically. The CPE cpe:2.3:a:rejetto:hfs:*:*:*:*:*:*:*:* confirms the affected component is the Rejetto HFS application itself across the 3.0.0-3.2.0 range.

RemediationAI

Vendor-released patch: 3.2.1 - upgrade every HFS instance to v3.2.1 or later immediately, as it addresses multiple session/authentication weaknesses affecting all prior 3.x versions (https://github.com/rejetto/hfs/releases/tag/v3.2.0...v3.2.1 changelog; release: https://github.com/rejetto/hfs/releases/tag/v3.2.1). Where an immediate upgrade is not possible, restrict network exposure of the admin interface by placing HFS behind a reverse proxy or firewall that limits access to the login/admin endpoints to trusted source IPs, which prevents an attacker from collecting the login responses needed to reconstruct the PRNG state (trade-off: legitimate remote administration must originate from allowlisted addresses). As a further compensating control, disable or lock down the server_code configuration feature and any admin account that is not strictly required, since server_code is the specific vector that converts a forged admin cookie into code execution (trade-off: loss of that scripting/automation capability). Rotate the server after patching, as any previously issued admin cookies should be considered compromised. Consult the VulnCheck advisory (https://www.vulncheck.com/advisories/rejetto-hfs-session-forgery-via-predictable-signing-key) for details.