Vulnerability report

Exploited in the wild High confidence Not in CISA KEV

CVE-2026-61500

Rejetto HFS < 3.2.1 Session Forgery Remote Code Execution

Rejetto / hfs · affected before 3.2.1

Watch CVE-2026-61500 Browse KEV feed
Severity
CVSS 9.3 · Critical
Confidence
High
Exploit status
Exploited in the wild
EPSS
1.0%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2026-61500 is an unauthenticated vulnerability affecting Rejetto hfs. 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...

Is it exploited?

Yes. Previdian tracks this CVE as a known exploited vulnerability. Confidence is high. Also confirmed by third-party sources.

Who is affected?

Rejetto / hfs affected before 3.2.1.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

Rejetto HFS < 3.2.1 Session Forgery Remote Code Execution

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.

rejetto Affected
hfs
Before 3.2.1
View vendor advisory (opens in new tab)
Published
13 Jul 2026
Exploitation Reported
05 Oct 2026
Attack vector
Remote
Complexity
Low
Privileges
None
User interaction
None

CVE References

Exploitation evidence

Why Previdian marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

The Hacker News

Recorded 05 Oct 2026

A trusted third party reported exploitation.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

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Source Added
The Hacker News First 2026-10-05 08:32 UTC
Security Affairs 2026-10-05 19:21 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
User-Agents
Callback hosts
0
0
0

Request targets and User-Agents available in Pro. Callback host details available in Enterprise.

Scanner coverage

No scanner integrations recorded yet.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

No detection artifacts or sensor request patterns are available for this CVE yet.

Check back as sensor telemetry and scanner integrations are updated.

Risk and context

Severity, weaknesses, and research context

CVSS v4.0

9.3 Critical

Potential damage if exploited. Separate from whether attackers are using it.

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

EPSS

1.0%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

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

Recent mention · The Hacker News

Anthropic Expands Claude Access for Vetted Cyber Teams as Glasswing Finds 129,000 Flaws

Ravie LakshmananOct 07, 2026Artificial Intelligence / Vulnerability Anthropic on Tuesday said it's expanding a program that allows vetted cybersecurity professionals to test its advanced artificial intelligence (AI) models with reduced safeguards and blocking classifiers, as...

Read full advisory

All CVSS Scores

CVSS v4.0 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

CVSS v3.1 9.8 Critical

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

All Mentions

Recent mention · The Hacker News

Anthropic Expands Claude Access for Vetted Cyber Teams as Glasswing Finds 129,000 Flaws

The Hacker News · 07 Oct 2026

Ravie LakshmananOct 07, 2026Artificial Intelligence / Vulnerability Anthropic on Tuesday said it's expanding a program that allows vetted cybersecurity professionals to test its advanced artificial intelligence (AI) models with reduced safeguards and blocking classifiers, as the company claimed its Project Glasswing initiative uncovered at least 129,000 verified software vulnerabilities between April and July 2026. The company said it also found an additional 5,500 verified software vulnerabilities between April and October 2026 through open-source scanning efforts. "Of these verified...

Recent mention · BleepingComputer

Rejetto HFS servers now actively scanned for critical RCE flaw

BleepingComputer · 05 Oct 2026

Hackers are actively scanning for a Rejetto HFS weak signing key vulnerability, tracked as CVE-2026-61500, that allows session forgery, account takeover, and remote code execution (RCE). [...]

Recent mention · Security Affairs

Anthropic Mythos Found A Bug in Rejetto HFS. Attackers Are Now Exploiting It.

Security Affairs · 05 Oct 2026

AI-assisted research uncovered a critical Rejetto HFS flaw that enables authentication bypass and remote code execution, now exploited in the wild. A Rejetto HFS vulnerability, tracked as CVE-2026-61500 (CVSS score of 9.3), discovered with the help of the Anthropic Mythos AI model is now being exploited in the wild, turning an interesting security research experiment […]

Recent mention · The Hacker News

Attackers Target Rejetto HFS Flaw That Enables Admin Session Forgery and RCE

The Hacker News · 05 Oct 2026

Ravie LakshmananOct 05, 2026Vulnerability / Web Security A critical security flaw impacting Rejetto HTTP File Server (HFS) is witnessing active exploitation attempts, according to VulnCheck. The vulnerability in question is CVE-2026-61500 (CVSS score: 9.3), a case of session forgery stemming from the use of a weak pseudo-random number generator (PRNG) that can lead to a predictable key, which an attacker can then use to gain unauthorized access and seize control of affected systems. "Rejetto HFS 3.0.0 through 3.2.0 derives its session-cookie signing key from the non-cryptographic...

Recent mention · TheRegister

Anthropic's super bug-hunting model Mythos is hardcore good at math, as latest vuln under attack shows

TheRegister · 03 Oct 2026

Exploitation attempts came from China-hosted IP, VulnCheck researcher says

Recent mention · Horizon3.ai Attack Research

Horizon3’s Tales from the Trenches: Anthropic’s Mythos and Rejetto HFS

Horizon3.ai Attack Research · 30 Sep 2026

Horizon3 researchers used Anthropic’s Mythos to uncover a chain of cryptographic weaknesses in Rejetto HFS, recover a predictable signing key, forge an admin session, and achieve remote code execution.

Timeline

From disclosure to observed exploitation

  1. 19:21 UTC

    KEV confirmed by Security Affairs

    Exploitation attested by an external source

  2. 08:32 UTC

    Added to Previdian KEV Feed

    High-confidence, third-party attested exploitation

  3. 17:21 UTC

    CVE published

    Vulnerability disclosed publicly

  4. 15:43 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

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  • Evidence confidence and provenance
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GET /api/v2/pro/kevs/CVE-2026-61500

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2026-61500",
  "confidence": "High",
  "cvss_score": 9.3,
  "cvss_estimated": false,
  "epss_score": 0.00987,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}