A severe flaw in the Erlang/OTP built-in SSH daemon allows unauthenticated remote attackers to achieve full remote code execution (RCE) by sending crafted SSH connection protocol messages before authentication completes.

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📋 Summary

CVE-2025-32433 is a critical vulnerability (CVSS 10.0) affecting the SSH server implementation in Erlang/OTP.

Attackers can exploit it by sending SSH messages with numbers ≥ 80 (reserved for post-authentication per RFC 4254) during the pre-authentication phase. The server fails to enforce proper state handling, leading to arbitrary code execution.

• Discovered by: Researchers from Ruhr University Bochum
• Disclosure Date: April 16, 2025
• CWE: CWE-306 — Missing Authentication for Critical Function
• Impact: Full system compromise (especially if the SSH daemon runs as root or with high privileges)
• Exploitation: Public PoCs available; observed in the wild since May 2025

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🛠 Affected Versions

Note: Only OTP 25, 26, and 27 receive official security patches. Older releases are end-of-life.

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🔧 Root Cause

According to the SSH protocol specification, connection protocol messages (message numbers 80+) must only be processed after successful user authentication.

The vulnerable Erlang/OTP SSH server did not properly disconnect or ignore such messages in the unauthenticated state, creating a path to RCE.

Official Patch: Added strict state checks in the message handling logic.

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🛡️ Remediation

Immediate Fix

Upgrade to a patched OTP release:

• OTP 27.3.3 or newer
• OTP 26.2.5.11 or newer
• OTP 25.3.2.20 or newer

Then rebuild and redeploy your Erlang/Elixir applications.

Official Erlang/OTP Advisory:
https://github.com/erlang/otp/security/advisories/GHSA-37cp-fgq5-7wc2

Key Patch Commits:

• https://github.com/erlang/otp/commit/6eef04130afc8b0ccb63c9a0d8650209cf54892f

Temporary Workarounds

• Disable the SSH server if not required (:ssh.stop() or remove ssh application)
• Restrict network access to the SSH port using firewalls (allow only trusted IPs)
• Run the SSH daemon with minimal privileges (never as root when possible)

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🧪 Exploitation

Public Proof-of-Concept exploits were released shortly after disclosure. Exploitation is straightforward and does not require credentials.

The vulnerability has been added to CISA KEV catalog and has been actively exploited in the wild (especially against OT networks and firewalls).

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📌 Who Is Affected?

Any system running an Erlang/OTP SSH server (ssh.daemon/1, ssh:daemon/2, etc.) is potentially vulnerable. This includes:

• Custom Erlang/Elixir services
• Telecom and networking equipment
• IoT/OT devices
• Some Cisco products (see their advisory for details)

Not affected: Pure SSH client usage or applications that do not start the built-in SSH daemon.

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🔍 References & Resources

• NVD Entry
• Erlang/OTP Official Advisory
• Unit 42 Analysis
• Cisco Security Advisory
• CISA KEV Listing

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✅ Recommendations

• Scan your infrastructure for exposed Erlang/OTP SSH services
• Patch immediately — public exploits are widely available
• Monitor SSH ports for suspicious pre-auth traffic
• Follow least-privilege principles for all services

Last Updated: April 2026

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