CVE-2024-2873: WolfSSH Auth Bypass Vulnerability

CVE-2024-2873 Overview

A critical authentication bypass vulnerability was discovered in wolfSSH's server-side state machine that affects versions prior to 1.4.17. The flaw allows malicious clients to create SSH channels without first completing the required user authentication process, resulting in unauthorized access to protected resources. This vulnerability stems from improper authentication enforcement (CWE-287) in the SSH protocol implementation.

Critical Impact

Unauthenticated remote attackers can bypass authentication entirely and establish unauthorized SSH channels, potentially gaining full access to systems protected by wolfSSH without providing valid credentials.

Affected Products

• wolfSSH versions prior to 1.4.17
• Systems implementing wolfSSH for SSH server functionality
• Embedded devices and IoT systems using wolfSSH for secure communications

Discovery Timeline

• 2024-03-25 - CVE-2024-2873 published to NVD
• 2025-12-05 - Last updated in NVD database

Technical Details for CVE-2024-2873

Vulnerability Analysis

The vulnerability resides in wolfSSH's server-side state machine implementation, which is responsible for managing the SSH connection lifecycle and enforcing the protocol's security requirements. In a properly functioning SSH implementation, the server must enforce a strict sequence of operations: connection establishment, key exchange, user authentication, and only then channel creation. This vulnerability allows attackers to circumvent this critical authentication gate.

The flaw enables a malicious client to send channel open requests before completing the user authentication phase of the SSH protocol. Because the server-side state machine fails to properly validate that authentication has been successfully completed before processing channel requests, unauthorized channels can be established.

This vulnerability is particularly severe because SSH is often used as a critical security boundary for remote administration and secure communications in embedded systems, IoT devices, and network infrastructure where wolfSSH is commonly deployed.

Root Cause

The root cause is an improper authentication check (CWE-287) in the wolfSSH server-side state machine. The state machine fails to enforce the proper sequence of SSH protocol operations, specifically not validating that user authentication has been completed before allowing channel operations. This represents a fundamental flaw in the protocol state management logic where the authentication state is not properly checked when processing SSH_MSG_CHANNEL_OPEN requests.

Attack Vector

The attack can be executed remotely over the network without requiring any prior authentication or user interaction. An attacker needs only network access to a wolfSSH server to exploit this vulnerability:

1. The attacker establishes a TCP connection to the target wolfSSH server
2. The attacker performs the initial SSH key exchange handshake
3. Instead of proceeding with user authentication, the attacker directly sends SSH channel open requests
4. The vulnerable state machine processes these requests without verifying authentication status
5. The attacker gains unauthorized channel access, bypassing authentication entirely

The vulnerability mechanism involves sending SSH protocol messages out of the expected sequence. When a malicious client sends SSH_MSG_CHANNEL_OPEN messages before completing authentication, the server's state machine fails to reject these requests. This allows the attacker to establish interactive sessions or execute commands without providing valid credentials. For detailed technical information about the fix, see GitHub Pull Request #670 and GitHub Pull Request #671.

Detection Methods for CVE-2024-2873

Indicators of Compromise

• SSH connections that establish channels without corresponding successful authentication log entries
• Unusual SSH session activity from clients that bypass normal authentication workflows
• SSH protocol anomalies where channel operations precede authentication completion
• Unexpected SSH sessions appearing without associated authentication events in logs

Detection Strategies

• Implement network-level monitoring to detect SSH protocol sequence anomalies where channel requests precede authentication
• Configure SSH logging to capture detailed protocol state transitions and flag sessions that skip authentication phases
• Deploy intrusion detection rules to identify SSH connections with out-of-sequence protocol messages
• Monitor for connections to wolfSSH servers that do not generate expected authentication log events

Monitoring Recommendations

• Enable verbose logging on wolfSSH servers to capture protocol state machine transitions
• Implement real-time alerting for SSH sessions that establish channels without authentication success logs
• Review SSH access logs for signs of unauthorized channel creation patterns
• Use network traffic analysis tools to monitor SSH protocol compliance and sequence integrity

How to Mitigate CVE-2024-2873

Immediate Actions Required

• Upgrade wolfSSH to version 1.4.17 or later immediately
• Audit existing SSH server deployments to identify all instances running vulnerable wolfSSH versions
• Implement network segmentation to limit exposure of wolfSSH servers while patches are applied
• Review SSH access logs for any signs of exploitation or unauthorized access attempts

Patch Information

wolfSSL has addressed this vulnerability in wolfSSH version 1.4.17 and later. The fix implements proper state machine validation to ensure channel operations are only permitted after successful user authentication. The patches are available through GitHub Pull Request #670 and GitHub Pull Request #671. For comprehensive security information, refer to the wolfSSL Security Vulnerabilities Documentation.

Workarounds

• Implement network-level access controls (firewalls, ACLs) to restrict SSH access to trusted IP addresses only
• Deploy a properly configured SSH proxy or jump server in front of vulnerable wolfSSH instances
• Enable additional authentication layers such as VPN requirements before SSH access is permitted
• Monitor and rate-limit incoming SSH connections to detect and block anomalous connection attempts

# Example: Restrict SSH access using iptables until patching is complete
# Only allow SSH from trusted management network
iptables -A INPUT -p tcp --dport 22 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP