CVE-2023-6004: Libssh RCE Vulnerability

CVE-2023-6004 Overview

A command injection vulnerability has been identified in libssh, a widely-used C library implementing the SSH protocol. The flaw exists in the ProxyCommand and ProxyJump features, where insufficient validation of hostname syntax allows attackers to inject malicious code through specially crafted hostname parameters. This vulnerability affects multiple Linux distributions including Fedora and Red Hat Enterprise Linux.

Critical Impact

Attackers with local access can potentially execute arbitrary commands on affected systems by exploiting improper hostname validation in libssh's proxy features, leading to code execution with the privileges of the SSH client process.

Affected Products

• libssh libssh (multiple versions)
• Fedora 38
• Red Hat Enterprise Linux 8.0 and 9.0

Discovery Timeline

• 2024-01-03 - CVE-2023-6004 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2023-6004

Vulnerability Analysis

This vulnerability falls under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component), commonly known as injection vulnerabilities. The flaw resides in how libssh processes hostname parameters when utilizing the ProxyCommand or ProxyJump SSH client features.

When a user configures SSH to connect through a proxy using these features, the hostname is incorporated into command-line arguments without adequate sanitization. An attacker who can influence the hostname value—whether through social engineering, malicious configuration files, or other means—can embed shell metacharacters or command sequences that will be executed when the proxy command is invoked.

The local attack vector requires the attacker to have some level of access to the target system or the ability to manipulate SSH configuration. While user interaction is required to trigger the vulnerability, successful exploitation can impact confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2023-6004 is insufficient input validation and sanitization of hostname parameters in libssh's ProxyCommand and ProxyJump implementation. The library fails to properly escape or validate special characters in hostnames before they are passed to shell commands, allowing shell metacharacters to be interpreted as command separators or operators.

Attack Vector

The attack requires local access and exploits the ProxyCommand or ProxyJump features in libssh. An attacker must be able to influence the hostname parameter that gets passed to these features. This could occur through:

1. Malicious SSH configuration: An attacker with write access to SSH configuration files could insert a crafted hostname containing injection payloads
2. Social engineering: Tricking a user into connecting to a maliciously crafted hostname
3. Application integration: Applications that use libssh and accept user-controlled hostnames without proper validation

When the vulnerable code processes a hostname containing shell metacharacters (such as ;, |, $(), or backticks), these characters are interpreted by the shell, allowing arbitrary command execution.

Detection Methods for CVE-2023-6004

Indicators of Compromise

• Unusual SSH connection attempts with hostnames containing shell metacharacters or command sequences
• Unexpected child processes spawned by SSH client processes
• SSH configuration files modified to include suspicious hostname entries with special characters

Detection Strategies

• Monitor SSH configuration files (~/.ssh/config, /etc/ssh/ssh_config) for entries containing shell metacharacters in hostname fields
• Implement file integrity monitoring on SSH configuration files to detect unauthorized modifications
• Review application logs for SSH connections to hostnames containing characters like ;, |, $, or backticks
• Use behavioral analysis to detect anomalous process execution chains originating from SSH client processes

Monitoring Recommendations

• Enable detailed logging for SSH client operations and review logs for suspicious hostname patterns
• Deploy endpoint detection and response (EDR) solutions to monitor for unusual command execution following SSH client invocation
• Implement network monitoring to detect SSH connections to malformed or suspicious hostnames

How to Mitigate CVE-2023-6004

Immediate Actions Required

• Update libssh to the latest patched version available for your distribution
• Audit existing SSH configurations for any potentially malicious hostname entries
• Restrict write access to SSH configuration files to prevent unauthorized modifications
• Review applications that integrate with libssh to ensure they validate hostname inputs before passing to the library

Patch Information

Security patches have been released by multiple vendors. Red Hat has published security advisories RHSA-2024:2504 and RHSA-2024:3233 addressing this vulnerability for Enterprise Linux users. Fedora has also released updated packages as documented in their package announcements. For the official libssh advisory, refer to the LibSSH Security Advisory. NetApp customers should consult NTAP-20240223-0004.

Workarounds

• Avoid using ProxyCommand or ProxyJump features until patches can be applied
• Implement strict input validation on any application that programmatically constructs SSH connections with user-supplied hostnames
• Use allowlisting for permitted SSH hostnames where possible to prevent connection to arbitrary hosts
• Consider using alternative SSH libraries or direct SSH connections without proxy features as a temporary measure

# Verify installed libssh version on Red Hat/Fedora systems
rpm -qa | grep libssh

# Check for available updates
dnf check-update libssh

# Apply security updates
sudo dnf update libssh