CVE-2023-6004 in libssh
Summary
by MITRE • 01/03/2024
A flaw was found in libssh. By utilizing the ProxyCommand or ProxyJump feature, users can exploit unchecked hostname syntax on the client. This issue may allow an attacker to inject malicious code into the command of the features mentioned through the hostname parameter.
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Analysis
by VulDB Data Team • 05/08/2025
The vulnerability identified as CVE-2023-6004 resides within the libssh library, a widely-used open-source implementation of the ssh protocol that serves as the foundation for secure remote access and communication in numerous enterprise and network infrastructure deployments. This flaw specifically targets the ProxyCommand and ProxyJump functionality, which are essential features that enable users to establish secure connections through intermediate jump hosts or proxy servers. The vulnerability stems from insufficient validation of hostname syntax when these proxy features are utilized, creating a potential attack surface where malicious input can be injected into command execution contexts. The affected implementation fails to properly sanitize or validate the hostname parameter, allowing crafted inputs to bypass normal parsing mechanisms and potentially execute unintended commands. This issue represents a critical security weakness that directly impacts the integrity and security posture of systems relying on libssh for secure communications, particularly in environments where proxy configurations are commonly employed for network segmentation and access control.
The technical exploitation of CVE-2023-6004 occurs through the manipulation of hostname parameters within the ProxyCommand or ProxyJump configuration directives. When a user configures these proxy features, the library processes the specified hostname without adequate validation of special characters or command injection sequences that could be interpreted by the underlying shell or command execution layer. Attackers can craft malicious hostnames containing shell metacharacters such as semicolons, pipes, or command substitution operators that get executed when the proxy connection is established. This vulnerability aligns with CWE-74, which describes improper neutralization of special elements used in a command, and CWE-94, which addresses the execution of arbitrary code due to improper control of generation of code. The flaw essentially creates a command injection vulnerability at the client-side configuration parsing level, where legitimate proxy functionality becomes a vector for malicious code execution. The attack requires minimal privileges and can be executed by any user who has access to configure proxy settings, making it particularly dangerous in multi-user environments where proxy configurations might be exposed to untrusted parties.
The operational impact of this vulnerability extends beyond simple command execution, potentially enabling attackers to perform privilege escalation, data exfiltration, or lateral movement within networks that rely on libssh for secure communications. When exploited, the vulnerability can allow an attacker to execute arbitrary commands on the client system with the privileges of the user running the ssh client, potentially leading to full system compromise if the user has elevated privileges. The vulnerability affects systems where libssh is used for client-side SSH operations, particularly in environments where jump hosts or proxy configurations are common, such as enterprise networks, cloud infrastructure deployments, and DevOps environments where automated tooling relies on SSH connections. Organizations using vulnerable versions of libssh may experience unauthorized access to their network resources, as attackers can leverage this vulnerability to bypass normal authentication mechanisms and establish persistent access through compromised proxy configurations. The impact is particularly severe because the vulnerability is present in widely-deployed software components and can be exploited without requiring network-level access or complex attack chains, making it a high-priority remediation item for security teams managing network infrastructure and remote access systems.
Organizations should immediately update to patched versions of libssh that implement proper hostname validation and input sanitization for proxy features. The recommended mitigation involves applying security patches from the libssh project maintainers or updating to versions that have addressed the unchecked hostname syntax issue. System administrators should conduct comprehensive inventory audits to identify all systems using vulnerable libssh versions and assess proxy configurations that might be exposed to this vulnerability. Additional defensive measures include implementing strict access controls for SSH configuration files, monitoring for unusual proxy command usage patterns, and reviewing network access controls to limit exposure of systems that might be vulnerable to this attack vector. The vulnerability demonstrates the importance of input validation and command execution sanitization in security-critical libraries, as highlighted by ATT&CK technique T1059.001 for command and script injection. Security teams should also consider implementing network segmentation and monitoring solutions that can detect anomalous proxy command execution patterns and provide visibility into potential exploitation attempts. Regular security assessments of third-party libraries and dependencies should include evaluation of input validation mechanisms and command execution contexts to prevent similar vulnerabilities from being introduced into production environments.