CVE-2018-7544 in OpenVPN
Summary
by MITRE
** DISPUTED ** A cross-protocol scripting issue was discovered in the management interface in OpenVPN through 2.4.5. When this interface is enabled over TCP without a password, and when no other clients are connected to this interface, attackers can execute arbitrary management commands, obtain sensitive information, or cause a denial of service (SIGTERM) by triggering XMLHttpRequest actions in a web browser. This is demonstrated by a multipart/form-data POST to http://localhost:23000 with a "signal SIGTERM" command in a TEXTAREA element. NOTE: The vendor disputes that this is a vulnerability. They state that this is the result of improper configuration of the OpenVPN instance rather than an intrinsic vulnerability, and now more explicitly warn against such configurations in both the management-interface documentation, and with a runtime warning.
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability described in CVE-2018-7544 relates to a cross-protocol scripting issue within the management interface of OpenVPN versions through 2.4.5. This flaw exists when the management interface is configured to operate over TCP without authentication credentials, creating a significant security risk that can be exploited by remote attackers. The issue stems from the improper handling of web-based management commands that can be triggered through XMLHttpRequest actions in a web browser environment, making it particularly concerning for systems where such interfaces are exposed to untrusted networks.
The technical implementation of this vulnerability involves the management interface accepting commands through HTTP POST requests to the default port 23000, specifically targeting the multipart/form-data content type. Attackers can submit malicious commands through a TEXTAREA element within the POST request payload, with the demonstration showing how a "signal SIGTERM" command can be executed to terminate the OpenVPN process. This represents a critical flaw in the interface's input validation and authentication mechanisms, allowing unauthorized command execution without proper authorization checks. The vulnerability operates at the application layer and demonstrates a classic case of insufficient access control and improper privilege management, which aligns with CWE-285 for insufficient authorization and CWE-352 for cross-site request forgery.
The operational impact of this vulnerability extends beyond simple command execution to include information disclosure and denial of service conditions. When an attacker successfully exploits this vulnerability, they can gain complete control over the OpenVPN management interface, potentially leading to unauthorized access to sensitive configuration data, disruption of network services through process termination, or even complete system compromise. The risk is particularly elevated in environments where the management interface is exposed to external networks without proper authentication mechanisms. This vulnerability directly impacts the availability and integrity of the OpenVPN service, as demonstrated by the SIGTERM command that can terminate the service process, creating a denial of service scenario. The attack vector requires no special privileges or complex exploitation techniques, making it particularly dangerous in misconfigured environments.
Security practitioners should consider this vulnerability in the context of the ATT&CK framework, specifically under the techniques of privilege escalation and defense evasion. The management interface configuration represents a common misconfiguration pattern that violates security best practices for network service hardening. Organizations should implement proper network segmentation to isolate management interfaces from public networks, enforce strong authentication mechanisms, and regularly audit their OpenVPN configurations. The vendor's response to this issue, while disputing its classification as a vulnerability, acknowledges the importance of proper configuration and has updated their documentation to warn against such setups. This highlights the critical importance of following security guidelines and proper system administration practices to prevent configuration-induced security weaknesses.
The remediation approach for this vulnerability involves several key steps including disabling the management interface when not required, implementing proper authentication mechanisms for management access, configuring the interface to listen only on localhost or trusted networks, and ensuring that no unauthenticated management interfaces are exposed to external access. System administrators should also implement network access controls and monitoring to detect unauthorized access attempts to management interfaces. The vulnerability demonstrates the importance of principle of least privilege and proper network architecture design, where management interfaces should never be exposed without proper authentication and authorization controls. Regular security assessments and configuration reviews should be conducted to ensure that management interfaces are properly secured and that no legacy configurations that could expose the system to this type of attack remain in place.