CVE-2019-17567 in HTTP Server
Summary
by MITRE • 06/10/2021
Apache HTTP Server versions 2.4.6 to 2.4.46 mod_proxy_wstunnel configured on an URL that is not necessarily Upgraded by the origin server was tunneling the whole connection regardless, thus allowing for subsequent requests on the same connection to pass through with no HTTP validation, authentication or authorization possibly configured.
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Analysis
by VulDB Data Team • 03/05/2025
The vulnerability identified as CVE-2019-17567 affects Apache HTTP Server versions ranging from 2.4.6 through 2.4.46 and specifically involves the mod_proxy_wstunnel module. This module is designed to handle WebSocket tunneling through proxy configurations, but the flaw manifests when the server is configured to handle WebSocket connections that may not be properly upgraded by the origin server. The issue stems from improper connection handling where the proxy continues to tunnel the entire connection regardless of whether the WebSocket upgrade has been successfully completed, creating a significant security gap in the HTTP request processing pipeline.
The technical flaw resides in the mod_proxy_wstunnel module's failure to properly validate the WebSocket upgrade process before establishing the tunnel. When a client establishes a connection through the proxy server, the module should verify that the WebSocket upgrade has been successfully negotiated with the origin server before allowing subsequent requests to pass through the tunnel. However, due to this vulnerability, the proxy maintains the tunnel connection even when the upgrade fails or is incomplete, permitting unauthorized access to resources that should require proper HTTP validation, authentication, and authorization. This behavior creates a persistent tunnel that bypasses normal security controls that would typically be enforced during HTTP request processing.
The operational impact of this vulnerability is substantial as it allows attackers to exploit the proxy configuration to gain unauthorized access to backend resources. Once an attacker establishes a connection through the vulnerable proxy, they can potentially make subsequent requests on the same connection without proper HTTP validation, authentication, or authorization checks. This means that sensitive resources behind the proxy could be accessed without proper credentials or access controls, potentially leading to data breaches, privilege escalation, or unauthorized system access. The vulnerability is particularly concerning in environments where the proxy serves as a security boundary between internal and external networks, as it effectively creates a backdoor that bypasses normal security mechanisms.
Organizations affected by this vulnerability should immediately upgrade their Apache HTTP Server installations to version 2.4.47 or later, which contains the necessary patches to address the WebSocket tunneling flaw. The mitigation strategy should also include reviewing and validating all proxy configurations, particularly those involving mod_proxy_wstunnel, to ensure that proper WebSocket upgrade validation is implemented. Security teams should conduct thorough audits of their proxy server configurations to identify any instances where the vulnerable module is active and where connections might be bypassing normal security controls. Additionally, network segmentation and access control measures should be strengthened to limit the potential impact of any successful exploitation attempts, as the vulnerability essentially allows for persistent tunneling that could remain undetected for extended periods. This issue aligns with CWE-284 (Improper Access Control) and represents a significant risk under the ATT&CK framework category of Privilege Escalation through Proxy Configuration flaws.
The vulnerability demonstrates a critical weakness in how proxy modules handle connection state management and security validation. The flaw essentially creates a persistent tunneling mechanism that bypasses normal HTTP security controls, making it particularly dangerous in environments where proxies are used as security gateways. Organizations should implement monitoring solutions to detect unusual proxy behavior and establish proper incident response procedures to address potential exploitation attempts. The root cause of the vulnerability highlights the importance of proper validation in proxy modules and the need for comprehensive security testing of all proxy-related functionality.