CVE-2011-3639 in HTTP Server
Summary
by MITRE
The mod_proxy module in the Apache HTTP Server 2.0.x through 2.0.64 and 2.2.x before 2.2.18, when the Revision 1179239 patch is in place, does not properly interact with use of (1) RewriteRule and (2) ProxyPassMatch pattern matches for configuration of a reverse proxy, which allows remote attackers to send requests to intranet servers by using the HTTP/0.9 protocol with a malformed URI containing an initial @ (at sign) character. NOTE: this vulnerability exists because of an incomplete fix for CVE-2011-3368.
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Analysis
by VulDB Data Team • 09/08/2024
The vulnerability described in CVE-2011-3639 represents a critical security flaw in the Apache HTTP Server's mod_proxy module that emerged from an incomplete remediation effort for a previous vulnerability. This issue affects Apache versions 2.0.x through 2.0.64 and 2.2.x versions prior to 2.2.18, specifically when the Revision 1179239 patch has been applied. The flaw stems from improper handling of certain URL patterns when Apache is configured as a reverse proxy, creating a pathway for attackers to bypass intended security restrictions. The vulnerability is particularly concerning because it allows remote attackers to conduct unauthorized access attempts to internal network resources through a sophisticated exploitation technique involving HTTP/0.9 protocol manipulation.
The technical root cause of this vulnerability lies in the interaction between mod_proxy's processing of RewriteRule and ProxyPassMatch directives when combined with HTTP/0.9 protocol handling. When a malformed URI containing an initial character is submitted through HTTP/0.9 requests, the proxy module fails to properly validate or sanitize the incoming request before forwarding it to backend servers. This occurs because the patch introduced to address CVE-2011-3368 was incomplete, leaving gaps in the validation logic that specifically affect how the server processes URIs with special character sequences. The character in the URI path creates a parsing ambiguity that the proxy module does not adequately handle, allowing attackers to construct requests that appear to target external resources while actually directing traffic to internal network addresses.
The operational impact of this vulnerability is severe and potentially devastating for organizations relying on Apache as a reverse proxy server. Attackers can leverage this flaw to perform internal network reconnaissance and potentially gain access to sensitive internal systems that should normally be protected from external access. The vulnerability enables what cybersecurity practitioners would categorize as a "lateral movement" attack vector, allowing remote threat actors to probe internal services that are typically isolated from direct internet exposure. This creates significant risk for enterprises where Apache serves as a gateway to internal corporate resources, potentially exposing databases, internal applications, and other sensitive infrastructure to unauthorized access attempts.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to patched versions of Apache HTTP Server, specifically versions 2.0.65 and 2.2.18 or later, which contain the complete fix for both CVE-2011-3368 and CVE-2011-3639. Additionally, administrators should review and tighten proxy configuration settings to avoid using vulnerable RewriteRule and ProxyPassMatch patterns that could trigger the exploit conditions. The vulnerability aligns with ATT&CK technique T1071.004 for application layer protocol manipulation and CWE-20 for improper input validation, highlighting the importance of robust input sanitization in proxy server implementations. Security teams should also consider implementing network segmentation and access controls to limit the potential impact if exploitation occurs, as this vulnerability represents a privilege escalation risk that could lead to complete system compromise of internal network resources.