CVE-2011-3368 in HTTP Server
Summary
by MITRE
The mod_proxy module in the Apache HTTP Server 1.3.x through 1.3.42, 2.0.x through 2.0.64, and 2.2.x through 2.2.21 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 via a malformed URI containing an initial @ (at sign) character.
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Analysis
by VulDB Data Team • 12/22/2024
The vulnerability described in CVE-2011-3368 represents a critical security flaw in the Apache HTTP Server's mod_proxy module that affects multiple versions across the 1.3, 2.0, and 2.2 release lines. This issue stems from improper handling of URI parsing when specific Apache configuration directives are used in conjunction with reverse proxy functionality. The flaw specifically manifests when the RewriteRule and ProxyPassMatch directives are configured together to establish reverse proxy behavior, creating a pathway for malicious actors to bypass normal access controls and potentially reach internal network resources that should remain protected from external access.
The technical root cause of this vulnerability lies in how Apache processes URIs that begin with the (at sign) character when these are processed through the mod_proxy module. When a malformed URI containing an initial character is submitted to an Apache server configured with specific proxy directives, the server fails to properly validate or sanitize the URI before forwarding the request to backend servers. This occurs because the mod_proxy module does not correctly handle the parsing of such URIs, allowing the @ character to be interpreted as part of the proxy destination specification rather than as part of the URI path. The vulnerability is particularly dangerous because it leverages legitimate Apache configuration features to create an unexpected behavior that enables unauthorized access to internal systems.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable complete network compromise through what is known as an "intranet probing" attack pattern. Attackers can craft malicious requests that appear to target external resources but are actually routed to internal servers within the organization's network, effectively creating a bridge between external network access and internal system enumeration. This type of vulnerability directly maps to attack techniques documented in the MITRE ATT&CK framework under the 'Initial Access' and 'Lateral Movement' phases, specifically targeting the 'Proxy" and 'Remote Services' tactics. The vulnerability's classification aligns with CWE-1287, which describes "Improper Handling of URLs that Contain a Double-Slash" and related URI parsing issues that can lead to access control bypasses.
Organizations affected by this vulnerability face significant risks including unauthorized access to internal web services, potential data exfiltration, and the ability to perform reconnaissance on internal network infrastructure. The attack vector is particularly insidious because it can be executed through standard HTTP requests without requiring special privileges or complex exploitation techniques. Security professionals should note that this vulnerability demonstrates the critical importance of proper URI sanitization in web server configurations and highlights the dangers of combining multiple Apache modules in ways that create unexpected interaction patterns. The vulnerability also underscores the need for comprehensive security testing of web server configurations, particularly those involving proxy and rewrite rules, as these combinations can introduce subtle but dangerous security flaws that are difficult to detect through routine scanning.
Mitigation strategies for CVE-2011-3368 require immediate attention from system administrators and security teams. The most effective approach involves updating Apache HTTP Server to versions that contain the necessary patches, specifically versions 2.2.22 or later for the 2.2.x branch, and 2.0.65 or later for the 2.0.x branch, with the 1.3.x branch being deprecated and requiring complete replacement. Organizations should also implement strict input validation for all proxy-related configurations and consider using alternative proxy directive combinations that do not exhibit this behavior. Network-level mitigations include implementing proper firewall rules to restrict access to internal proxy endpoints and monitoring for unusual proxy request patterns that might indicate exploitation attempts. The vulnerability's resolution through official patches aligns with industry best practices for vulnerability management and demonstrates the importance of maintaining current software versions to protect against known security flaws.