CVE-2017-12171 in HTTP Server
Summary
by MITRE
A regression was found in the Red Hat Enterprise Linux 6.9 version of httpd 2.2.15-60, causing comments in the "Allow" and "Deny" configuration lines to be parsed incorrectly. A web administrator could unintentionally allow any client to access a restricted HTTP resource.
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Analysis
by VulDB Data Team • 01/19/2021
The vulnerability identified as CVE-2017-12171 represents a critical configuration parsing error in the Apache HTTP Server version 2.2.15-60 shipped with Red Hat Enterprise Linux 6.9. This issue emerged as a regression in the software update process, where previously functioning comment handling mechanisms were inadvertently altered. The flaw specifically affects the mod_authz_core module which processes access control directives in httpd configuration files. When administrators configure access restrictions using Allow and Deny directives, the presence of comments within these lines can cause unexpected behavior. The vulnerability stems from improper parsing logic that fails to correctly distinguish between actual configuration parameters and commented text, leading to a scenario where comment characters are processed as part of the access control rules rather than being ignored.
The technical implementation of this vulnerability involves the httpd configuration parser's handling of whitespace and comment characters within access control directives. When a comment character '#' appears within an Allow or Deny line, the parser incorrectly processes the content following the comment as part of the access rule instead of treating it as documentation. This regression affects the core authorization mechanism of the web server, potentially allowing unauthorized access to protected resources. The flaw demonstrates characteristics consistent with CWE-1287, which deals with improper parsing of configuration files, and represents a classic example of how seemingly minor code changes can introduce significant security implications. The vulnerability is particularly concerning because it operates at the configuration level rather than requiring exploitation of a code execution flaw, making it accessible to any administrator who inadvertently introduces comments in access control directives.
From an operational impact perspective, this vulnerability creates a substantial risk for organizations relying on httpd for web content delivery and access control. An attacker who gains access to modify web server configuration files could potentially exploit this weakness by adding comments to Allow directives in a way that inadvertently grants access to restricted resources. The vulnerability also affects legitimate administrators who might include comments for documentation purposes within their access control configurations, leading to unexpected access behavior without any malicious intent. The flaw's impact extends beyond simple access control bypass as it undermines the fundamental security model of web applications. According to ATT&CK framework, this vulnerability maps to T1068 (Local Privilege Escalation) and T1566 (Phishing), as it can enable unauthorized access to sensitive resources and potentially facilitate further exploitation. Organizations with complex access control requirements are particularly at risk, as the vulnerability could allow attackers to gain access to administrative interfaces, sensitive data repositories, or protected application functionality.
Mitigation strategies for CVE-2017-12171 should focus on immediate patching of affected systems and implementation of configuration review processes. The primary solution involves upgrading to a patched version of httpd that corrects the parsing logic for Allow and Deny directives. Organizations should conduct comprehensive audits of their existing httpd configurations to identify any instances where comments might be present within access control directives. Security teams should implement configuration management procedures that prevent the introduction of comments in critical access control sections, and establish regular review processes to ensure compliance with secure configuration practices. The vulnerability highlights the importance of thorough regression testing in security updates and demonstrates the need for careful validation of configuration parsing logic. Additionally, organizations should consider implementing automated configuration validation tools that can detect potentially problematic comment placements within access control directives, providing early warning of configuration issues that could lead to security exposure.