CVE-2016-4975 in HTTP Server
Summary
by MITRE
Possible CRLF injection allowing HTTP response splitting attacks for sites which use mod_userdir. This issue was mitigated by changes made in 2.4.25 and 2.2.32 which prohibit CR or LF injection into the "Location" or other outbound header key or value. Fixed in Apache HTTP Server 2.4.25 (Affected 2.4.1-2.4.23). Fixed in Apache HTTP Server 2.2.32 (Affected 2.2.0-2.2.31).
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Analysis
by VulDB Data Team • 03/15/2020
The vulnerability identified as CVE-2016-4975 represents a critical security flaw in the Apache HTTP Server that enables CRLF injection attacks leading to HTTP response splitting. This vulnerability specifically affects installations using mod_userdir module, which allows users to have personalized web directories accessible via URLs. The flaw stems from inadequate input validation and sanitization of user-supplied data within the server's header processing mechanisms, creating an avenue for malicious actors to inject carriage return and line feed characters into HTTP headers.
The technical exploitation of this vulnerability occurs through the manipulation of user directory paths or other input fields that are subsequently used in HTTP response headers. When mod_userdir processes user requests, it incorporates user-provided data into the Location header or other outbound headers without proper sanitization. This allows attackers to inject CRLF sequences that can terminate the current HTTP response and begin a new one, effectively enabling HTTP response splitting attacks. The vulnerability is categorized under CWE-113, which specifically addresses "Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')", a well-documented weakness in web application security.
The operational impact of this vulnerability is severe and multifaceted. Attackers can leverage HTTP response splitting to perform various malicious activities including cache poisoning, session hijacking, and cross-site scripting attacks. By injecting malicious content into HTTP headers, an attacker can manipulate how web browsers process responses, potentially redirecting users to malicious sites or injecting harmful content into web pages. This vulnerability particularly affects web applications that rely on mod_userdir for user directory access, making it a significant concern for hosting providers and organizations that utilize this Apache module for user personalization.
The Apache HTTP Server development team addressed this vulnerability through comprehensive fixes implemented in versions 2.4.25 and 2.2.32. These updates introduced strict validation mechanisms that prohibit carriage return and line feed characters from being injected into Location headers and other outbound header keys or values. The mitigation strategy involves implementing robust input sanitization and header validation routines that filter out potentially dangerous characters before they can be processed in HTTP responses. Organizations using affected Apache versions should immediately upgrade to the patched releases to eliminate this security risk, as the vulnerability has been classified under the ATT&CK technique T1190 for "Exploit Public-Facing Application" and T1071.004 for "Application Layer Protocol: DNS" due to its potential for enabling further attack vectors.
The remediation approach recommended for organizations involves comprehensive security audits of Apache configurations, particularly those utilizing mod_userdir functionality. System administrators should implement strict input validation policies, deploy web application firewalls that can detect and block CRLF injection attempts, and conduct regular security assessments to identify potential vulnerabilities in HTTP header processing. Additionally, organizations should establish monitoring protocols to detect unusual patterns in HTTP response headers that might indicate exploitation attempts. The vulnerability serves as a critical reminder of the importance of proper input validation and header sanitization in web server implementations, highlighting the necessity of adhering to security best practices and maintaining up-to-date software versions to prevent exploitation of known vulnerabilities.