CVE-2001-1556 in HTTP Server
Summary
by MITRE
The log files in Apache web server contain information directly supplied by clients and does not filter or quote control characters, which could allow remote attackers to hide HTTP requests and spoof source IP addresses when logs are viewed with UNIX programs such as cat, tail, and grep.
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Analysis
by VulDB Data Team • 04/11/2019
The vulnerability described in CVE-2001-1556 represents a significant logging security flaw within the Apache web server implementation that directly impacts the integrity and trustworthiness of server log data. This issue stems from the fundamental design approach where Apache logs contain raw client-supplied data without proper sanitization or escaping of control characters. The flaw exists specifically in how the web server processes and records HTTP request information, creating an environment where malicious actors can exploit the lack of input validation to manipulate log file contents. When users or automated systems process these log files using standard UNIX utilities, the unescaped control characters can cause the display and interpretation of log data to behave unexpectedly, potentially leading to serious security implications.
The technical root cause of this vulnerability lies in the absence of proper input filtering mechanisms within Apache's logging subsystem. Control characters such as carriage return, line feed, tab, and other non-printable ASCII values are directly embedded into log entries without any form of escaping or quoting. This creates a condition where attackers can inject malicious sequences that alter how log viewers interpret the data. When log files are processed by standard UNIX tools like cat, tail, or grep, these unescaped control characters can cause terminal output to be manipulated, making it appear as though different requests occurred than actually did. The vulnerability specifically affects the presentation layer of log data rather than the actual web server functionality, but this presentation manipulation can be exploited to hide malicious activities or forge legitimate-looking log entries.
The operational impact of this vulnerability extends beyond simple log file manipulation to potentially compromise security monitoring and incident response capabilities. Security administrators who rely on log analysis for threat detection and forensic investigations may be misled by manipulated log entries that obscure actual attack patterns or legitimate user activities. Attackers can exploit this flaw to hide their presence by crafting requests that, when logged, appear to contain benign content or even legitimate system commands when viewed through certain log viewing tools. The vulnerability is particularly dangerous because it leverages the trust placed in log files as authoritative records of system activity, making it difficult for security teams to accurately assess system health or detect ongoing attacks. This issue can also impact compliance requirements where audit trails must be trustworthy and tamper-proof.
This vulnerability maps directly to CWE-116, which describes the weakness of inadequate escaping of control characters in output, and relates to ATT&CK technique T1070.004, which covers the use of log manipulation to evade detection. The flaw demonstrates poor input validation practices and highlights the importance of proper data sanitization in security-critical components. Organizations should implement comprehensive log sanitization measures that escape or remove control characters before logging, ensure proper access controls on log files to prevent tampering, and consider using centralized logging solutions that provide better data integrity guarantees. Regular log file audits should be conducted to detect potential manipulation attempts, and security monitoring systems should be configured to alert on unusual log patterns or potential control character sequences that might indicate exploitation attempts. The vulnerability underscores the critical need for secure coding practices and input validation in all components of web applications and infrastructure, particularly those handling user-supplied data in logging contexts.