CVE-2009-4495 in Web Server
Summary
by MITRE
Yaws 1.85 writes data to a log file without sanitizing non-printable characters, which might allow remote attackers to modify a window s title, or possibly execute arbitrary commands or overwrite files, via an HTTP request containing an escape sequence for a terminal emulator.
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Analysis
by VulDB Data Team • 11/18/2025
CVE-2009-4495 represents a significant security vulnerability in Yaws web server version 1.85 that stems from inadequate input sanitization within its logging mechanism. This flaw occurs when the server processes HTTP requests and writes received data directly to log files without proper sanitization of non-printable characters and escape sequences. The vulnerability falls under the category of improper input validation and can be classified as a CWE-172 - "Improper Handling of Unicode Encoding" or more specifically related to CWE-77 - "Improper Neutralization of Special Elements used in a Command" when considering the potential for command execution through terminal escape sequences. The issue demonstrates a classic example of how seemingly benign logging operations can become attack vectors when proper data sanitization is omitted.
The technical exploitation of this vulnerability relies on the fact that Yaws does not properly sanitize data before writing it to log files, creating opportunities for attackers to inject terminal escape sequences that can manipulate terminal emulators. When these log files are later viewed or processed by terminal applications, the embedded escape sequences can trigger unintended behaviors including window title modification, arbitrary command execution, or even file overwrite operations. Attackers can craft HTTP requests containing specially formatted escape sequences that when logged will be interpreted by terminal emulators as actual commands rather than plain text. This type of vulnerability aligns with ATT&CK technique T1059 - "Command and Scripting Interpreter" where adversaries leverage command injection capabilities to execute malicious code. The vulnerability particularly affects environments where log files are viewed in terminal emulators or processed by applications that interpret escape sequences.
The operational impact of CVE-2009-4495 extends beyond simple data corruption or display manipulation. When attackers successfully exploit this vulnerability, they can potentially achieve full system compromise through command execution capabilities. The attack surface is particularly concerning in web server environments where log files are frequently monitored or accessed by system administrators, as these files become potential entry points for privilege escalation. The vulnerability can be leveraged in various attack scenarios including privilege escalation through log file manipulation, persistent backdoor establishment, and information disclosure through command execution capabilities. Organizations using Yaws 1.85 are particularly vulnerable as this flaw allows attackers to manipulate system behavior through simple HTTP requests without requiring elevated privileges. The vulnerability also impacts defense-in-depth strategies since log files, traditionally considered safe audit trails, become potential attack vectors that can be weaponized to compromise system integrity.
Mitigation strategies for CVE-2009-4495 require immediate implementation of proper input sanitization within the Yaws logging subsystem and comprehensive security hardening of the web server environment. Organizations should implement strict character filtering and sanitization for all data written to log files, particularly focusing on non-printable characters and escape sequences that could be interpreted by terminal emulators. The solution involves configuring the web server to sanitize input data before logging, which can be achieved through proper encoding of special characters and implementation of input validation routines that prevent the injection of terminal escape sequences. Additionally, system administrators should implement regular log file monitoring and access controls to prevent unauthorized modification of log files, while also ensuring that log viewing applications properly handle escape sequences without interpreting them as commands. Security patches should be applied immediately to upgrade to versions of Yaws that address this vulnerability, and organizations should conduct thorough security assessments of their logging infrastructure to identify similar vulnerabilities in other applications. The remediation process should also include implementing proper log management practices that separate log processing from log viewing to prevent terminal-based command injection scenarios.