CVE-2003-0843 in mod_gzip
Summary
by MITRE
Format string vulnerability in mod_gzip_printf for mod_gzip 1.3.26.1a and earlier, and possibly later official versions, when running in debug mode and using the Apache log, allows remote attackers to execute arbitrary code via format string characters in an HTTP GET request with an "Accept-Encoding: gzip" header.
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Analysis
by VulDB Data Team • 05/17/2019
The vulnerability described in CVE-2003-0843 represents a critical format string flaw within the mod_gzip Apache module, specifically affecting versions 1.3.26.1a and earlier, with potential exposure in later official releases. This vulnerability resides in the mod_gzip_printf function which handles printf-style formatting operations. The flaw becomes exploitable when the module operates in debug mode and processes Apache log entries, creating a dangerous condition where attacker-controlled data can be interpreted as format specifiers rather than literal text. The vulnerability is triggered through HTTP GET requests that include the specific "Accept-Encoding: gzip" header, which causes the module to process compressed content and subsequently invoke the vulnerable printf function with attacker-supplied data.
The technical nature of this vulnerability stems from improper input validation within the mod_gzip module's printf implementation. When the module receives a request with gzip encoding, it processes the Accept-Encoding header and potentially passes user-supplied data directly into printf functions without proper sanitization. This allows attackers to inject format string specifiers such as %s, %d, or %x which can then be interpreted by the printf function to read from memory locations or write to arbitrary addresses. The debug mode operation amplifies the risk as additional logging functionality is enabled, providing more opportunities for attacker-controlled data to be processed through vulnerable code paths. The vulnerability maps directly to CWE-134, which specifically addresses the use of user-supplied format strings in printf functions, and represents a classic example of improper input validation leading to arbitrary code execution.
The operational impact of this vulnerability is severe and far-reaching within Apache web server environments. Remote attackers can leverage this flaw to execute arbitrary code on vulnerable systems with the privileges of the Apache process, typically running as the web server user. This could lead to complete system compromise, allowing attackers to establish persistent backdoors, escalate privileges, or launch further attacks against internal network resources. The vulnerability affects not just individual servers but entire web applications that rely on mod_gzip for content compression, making it particularly dangerous in high-traffic environments where multiple users can potentially exploit the flaw. The attack vector is relatively simple and does not require authentication, making it particularly attractive to automated exploitation tools, and the impact extends beyond just the immediate server to potentially compromise the entire hosting infrastructure.
Mitigation strategies for this vulnerability should focus on immediate patching and configuration hardening. The primary remediation involves upgrading to mod_gzip versions that have addressed this vulnerability, specifically those beyond 1.3.26.1a, and ensuring that debug mode is disabled in production environments. System administrators should also implement proper input validation at the web server level and consider disabling mod_gzip entirely if compression functionality is not critical for the application's performance requirements. Network-level protections can include implementing web application firewalls that monitor for suspicious format string patterns in HTTP requests, and regular security audits should verify that debug modes are not enabled in production systems. Additionally, organizations should follow ATT&CK framework guidance for defensive measures against code injection vulnerabilities, particularly focusing on techniques related to command and control through web server exploitation. The vulnerability underscores the importance of proper input sanitization and the dangers of enabling debug functionality in production environments, serving as a reminder of the critical security considerations in web server module configurations.