CVE-2013-5705 in ModSecurity
Summary
by MITRE
apache2/modsecurity.c in ModSecurity before 2.7.6 allows remote attackers to bypass rules by using chunked transfer coding with a capitalized Chunked value in the Transfer-Encoding HTTP header.
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Analysis
by VulDB Data Team • 05/10/2026
The vulnerability identified as CVE-2013-5705 affects ModSecurity versions prior to 2.7.6, specifically within the apache2/modsecurity.c component. This issue represents a significant bypass flaw in web application firewall implementations that could allow malicious actors to circumvent security rules designed to protect web applications from various attack vectors. The vulnerability stems from improper handling of HTTP transfer encoding mechanisms within the ModSecurity module that processes web traffic for Apache web servers.
The technical flaw occurs when ModSecurity encounters HTTP requests with chunked transfer coding where the Transfer-Encoding header value is formatted with capitalized letters, specifically "Chunked" rather than the standard lowercase "chunked" or mixed case variations. This capitalization causes the ModSecurity module to fail to properly recognize and process the chunked transfer encoding, effectively bypassing the security rules that would normally be applied to such requests. The issue demonstrates a classic input validation weakness where the system fails to normalize or properly interpret HTTP headers according to RFC specifications, creating a path for attackers to evade detection mechanisms.
From an operational impact perspective, this vulnerability allows remote attackers to execute attacks that would normally be blocked by ModSecurity rules, potentially enabling various malicious activities including but not limited to cross-site scripting attempts, SQL injection attacks, and other web application exploits. The bypass occurs at the HTTP protocol level where the firewall fails to properly parse and handle the chunked transfer encoding, which is a legitimate HTTP feature used for streaming data in HTTP/1.1 communications. This weakness essentially creates a tunnel through which malicious payloads can pass undetected by the security monitoring systems that rely on ModSecurity for protection.
The vulnerability aligns with CWE-20, which describes improper input validation, and demonstrates how insufficient normalization of HTTP headers can create security gaps in web application firewalls. From an ATT&CK framework perspective, this issue maps to techniques involving evasion and command and control communications where adversaries can bypass network-based defenses to maintain persistence and execute malicious activities. Organizations using affected ModSecurity versions face significant risk as this bypass can be easily automated and does not require specialized knowledge to exploit, making it particularly dangerous in production environments where web application security is paramount.
The recommended mitigation involves upgrading to ModSecurity version 2.7.6 or later, where the issue has been addressed through proper handling of capitalized transfer encoding values. Additionally, organizations should implement comprehensive monitoring of HTTP headers to detect unusual patterns and maintain regular security updates for all web application firewall components. Security teams should also consider implementing additional detection mechanisms beyond ModSecurity to provide defense in depth, as this vulnerability demonstrates how protocol-level parsing issues can create exploitable gaps in security infrastructure.