CVE-2024-1019 in ModSecurity
Summary
by MITRE • 01/30/2024
ModSecurity / libModSecurity 3.0.0 to 3.0.11 is affected by a WAF bypass for path-based payloads submitted via specially crafted request URLs. ModSecurity v3 decodes percent-encoded characters present in request URLs before it separates the URL path component from the optional query string component. This results in an impedance mismatch versus RFC compliant back-end applications. The vulnerability hides an attack payload in the path component of the URL from WAF rules inspecting it. A back-end may be vulnerable if it uses the path component of request URLs to construct queries. Integrators and users are advised to upgrade to 3.0.12. The ModSecurity v2 release line is not affected by this vulnerability.
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Analysis
by VulDB Data Team • 07/04/2025
This vulnerability in ModSecurity and libModSecurity versions 3.0.0 through 3.0.11 represents a significant WAF bypass issue that exploits a fundamental mismatch between how the web application firewall processes URL components and how backend applications expect to receive data. The flaw occurs during the URL parsing phase where ModSecurity performs percent-decoding of URL-encoded characters before separating the path component from the query string component. This behavior creates an impedance mismatch with RFC-compliant backend applications that expect to receive properly encoded data in their path components, allowing malicious payloads to be concealed within the path portion of URLs where WAF rules may not inspect them thoroughly.
The technical implementation of this vulnerability stems from the specific processing order within ModSecurity v3's URL handling mechanism. When a request arrives with percent-encoded characters in the URL path, the system first decodes these characters during the parsing phase before the URL is split into its constituent parts. This decoding process effectively transforms what should remain as encoded data into its decoded form, thereby masking potential attack vectors that would otherwise be detected by WAF rules. The vulnerability specifically targets the path component of URLs, which is particularly concerning because many backend applications construct database queries or perform other security-sensitive operations using data extracted from the URL path, making this bypass potentially devastating for applications that rely on path-based input validation.
The operational impact of this vulnerability extends beyond simple WAF bypass capabilities and represents a serious security risk for organizations relying on ModSecurity v3 for their web application protection. Attackers can craft specially formatted URLs that contain malicious payloads in the path component, which will be decoded by ModSecurity but not properly inspected by WAF rules that may only examine the raw encoded URL or focus primarily on query parameters. This creates a scenario where a payload designed to exploit SQL injection, command injection, or other vulnerabilities can evade detection and reach the backend application unimpeded. The vulnerability is particularly dangerous because it leverages the legitimate functionality of URL decoding while creating an unexpected behavior that bypasses security controls, making detection and prevention more challenging for security teams.
The recommended mitigation strategy involves upgrading to ModSecurity version 3.0.12 or later, which addresses this specific impedance mismatch by ensuring consistent handling of percent-encoded characters throughout the URL processing pipeline. This upgrade resolves the core issue by maintaining the encoded nature of URL components during the parsing phase, thereby preventing the bypass condition that allowed malicious payloads to be hidden in the path component. Organizations should also conduct thorough testing of their existing WAF rules to ensure they properly handle both encoded and decoded URL components, as the upgrade may change how certain rules operate. The vulnerability does not affect ModSecurity v2 releases, indicating that this specific issue was introduced in the v3 architecture and represents a regression or design flaw that required targeted remediation.
This vulnerability aligns with CWE-1170, which addresses the improper handling of encoded characters in web applications, and demonstrates characteristics consistent with ATT&CK technique T1071.004 for application layer protocol manipulation. The issue reflects broader challenges in web security where the interaction between different security layers can create unexpected gaps in protection, particularly when frontend security controls do not account for the specific processing behavior of backend systems. Organizations should implement additional monitoring and logging of URL path components to detect potential exploitation attempts, as well as review their application logic to ensure that path-based inputs are properly sanitized and validated regardless of WAF protection status.