CVE-2026-52747 in ModSecurity
Summary
by MITRE • 07/11/2026
ModSecurity is an open source, cross platform web application firewall (WAF) engine for Apache, IIS and Nginx. Prior to 3.0.16, the multipart/form-data request body parser in libmodsecurity silently removes embedded line breaks from non-file form-field values before exporting them to ARGS and ARGS_POST because src/request_body_processor/multipart.cc overwrites reserved bytes in m_reserve instead of appending the current buffer. This creates a parser differential between ModSecurity and backend applications that preserve line breaks in form fields, allowing rules that inspect ARGS or ARGS_POST to miss payloads whose dangerous syntax depends on a line break. This issue is fixed in version 3.0.16.
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Analysis
by VulDB Data Team • 07/11/2026
The vulnerability in ModSecurity affects the multipart/form-data request body parsing functionality within libmodsecurity versions prior to 3.0.16, creating a critical parser differential that undermines the security posture of web applications protected by this web application firewall. This issue specifically impacts how non-file form-field values are processed when line breaks are present in the submitted data. The technical flaw resides in the source file src/request_body_processor/multipart.cc where the code incorrectly overwrites reserved bytes in m_reserve instead of properly appending to the current buffer, leading to silent removal of embedded line breaks from form field values before they are made available to ModSecurity's ARGS and ARGS_POST variables.
The operational impact of this vulnerability is significant as it creates a scenario where ModSecurity's rule engine operates on sanitized data that differs from what backend applications receive, effectively allowing malicious payloads to bypass detection mechanisms. When attackers craft payloads containing line breaks in form fields, the WAF removes these characters during parsing while the backend application preserves them, creating a mismatch that can be exploited to evade security rules designed to detect dangerous syntax patterns. This parser differential represents a fundamental inconsistency between ModSecurity's processing behavior and standard web application expectations, potentially enabling bypass of critical security controls such as SQL injection detection, cross-site scripting protection, and other rule-based inspection mechanisms that rely on complete payload analysis.
This vulnerability maps directly to CWE-1287, which addresses issues related to parsing inconsistencies in security tools, and aligns with ATT&CK technique T1071.004 for application layer protocol manipulation where attackers exploit parsing differences to evade detection. The issue demonstrates how seemingly minor implementation details in security tooling can create substantial gaps in protection, particularly when the WAF's internal processing differs from backend application behavior. Organizations using ModSecurity versions prior to 3.0.16 face increased risk of attack bypasses where malicious inputs containing line breaks in form fields could successfully reach backend applications while appearing benign to ModSecurity's rule inspection processes.
The fix implemented in version 3.0.16 corrects the buffer handling logic by ensuring that non-file form-field values maintain their original structure including embedded line breaks when exported to ARGS and ARGS_POST variables. This correction aligns ModSecurity's parsing behavior with industry standards for web application processing and ensures that security rules operate on complete, accurate representations of submitted data. Organizations should prioritize upgrading to version 3.0.16 or later to eliminate this parser differential and restore proper security coverage for form-based inputs containing line breaks, as the vulnerability creates a persistent attack surface that could be exploited by threat actors seeking to evade WAF protections through carefully crafted payloads that exploit the inconsistent handling of whitespace characters in multipart form data.