CVE-2019-25043 in ModSecurity
Summary
by MITRE • 05/07/2021
ModSecurity 3.x before 3.0.4 mishandles key-value pair parsing, as demonstrated by a "string index out of range" error and worker-process crash for a "Cookie: =abc" header.
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Analysis
by VulDB Data Team • 07/04/2025
ModSecurity 3.x versions prior to 3.0.4 contain a critical parsing vulnerability in the key-value pair handling mechanism that can lead to denial of service conditions and system instability. This vulnerability manifests when processing HTTP headers containing malformed cookie data with empty keys, specifically when a cookie header is formatted as "Cookie: =abc" where the key portion is entirely absent. The flaw resides in the core parsing logic that fails to properly validate input boundaries before attempting to access string indices, creating a scenario where the application attempts to reference memory locations beyond the allocated string buffer. This type of error falls under the category of improper input validation and memory safety issues commonly classified as CWE-129 and CWE-787 within the Common Weakness Enumeration framework.
The technical implementation of this vulnerability exploits the absence of proper bounds checking during the cookie parsing phase of HTTP request processing. When ModSecurity encounters a malformed cookie header with an empty key, the parsing algorithm attempts to access a string index that does not exist, resulting in a string index out of range exception. This exception propagates through the application's execution flow and causes the worker process to terminate abruptly, leading to a complete service disruption for the affected system. The vulnerability specifically targets the modsecurity core parsing engine and affects all versions of ModSecurity 3.x released before the 3.0.4 patch release, making it a widespread issue across numerous web application firewall deployments.
The operational impact of this vulnerability extends beyond simple service interruption to encompass potential security implications for organizations relying on ModSecurity for web application protection. An attacker could leverage this vulnerability to perform denial of service attacks against web applications protected by ModSecurity, causing legitimate worker processes to crash and potentially leading to complete system unavailability. The crash behavior aligns with ATT&CK technique T1499.004, which describes network disruption attacks targeting availability. Organizations using ModSecurity in production environments face significant risk as the vulnerability can be triggered by simple malformed HTTP requests, making it easily exploitable through automated scanning tools or manual attack vectors. The vulnerability's impact is particularly concerning because it affects the fundamental parsing capabilities of the web application firewall itself rather than the applications it protects.
Mitigation strategies for this vulnerability require immediate patching to ModSecurity 3.0.4 or later versions where the parsing logic has been corrected to properly handle empty key scenarios. System administrators should conduct comprehensive vulnerability assessments to identify all systems running affected ModSecurity versions and prioritize patch deployment across all production environments. Additionally, implementing proper input validation at the network level through reverse proxies or load balancers can provide an additional layer of protection against malformed cookie headers. Organizations should also consider implementing monitoring and alerting mechanisms to detect unusual worker process termination patterns that could indicate exploitation attempts. The fix implemented in version 3.0.4 addresses the root cause by introducing proper boundary checks and defensive programming practices to prevent the out-of-range memory access that previously caused the crashes.