CVE-2011-5021 in PHPIDS
Summary
by MITRE
PHPIDS before 0.7 does not properly implement Regular Expression Denial of Service (ReDoS) filters, which allows remote attackers to bypass rulesets and add PHP sequences to a file via unspecified vectors.
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Analysis
by VulDB Data Team • 02/13/2019
The vulnerability identified as CVE-2011-5021 represents a critical weakness in the PHPIDS (PHP Intrusion Detection System) framework version 0.7 and earlier. This flaw resides in the system's implementation of Regular Expression Denial of Service (ReDoS) protection mechanisms, which are fundamental to preventing malicious input patterns from overwhelming the intrusion detection system. The vulnerability specifically affects the filtering logic that should prevent attackers from crafting malicious input that could bypass security rulesets and potentially inject harmful PHP sequences into files. This issue creates a dangerous scenario where the very protection mechanisms designed to defend against attacks become ineffective, allowing adversaries to exploit the system's weaknesses.
The technical implementation flaw stems from improper handling of regular expression patterns within the PHPIDS framework's rule evaluation system. When processing incoming data, the system fails to adequately sanitize or validate regular expression inputs that could be used to craft malicious payloads. This improper implementation creates a condition where attackers can submit specially crafted input that either causes the regular expression engine to consume excessive computational resources or bypasses the intended filtering logic entirely. The vulnerability operates through unspecified vectors, meaning the exact attack pathways are not clearly defined but likely involve manipulation of input parameters that get processed through the vulnerable regex patterns. This weakness aligns with CWE-1321, which specifically addresses improper handling of regular expressions in security systems.
The operational impact of CVE-2011-5021 extends beyond simple bypass of security rulesets, creating potential for more severe consequences in web application environments. Attackers who successfully exploit this vulnerability can potentially inject malicious PHP code into files, which could lead to complete system compromise, data exfiltration, or unauthorized access to sensitive information. The vulnerability undermines the core security posture of applications relying on PHPIDS for protection, as it allows attackers to circumvent the very mechanisms designed to detect and prevent malicious activity. This creates a false sense of security for system administrators and developers who depend on the framework to protect against common web attacks such as SQL injection, cross-site scripting, and other malicious input patterns. The vulnerability also aligns with ATT&CK technique T1059.007, which covers the use of scripting languages to execute malicious code.
Mitigation strategies for CVE-2011-5021 require immediate attention and involve multiple layers of defensive measures. The primary recommendation is to upgrade to PHPIDS version 0.7 or later, where the ReDoS filtering mechanisms have been properly implemented to prevent the bypass conditions. Organizations should also implement additional input validation measures beyond the framework's built-in protections, including thorough sanitization of all user inputs and monitoring for anomalous patterns that could indicate exploitation attempts. Network-level protections such as web application firewalls can provide additional defense in depth, while regular security audits should verify that no malicious code has been injected through this vulnerability. The remediation process should include comprehensive testing of the updated system to ensure that legitimate security rules continue to function properly while eliminating the bypass capabilities that the vulnerability provided.