CVE-2004-0595 in PHP
Summary
by MITRE
The strip_tags function in PHP 4.x up to 4.3.7, and 5.x up to 5.0.0RC3, does not filter null (\0) characters within tag names when restricting input to allowed tags, which allows dangerous tags to be processed by web browsers such as Internet Explorer and Safari, which ignore null characters and facilitate the exploitation of cross-site scripting (XSS) vulnerabilities.
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Analysis
by VulDB Data Team • 02/01/2025
The strip_tags function in php versions 4.x up to 4.3.7 and 5.x up to 5.0.0rc3 contained a critical security flaw that fundamentally undermined input sanitization mechanisms designed to prevent cross site scripting attacks. this vulnerability arose from the function's improper handling of null characters within html tag names during the filtering process. when developers used strip_tags to remove potentially dangerous html content from user inputs, the function failed to properly sanitize null bytes embedded within tag names, creating a pathway for malicious code execution.
the technical flaw specifically manifested when php encountered html tags containing null characters within their names such as <scriptsrc="malicious.js"> or <imgonload="alert('xss')">. the strip_tags function would process these inputs and remove the outer tag structure while leaving the null character intact, resulting in malformed html that browsers like internet explorer and safari would interpret differently than intended. these browsers would effectively ignore the null characters during parsing, allowing the malicious code to execute within the context of the web application. this behavior directly violated the expected security properties of input filtering and created a vector for persistent cross site scripting attacks.
the operational impact of this vulnerability was significant for web applications relying on php's strip_tags function for security purposes. attackers could exploit this weakness to inject malicious javascript code that would bypass standard input sanitization measures, potentially leading to session hijacking, data theft, or full compromise of user accounts. the vulnerability was particularly dangerous because it affected widely deployed php versions that were commonly used in web applications, making it a prime target for automated exploitation. organizations running vulnerable php installations faced increased risk of successful cross site scripting attacks, with the potential for widespread impact across their user bases.
the security implications extend beyond simple input filtering failures and align with common weakness enumeration cwe-79 which describes cross site scripting vulnerabilities. this vulnerability also maps to attack technique t1059 in the attack framework, specifically targeting command and scripting interpreters where malicious code execution occurs through browser-based vectors. mitigation strategies required immediate patching of affected php versions, with administrators implementing additional input validation layers and considering alternative sanitization approaches. the vulnerability highlighted the importance of thorough testing for edge cases in security functions and demonstrated how seemingly minor implementation flaws could create significant security risks in widely used software components.
organizations affected by this vulnerability should have implemented comprehensive security assessments to identify applications using vulnerable php versions, while also reviewing their input handling processes for similar edge case issues. the incident underscored the critical need for security-conscious development practices and thorough testing of security-relevant functions. proper patch management and regular security updates became essential practices for preventing similar vulnerabilities from compromising web application security. the vulnerability also emphasized the importance of understanding how different browsers handle malformed html and null character processing, which became a crucial consideration for web security professionals developing robust input sanitization strategies.