CVE-2009-4142 in PHP
Summary
by MITRE
The htmlspecialchars function in PHP before 5.2.12 does not properly handle (1) overlong UTF-8 sequences, (2) invalid Shift_JIS sequences, and (3) invalid EUC-JP sequences, which allows remote attackers to conduct cross-site scripting (XSS) attacks by placing a crafted byte sequence before a special character.
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Analysis
by VulDB Data Team • 02/01/2025
The vulnerability described in CVE-2009-4142 represents a critical flaw in PHP's htmlspecialchars function that existed in versions prior to 5.2.12. This issue specifically targets the function's inability to properly sanitize certain UTF-8 and multibyte character sequences, creating a pathway for malicious actors to bypass security controls designed to prevent cross-site scripting attacks. The vulnerability stems from insufficient validation of character encoding sequences, particularly when processing user-supplied input that contains overlong UTF-8 sequences, invalid Shift_JIS sequences, and invalid EUC-JP sequences. These encoding anomalies occur when malformed byte sequences are processed without proper validation, allowing attackers to craft inputs that can evade standard sanitization measures.
The technical implementation of this vulnerability exploits the way PHP's htmlspecialchars function processes multibyte character encodings. When encountering overlong UTF-8 sequences, the function fails to properly normalize these sequences before applying HTML entity encoding, which can result in the bypass of security checks. Invalid Shift_JIS and EUC-JP sequences similarly create opportunities for attackers to inject malicious content that appears legitimate to the sanitization function but can be interpreted differently by web browsers. The specific attack vector involves placing crafted byte sequences before special characters such as angle brackets, quotes, or other HTML metacharacters that would normally be escaped during the sanitization process. This allows attackers to inject script tags or other malicious content that can execute in the context of other users' browsers.
The operational impact of CVE-2009-4142 is significant, as it directly enables persistent cross-site scripting attacks that can compromise user sessions, steal sensitive information, or redirect users to malicious websites. Attackers can leverage this vulnerability to execute arbitrary JavaScript code in victims' browsers, potentially leading to session hijacking, data theft, or full compromise of web applications. The vulnerability is particularly dangerous because it operates at the input sanitization layer, meaning that even applications that properly implement other security measures can be compromised if they rely on htmlspecialchars for XSS protection. The attack requires no special privileges and can be executed through standard web application interfaces, making it a widespread threat to PHP-based web applications.
Mitigation strategies for this vulnerability center on immediate software updates to PHP versions 5.2.12 or later, which contain the necessary fixes for proper UTF-8 sequence handling. Organizations should also implement additional input validation layers and consider using alternative sanitization functions that provide more robust handling of multibyte character sequences. Security monitoring should focus on detecting unusual character sequences in user input, particularly those that might indicate attempts to exploit this vulnerability. The flaw aligns with CWE-116, which addresses improper encoding of data, and maps to ATT&CK technique T1059.007 for script injection, specifically targeting web application vulnerabilities. Organizations should also review their input sanitization practices and ensure that all character encoding validation occurs before any HTML entity encoding is applied to prevent similar issues in other components of their web applications.