CVE-2010-3870 in PHP
Summary
by MITRE
The utf8_decode function in PHP before 5.3.4 does not properly handle non-shortest form UTF-8 encoding and ill-formed subsequences in UTF-8 data, which makes it easier for remote attackers to bypass cross-site scripting (XSS) and SQL injection protection mechanisms via a crafted string.
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Analysis
by VulDB Data Team • 01/04/2025
The vulnerability described in CVE-2010-3870 represents a critical flaw in PHP's handling of UTF-8 encoded data within the utf8_decode function. This issue affects PHP versions prior to 5.3.4 and stems from the function's inadequate validation of UTF-8 encoding sequences. The vulnerability specifically targets the function's inability to properly process non-shortest form UTF-8 encoding, where UTF-8 characters can be represented using multiple byte sequences, and ill-formed subsequences that do not conform to valid UTF-8 standards. Such improper handling creates exploitable conditions that can be leveraged by malicious actors to circumvent security controls designed to prevent cross-site scripting and SQL injection attacks.
The technical flaw in this vulnerability resides in the utf8_decode function's failure to correctly validate UTF-8 byte sequences according to RFC 3629 standards. When processing UTF-8 data, PHP's implementation does not adequately check for non-shortest form encodings, where characters that could be represented with fewer bytes are instead encoded using the maximum number of bytes allowed. Additionally, the function fails to properly handle ill-formed subsequences that contain invalid byte patterns, allowing malformed UTF-8 data to be processed without proper validation. This behavior creates a scenario where attackers can craft specially designed strings that appear valid to the application's security mechanisms but are actually processed in ways that bypass intended protections. The vulnerability operates at the input validation layer, where the function should normalize UTF-8 data but instead allows malformed sequences to pass through unfiltered.
The operational impact of CVE-2010-3870 extends beyond simple data processing errors to create significant security risks for web applications. Attackers can exploit this vulnerability to bypass XSS protection mechanisms by crafting UTF-8 encoded strings that, when processed by utf8_decode, result in different character representations than what security filters expect. This discrepancy allows malicious scripts to be embedded in seemingly harmless data, potentially leading to unauthorized code execution in user browsers. Similarly, the vulnerability enables SQL injection attacks by allowing attackers to construct database queries that bypass input sanitization checks. The attack vector is particularly dangerous because it can be exploited through various input points including form fields, URL parameters, and HTTP headers, making it difficult to fully protect against without proper patching or input validation measures. The vulnerability aligns with CWE-180, which describes "Incorrect Behavior Order" in input validation, and represents a classic case of improper encoding handling that violates fundamental security principles.
Organizations affected by this vulnerability should prioritize immediate patching of their PHP installations to version 5.3.4 or later, where the utf8_decode function properly handles UTF-8 encoding validation. Additional mitigations include implementing comprehensive input validation at multiple layers of the application architecture, including character encoding normalization before processing sensitive data, and employing strict content security policies to prevent XSS attacks. Security teams should also consider implementing web application firewalls that can detect and block malformed UTF-8 sequences, as well as conducting thorough security testing to identify potential exploitation points within their applications. The vulnerability demonstrates the critical importance of proper encoding validation in security-critical applications and highlights the need for adherence to established security standards such as those defined in the OWASP Top Ten and NIST guidelines for secure coding practices. This issue also relates to ATT&CK technique T1071.004, which covers application layer protocol manipulation, as attackers can exploit encoding vulnerabilities to manipulate application behavior and bypass security controls.