CVE-2011-0420 in PHP
Summary
by MITRE
The grapheme_extract function in the Internationalization extension (Intl) for ICU for PHP 5.3.5 allows context-dependent attackers to cause a denial of service (crash) via an invalid size argument, which triggers a NULL pointer dereference.
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Analysis
by VulDB Data Team • 01/19/2025
The vulnerability identified as CVE-2011-0420 resides within the Internationalization extension of PHP, specifically within the ICU (International Components for Unicode) library implementation. This flaw affects PHP versions up to and including 5.3.5, making it a significant concern for web applications that rely on international text processing capabilities. The issue manifests through the grapheme_extract function, which is designed to handle Unicode text segmentation and character extraction operations. When this function receives an invalid size argument during processing, it fails to properly validate input parameters, leading to a critical system failure.
The technical root cause of this vulnerability can be classified as a NULL pointer dereference, a condition that occurs when a program attempts to access memory through a pointer that has not been properly initialized or has been set to NULL. This particular flaw represents a classic software bug pattern that falls under CWE-476, which specifically addresses NULL pointer dereferences in software implementations. The vulnerability is context-dependent, meaning that attackers must carefully construct malicious input to trigger the specific conditions that lead to the crash. The grapheme_extract function's failure to validate the size argument parameter creates an exploitable path where malformed input can cause the PHP interpreter to crash and terminate unexpectedly.
From an operational perspective, this vulnerability presents a substantial denial of service risk to web applications running affected PHP versions. When exploited, the vulnerability causes the web server process to crash, resulting in immediate service unavailability for users attempting to access the affected application. The impact extends beyond simple service interruption as it can potentially be leveraged in distributed denial of service attacks where multiple instances of the vulnerability are exploited across different targets. This type of vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a fundamental weakness in input validation that can compromise system availability. The vulnerability is particularly concerning because it affects core internationalization functionality that many web applications depend upon for handling multilingual content, making it a critical target for attackers seeking to disrupt services.
The mitigation strategies for this vulnerability primarily focus on immediate patching and updating of affected PHP installations to versions that contain the necessary fixes. System administrators should prioritize upgrading to PHP 5.3.6 or later, which includes the corrected implementation of the grapheme_extract function. Additionally, implementing input validation controls at the application level can provide an additional layer of defense, though this approach is less reliable than the official patch. Organizations should also consider deploying intrusion detection systems that can monitor for suspicious patterns of requests that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper input validation and error handling in internationalization libraries, highlighting how seemingly minor implementation flaws can result in significant security implications. Security teams should conduct comprehensive testing of their PHP applications to identify any other potential uses of the ICU functions that might be vulnerable to similar issues, ensuring that all affected components receive appropriate updates and patches to maintain system integrity and availability.