CVE-2015-4602 in PHP
Summary
by MITRE
The __PHP_Incomplete_Class function in ext/standard/incomplete_class.c in PHP before 5.4.40, 5.5.x before 5.5.24, and 5.6.x before 5.6.8 allows remote attackers to cause a denial of service (application crash) or possibly execute arbitrary code via an unexpected data type, related to a "type confusion" issue.
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Analysis
by VulDB Data Team • 05/22/2022
The CVE-2015-4602 vulnerability represents a critical type confusion flaw within PHP's serialization handling mechanism that affects multiple versions of the PHP runtime environment. This vulnerability specifically targets the __PHP_Incomplete_Class function located in the ext/standard/incomplete_class.c file, which is responsible for managing incomplete class objects during the unserialization process. The flaw arises when PHP encounters unexpected data types during object deserialization, creating a scenario where the application's memory management becomes compromised due to improper type handling. The vulnerability exists across PHP versions 5.4.40 and earlier, 5.5.24 and earlier, and 5.6.8 and earlier, making it a widespread issue affecting a significant portion of web applications built on PHP platforms.
The technical exploitation of this vulnerability stems from a type confusion issue that occurs when PHP's unserialization process encounters malformed data structures. During the deserialization of objects, PHP expects specific data types to maintain object integrity and memory safety. However, when unexpected data types are introduced, particularly those that do not match the expected class structure, the __PHP_Incomplete_Class function fails to properly validate the incoming data. This type confusion allows attackers to manipulate the memory layout of objects in ways that were not anticipated by the original implementation. The vulnerability can be triggered through crafted serialized data that, when processed by PHP's unserialize function, causes the interpreter to attempt operations on data that it believes to be of one type while it is actually of another type. This fundamental mismatch in type expectations creates a condition where memory corruption can occur, leading to either application crashes or potentially more severe consequences including arbitrary code execution.
The operational impact of CVE-2015-4602 extends beyond simple denial of service scenarios, as the vulnerability can potentially enable remote code execution in certain conditions. When exploited successfully, attackers can cause PHP applications to crash, resulting in denial of service that affects legitimate users and can be used as part of broader attack campaigns. However, the more concerning aspect of this vulnerability is its potential for arbitrary code execution, which would allow attackers to execute malicious commands on the affected system with the privileges of the web server process. This makes the vulnerability particularly dangerous in web hosting environments where PHP applications are deployed, as successful exploitation could lead to complete system compromise. The vulnerability affects any PHP application that utilizes the unserialize function with user-controllable input, making it a critical concern for web applications that process serialized data from external sources such as cookies, session data, or API responses. The impact is further amplified because PHP's serialization mechanism is commonly used throughout web applications for data persistence and communication between different parts of an application or system components.
The vulnerability aligns with CWE-129, which addresses improper validation of array indices, and CWE-131, which deals with improper handling of buffer sizes, as both relate to memory safety issues in PHP's object handling. From an ATT&CK framework perspective, this vulnerability maps to T1203 - Exploitation for Client Execution and T1059 - Command and Scripting Interpreter, as it enables attackers to execute arbitrary commands through the compromised PHP application. Organizations should implement immediate mitigations including updating PHP to versions 5.4.40, 5.5.24, or 5.6.8 and later, depending on their current PHP version. Additional protective measures include implementing proper input validation for serialized data, avoiding the use of unserialize with untrusted data sources, and implementing web application firewalls to detect and block suspicious serialized data patterns. Security monitoring should focus on detecting unusual application behavior, memory access patterns, and potential exploitation attempts related to serialization functions. The vulnerability also highlights the importance of proper software supply chain security and regular patch management, as this issue affected PHP versions that were widely deployed across the internet. Organizations should also consider implementing runtime protections and application sandboxing to limit the potential impact of such vulnerabilities even if they are not immediately patched.