CVE-2015-2787 in PHP
Summary
by MITRE
Use-after-free vulnerability in the process_nested_data function in ext/standard/var_unserializer.re in PHP before 5.4.39, 5.5.x before 5.5.23, and 5.6.x before 5.6.7 allows remote attackers to execute arbitrary code via a crafted unserialize call that leverages use of the unset function within an __wakeup function, a related issue to CVE-2015-0231.
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Analysis
by VulDB Data Team • 05/02/2022
The CVE-2015-2787 vulnerability represents a critical use-after-free flaw in PHP's serialization and deserialization mechanisms that has significant implications for web application security. This vulnerability specifically affects PHP versions prior to 5.4.39, 5.5.23, and 5.6.7, making it a widespread concern across multiple PHP version lines. The flaw exists within the process_nested_data function in the ext/standard/var_unserializer.re file, which handles the deserialization of serialized data structures. The vulnerability is particularly dangerous because it allows remote attackers to execute arbitrary code through carefully crafted unserialize calls, effectively bypassing normal security boundaries and potentially leading to complete system compromise.
The technical exploitation of this vulnerability relies on a specific sequence involving the unset function within an __wakeup magic method. When PHP deserializes an object, it first calls the __wakeup method if one exists, and if this method contains calls to unset on object properties, it can create a scenario where freed memory is accessed after the object has been partially destroyed. This creates a use-after-free condition where an attacker can manipulate the memory layout to inject and execute malicious code. The vulnerability is classified under CWE-416 as a Use After Free condition, which occurs when a program continues to use a pointer after the memory it points to has been freed. The attack vector is particularly insidious because it leverages the legitimate PHP serialization functionality that is commonly used in web applications for session management, data persistence, and inter-component communication.
The operational impact of CVE-2015-2787 extends beyond simple code execution, as it can lead to complete system compromise when exploited in real-world scenarios. Web applications that deserialize user-supplied data without proper validation are particularly vulnerable, including those using PHP sessions, API endpoints that accept serialized data, or any application that relies on PHP's unserialize function with untrusted input. Attackers can craft serialized objects that, when processed by the vulnerable PHP version, will trigger the use-after-free condition and execute arbitrary commands with the privileges of the web server process. This vulnerability also relates to the broader category of deserialization attacks that fall under the ATT&CK framework's T1203 - Exploitation for Client Execution and T1059 - Command and Scripting Interpreter techniques, where attackers leverage application flaws to execute malicious code.
The mitigation strategies for this vulnerability involve immediate patching of affected PHP versions to the recommended secure releases, which address the underlying use-after-free condition in the serialization code. Organizations should also implement strict input validation and sanitization for any data that will be passed to unserialize functions, particularly when dealing with user-supplied data. Additional protective measures include disabling the unserialize function entirely when possible, implementing proper access controls and input filtering, and monitoring application logs for suspicious deserialization patterns. The vulnerability serves as a reminder of the critical importance of secure coding practices in serialization and deserialization operations, and the need for regular security updates and vulnerability assessments in PHP environments.