CVE-2015-6834 in Mac OS X
Summary
by MITRE
Multiple use-after-free vulnerabilities in PHP before 5.4.45, 5.5.x before 5.5.29, and 5.6.x before 5.6.13 allow remote attackers to execute arbitrary code via vectors related to (1) the Serializable interface, (2) the SplObjectStorage class, and (3) the SplDoublyLinkedList class, which are mishandled during unserialization.
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Analysis
by VulDB Data Team • 01/05/2025
The vulnerability CVE-2015-6834 represents a critical security flaw in PHP versions prior to specific patch releases, affecting the serialization and unserialization processes within the language. This vulnerability manifests as multiple use-after-free conditions that can be exploited by remote attackers to execute arbitrary code on affected systems. The flaw specifically impacts PHP versions before 5.4.45, 5.5.x before 5.5.29, and 5.6.x before 5.6.13, making it a widespread concern across multiple PHP release lines. The vulnerability is categorized under CWE-416 as use-after-free conditions, which occur when a program continues to use a pointer after the memory it points to has been freed, creating potential for memory corruption and code execution.
The technical implementation of this vulnerability involves three distinct classes that handle serialization operations in PHP: the Serializable interface, the SplObjectStorage class, and the SplDoublyLinkedList class. During the unserialization process, these components fail to properly manage memory references, leading to situations where freed memory blocks are accessed after being reallocated. When an attacker crafts malicious serialized data containing these specific class references, the unserialization routine executes code that leverages the freed memory to overwrite critical program structures. This memory corruption can be manipulated to redirect program execution flow, ultimately allowing arbitrary code execution with the privileges of the PHP process. The vulnerability is particularly dangerous because it operates during the deserialization phase, which is a common operation in web applications that process user input or data from external sources.
The operational impact of CVE-2015-6834 extends beyond simple code execution, as it can enable attackers to establish persistent access to affected systems. When exploited successfully, this vulnerability allows remote code execution that can be used to install backdoors, escalate privileges, or compromise entire web applications. The attack vector requires an attacker to send specially crafted serialized data to a PHP application that processes user input through unserialization functions. This scenario commonly occurs in web applications that accept serialized data from users, such as session data, configuration settings, or API responses. The vulnerability can be particularly devastating in environments where PHP applications handle sensitive data or where the web server has elevated privileges. Security researchers have classified this vulnerability as high-risk due to its remote exploitability and the potential for privilege escalation.
Mitigation strategies for CVE-2015-6834 primarily focus on immediate patching of affected PHP installations. System administrators should upgrade to PHP versions 5.4.45, 5.5.29, or 5.6.13 respectively, which contain the necessary fixes for the use-after-free conditions. Additionally, organizations should implement input validation and sanitization measures to prevent untrusted data from reaching unserialization functions. The principle of least privilege should be enforced by ensuring that PHP applications run with minimal required permissions. Security monitoring should include detection of suspicious serialized data patterns and anomalous behavior in applications that process user input. Organizations should also consider implementing web application firewalls and intrusion detection systems to help identify and block exploitation attempts. This vulnerability demonstrates the importance of secure coding practices in serialization routines and highlights the need for regular security updates and vulnerability assessments in PHP environments. The ATT&CK framework categorizes this vulnerability under T1059 for command and scripting interpreter and T1078 for valid accounts, as exploitation typically involves executing code through legitimate PHP processes and potentially leveraging compromised accounts.