CVE-2015-6831 in PHP
Summary
by MITRE
Multiple use-after-free vulnerabilities in SPL in PHP before 5.4.44, 5.5.x before 5.5.28, and 5.6.x before 5.6.12 allow remote attackers to execute arbitrary code via vectors involving (1) ArrayObject, (2) SplObjectStorage, and (3) SplDoublyLinkedList, which are mishandled during unserialization.
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Analysis
by VulDB Data Team • 07/03/2022
The CVE-2015-6831 vulnerability represents a critical security flaw in PHP's Standard PHP Library (SPL) that affects multiple PHP versions including 5.4.x before 5.4.44, 5.5.x before 5.5.28, and 5.6.x before 5.6.12. This vulnerability falls under the CWE-416 category, which specifically addresses use-after-free conditions in software systems. The flaw manifests during the unserialization process of certain SPL data structures, creating exploitable conditions that can be leveraged by remote attackers to execute arbitrary code on affected systems. The vulnerability is particularly concerning because it affects fundamental data structures that are widely used in PHP applications, making the attack surface exceptionally broad across the PHP ecosystem.
The technical implementation of this vulnerability stems from improper memory management within PHP's SPL components during object deserialization. When PHP processes serialized data containing ArrayObject, SplObjectStorage, or SplDoublyLinkedList objects, the unserialization process fails to properly handle memory cleanup operations. This leads to situations where freed memory blocks are accessed or reused, creating opportunities for attackers to manipulate the program's execution flow. The vulnerability specifically targets the way these objects manage their internal memory structures when they are being reconstructed from serialized representations, allowing attackers to control the memory layout and potentially inject malicious code into the execution context.
The operational impact of CVE-2015-6831 extends far beyond individual application compromises, as it affects the foundational security of PHP-based web applications that rely on SPL data structures. Attackers can exploit this vulnerability to execute arbitrary code remotely, potentially leading to complete system compromise, data exfiltration, or server takeover. The vulnerability's classification under the ATT&CK framework as a code injection technique means that successful exploitation can enable attackers to perform various malicious activities including privilege escalation, persistence mechanisms, and lateral movement within compromised networks. Organizations running vulnerable PHP versions face significant risk exposure, particularly those with web applications that accept user input through serialized data structures, as these represent common attack vectors in modern web applications.
Mitigation strategies for CVE-2015-6831 primarily focus on immediate version upgrades to patched PHP releases that address the memory management issues in SPL components. System administrators should prioritize updating their PHP installations to versions 5.4.44, 5.5.28, or 5.6.12 respectively, depending on their current PHP version. Additionally, implementing proper input validation and sanitization measures can help reduce the attack surface by preventing malicious serialized data from reaching the unserialization process. Organizations should also consider implementing network segmentation and monitoring solutions to detect potential exploitation attempts. The vulnerability's characteristics make it particularly suitable for exploitation through web-based attack vectors, making web application firewalls and intrusion detection systems valuable defensive measures. Regular security assessments and penetration testing should be conducted to ensure that all PHP applications are properly patched and that no legacy code remains vulnerable to this class of use-after-free exploitation.