CVE-2016-9936 in PHP
Summary
by MITRE
The unserialize implementation in ext/standard/var.c in PHP 7.x before 7.0.14 allows remote attackers to cause a denial of service (use-after-free) or possibly have unspecified other impact via crafted serialized data. NOTE: this vulnerability exists because of an incomplete fix for CVE-2015-6834.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 10/27/2022
The vulnerability described in CVE-2016-9936 represents a critical use-after-free condition within PHP's serialization mechanism that affects versions 7.x before 7.0.14. This flaw specifically resides in the ext/standard/var.c file where the unserialize function processes crafted input data. The vulnerability emerged as an incomplete remediation for CVE-2015-6834, demonstrating how security fixes can sometimes introduce new attack vectors when not thoroughly validated. The core issue manifests when maliciously crafted serialized data is processed by PHP's unserialize function, creating conditions where memory locations are accessed after they have been freed, leading to unpredictable behavior and potential system compromise.
The technical implementation of this vulnerability exploits the improper handling of object references during the deserialization process. When PHP encounters serialized data containing specific object structures, the unserialize function fails to properly manage memory allocation and deallocation sequences, particularly in scenarios involving circular references or complex object hierarchies. This improper memory management creates a use-after-free condition where an attacker can manipulate the memory layout to execute arbitrary code or cause system instability. The flaw operates at the core level of PHP's object serialization engine, making it particularly dangerous as it can be triggered through any application that processes untrusted serialized input, including web applications, APIs, and backend services.
The operational impact of CVE-2016-9936 extends beyond simple denial of service to potentially enable remote code execution in certain configurations. Attackers can leverage this vulnerability to cause system crashes, memory corruption, or in some cases achieve arbitrary code execution by carefully crafting serialized data that manipulates the freed memory locations. The vulnerability's exploitation requires minimal privileges and can be delivered through various attack vectors including web requests, file uploads, or database inputs that are later deserialized. The risk is particularly elevated in environments where PHP applications process user-supplied data without proper sanitization or validation, making this vulnerability a prime target for automated exploitation tools and advanced persistent threats.
Organizations should prioritize immediate patching of affected PHP installations to address this vulnerability, as the incomplete fix for CVE-2015-6834 created a new attack surface that remains exploitable. Security teams should implement comprehensive monitoring for unusual memory access patterns and serialization-related errors in application logs. Additionally, defensive measures including input validation, sanitization of serialized data, and implementation of secure deserialization practices should be enforced throughout the application stack. The vulnerability aligns with CWE-416, which specifically addresses use-after-free conditions, and represents a significant risk under ATT&CK framework category T1203 for legitimate program execution and T1059 for command and scripting interpreter attacks. Organizations must also consider implementing application firewalls and runtime protection mechanisms to detect and prevent exploitation attempts targeting this specific memory corruption vulnerability.