CVE-2017-12934 in PHP
Summary
by MITRE
ext/standard/var_unserializer.re in PHP 7.0.x before 7.0.21 and 7.1.x before 7.1.7 is prone to a heap use after free while unserializing untrusted data, related to the zval_get_type function in Zend/zend_types.h. Exploitation of this issue can have an unspecified impact on the integrity of PHP.
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Analysis
by VulDB Data Team • 12/16/2022
The vulnerability identified as CVE-2017-12934 represents a critical heap use after free condition within PHP's serialization unserialization mechanism, specifically affecting versions prior to 7.0.21 and 7.1.7. This flaw resides in the ext/standard/var_unserializer.re component which handles the deserialization of untrusted data streams, making it particularly dangerous in web applications that process user input through PHP's unserialize function. The vulnerability is directly linked to the zval_get_type function located in Zend/zend_types.h, which manages the internal representation of PHP variables during runtime operations.
The technical implementation of this vulnerability exploits the improper handling of memory management during the unserialization process. When PHP encounters serialized data containing malicious payloads, the var_unserializer component fails to properly validate the structure of the serialized data before attempting to free memory associated with zval structures. This improper memory management allows attackers to manipulate the heap memory layout, potentially leading to arbitrary code execution or denial of service conditions. The heap use after free condition occurs because the system attempts to access memory that has already been freed, creating opportunities for memory corruption that can be leveraged by malicious actors.
The operational impact of this vulnerability extends beyond simple exploitation scenarios, as it fundamentally compromises the integrity of PHP applications that rely on unserialize functions for processing external data. Applications processing user input, API responses, or session data through PHP's unserialization mechanisms become vulnerable to remote code execution attacks when exposed to maliciously crafted serialized objects. The unspecified impact mentioned in the CVE description reflects the potential for various outcomes including complete system compromise, data exfiltration, or service disruption depending on the specific attack vector and target environment. This vulnerability particularly affects web applications where user-controllable data is processed through PHP's serialization functions, making it a significant concern for applications handling any form of external data input.
Organizations should immediately implement mitigation strategies focusing on version updates to PHP 7.0.21 or 7.1.7 and later releases where this vulnerability has been addressed. Additionally, developers should avoid using unserialize functions with untrusted input data and implement proper input validation and sanitization measures. The vulnerability aligns with CWE-416, which describes the use of freed memory condition, and can be mapped to ATT&CK technique T1059.007 for command and scripting interpreter usage. Security practitioners should also consider implementing network segmentation, input validation firewalls, and regular security assessments to prevent exploitation of this and similar vulnerabilities in PHP applications.