CVE-2017-12933 in PHP
Summary
by MITRE
The finish_nested_data function in ext/standard/var_unserializer.re in PHP before 5.6.31, 7.0.x before 7.0.21, and 7.1.x before 7.1.7 is prone to a buffer over-read while unserializing untrusted data. 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-2017-12933 represents a critical buffer over-read flaw within PHP's serialization unmarshaling process that affects multiple versions of the PHP runtime environment. This issue resides in the finish_nested_data function located within the ext/standard/var_unserializer.re file, which is responsible for processing serialized data structures during deserialization operations. The flaw manifests when PHP processes untrusted serialized input, creating a scenario where the application reads beyond the bounds of allocated memory buffers. This type of vulnerability falls under the category of memory safety issues and is classified as CWE-125 in the Common Weakness Enumeration catalog, specifically addressing out-of-bounds read conditions that can lead to information disclosure, application crashes, or potentially arbitrary code execution depending on the exploitation context.
The technical implementation of this vulnerability exploits the way PHP handles nested data structures during unserialization, particularly when dealing with malformed or maliciously crafted serialized data. During the deserialization process, the finish_nested_data function fails to properly validate the boundaries of memory allocations when processing nested arrays or objects, allowing an attacker to craft serialized payloads that cause the parser to read memory locations beyond the intended data boundaries. This over-read condition can expose sensitive information from adjacent memory regions, potentially revealing stack contents, heap metadata, or other confidential data that may aid in further exploitation attempts. The vulnerability is particularly concerning because it operates at the core serialization layer of PHP, making it applicable to any application that processes untrusted serialized data, including user inputs, session data, or database records containing serialized objects.
The operational impact of CVE-2017-12933 extends beyond simple information disclosure, as it can potentially enable more sophisticated attack vectors within the context of the broader ATT&CK framework. While the immediate effect may appear limited to memory over-read conditions, attackers can leverage this vulnerability as part of a multi-stage attack approach to gather intelligence about the target system's memory layout, which can then be used to refine subsequent exploitation techniques. The vulnerability's presence in PHP's core serialization functionality means that any application utilizing PHP's unserialize() function with untrusted input is at risk, creating widespread potential for exploitation across web applications, content management systems, and framework-based applications that rely on PHP's native serialization mechanisms. The unspecified impact mentioned in the original description reflects the potential for various outcomes including application instability, denial of service conditions, or even remote code execution depending on the specific memory corruption patterns and the target environment's security mitigations.
Mitigation strategies for CVE-2017-12933 primarily focus on immediate version upgrades to patched PHP releases, specifically PHP 5.6.31, 7.0.21, or 7.1.7, which contain the necessary code fixes to prevent the buffer over-read condition. Organizations should also implement defensive programming practices such as avoiding direct unserialization of user-supplied data, implementing strict input validation, and utilizing alternative serialization formats that do not rely on PHP's native unserialize() function. Additional protective measures include employing input sanitization techniques, using application firewalls to filter suspicious serialized data patterns, and implementing proper error handling to prevent information leakage during deserialization failures. Security monitoring should be enhanced to detect unusual patterns in serialization operations that might indicate exploitation attempts, while regular security audits should verify that no applications are using deprecated or vulnerable PHP versions that may still be exposed to this or related vulnerabilities. The remediation process should also include comprehensive testing of serialized data handling components to ensure that the patched versions function correctly without introducing regressions in application behavior.