CVE-2018-19396 in PHP
Summary
by MITRE
ext/standard/var_unserializer.c in PHP 5.x through 7.1.24 allows attackers to cause a denial of service (application crash) via an unserialize call for the com, dotnet, or variant class.
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Analysis
by VulDB Data Team • 06/11/2023
The vulnerability identified as CVE-2018-19396 represents a critical denial of service flaw within the PHP serialization mechanism that affects versions 5.x through 7.1.24. This issue resides in the ext/standard/var_unserializer.c file which handles the deserialization process for PHP applications. The vulnerability specifically targets the com, dotnet, and variant classes that are part of PHP's COM extension, which provides interoperability with Windows COM components and .NET objects. When an attacker crafts a malicious serialized payload that attempts to unserialize objects of these specific classes, the application experiences a crash due to improper handling of the deserialization process.
The technical flaw manifests when PHP's unserialize function encounters serialized data containing references to com, dotnet, or variant classes that have not been properly initialized or validated. These classes are designed to interface with Windows-specific components and require specific runtime environments to function correctly. During the deserialization process, the PHP engine attempts to validate and instantiate these objects, but encounters malformed or unexpected data structures that cause memory corruption or invalid memory access patterns. This leads to application crashes and can be exploited to cause persistent denial of service conditions against PHP applications that process untrusted input through serialization mechanisms.
The operational impact of this vulnerability extends beyond simple application instability as it can be leveraged by attackers to disrupt services continuously. Web applications that accept user input for serialization purposes, such as session data storage, API endpoints, or data exchange mechanisms, become vulnerable to this attack vector. The vulnerability is particularly dangerous in environments where PHP applications handle user-supplied data without proper sanitization or validation before deserialization occurs. Attackers can construct malicious payloads that, when processed through the unserialize function, will cause the PHP process to terminate abruptly, leading to service unavailability for legitimate users and potential resource exhaustion attacks.
Mitigation strategies for CVE-2018-19396 should prioritize immediate patching of affected PHP versions to the latest stable releases that contain the necessary fixes. Organizations should implement strict input validation and sanitization measures for all data that undergoes serialization and deserialization processes, particularly when dealing with user-supplied content. The use of alternative serialization formats such as JSON instead of PHP's native serialize/unserialize functions can significantly reduce exposure to this class of vulnerability. Additionally, implementing proper error handling and monitoring mechanisms can help detect and respond to exploitation attempts. This vulnerability aligns with CWE-400, which addresses unchecked resource consumption, and maps to ATT&CK technique T1499.004 for network denial of service, highlighting the importance of proper input validation and resource management in preventing such attacks.