CVE-2017-6441 in PHP
Summary
by MITRE
** DISPUTED ** The _zval_get_long_func_ex in Zend/zend_operators.c in PHP 7.1.2 allows attackers to cause a denial of service (NULL pointer dereference and application crash) via crafted use of "declare(ticks=" in a PHP script. NOTE: the vendor disputes the classification of this as a vulnerability, stating "Please do not request CVEs for ordinary bugs. CVEs are relevant for security issues only."
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability described in CVE-2017-6441 involves a NULL pointer dereference condition within PHP's Zend engine, specifically in the _zval_get_long_func_ex function located in Zend/zend_operators.c. This flaw manifests when PHP scripts utilize the declare(ticks=) directive in a particular manner that triggers unexpected behavior in the interpreter's handling of variable types and execution flow. The issue demonstrates a classic denial of service vulnerability where malicious input can cause the PHP application to crash and terminate unexpectedly. The vulnerability occurs during the parsing and execution phase of PHP scripts, specifically when the interpreter attempts to process type conversions involving the ticks directive, which is used to set up signal handling within PHP scripts.
The technical implementation of this vulnerability stems from improper input validation and error handling within PHP's internal type system. When the declare(ticks=) directive is used with crafted parameters, the _zval_get_long_func_ex function fails to properly check for NULL pointer conditions before attempting to dereference pointers. This type of flaw falls under CWE-476 which specifically addresses NULL pointer dereference conditions in software systems. The vulnerability represents a weakness in the interpreter's defensive programming practices, where insufficient validation of input parameters leads to execution flow disruption. The condition occurs during the Zend engine's variable handling operations, where the system attempts to convert variable values to long integer types without proper null checking mechanisms in place.
From an operational impact perspective, this vulnerability creates significant disruption for PHP applications that rely on the declare directive for signal handling or execution timing control. Attackers can craft malicious PHP scripts that, when processed by the affected PHP versions, will cause the web server or command line interpreter to crash. This denial of service condition affects availability of the application and can be exploited to disrupt legitimate service operations. The vulnerability is particularly concerning in web environments where PHP scripts are executed dynamically, as it allows for remote code execution disruption and can be used to overwhelm server resources through repeated triggering of the crash condition. The impact extends beyond simple service interruption to potentially affect system stability and availability of critical web applications.
Security practitioners should note that while the vendor has disputed the classification of this issue as a vulnerability, the technical nature of the flaw still presents operational risks that organizations must consider. The vulnerability demonstrates poor defensive programming practices within PHP's core interpreter and highlights the importance of comprehensive input validation even for seemingly benign features. Organizations should consider implementing proper PHP version management and ensure they are running patched versions of the software to prevent exploitation of this type of condition. The flaw also underscores the importance of following ATT&CK framework concepts related to privilege escalation and denial of service through software exploitation, as this vulnerability could potentially be chained with other exploits to create more severe security incidents. Mitigation strategies should focus on proper PHP version updates, input sanitization, and monitoring for unusual script execution patterns that might indicate exploitation attempts.