CVE-2017-8923 in PHP
Summary
by MITRE
The zend_string_extend function in Zend/zend_string.h in PHP through 7.1.5 does not prevent changes to string objects that result in a negative length, which allows remote attackers to cause a denial of service (application crash) or possibly have unspecified other impact by leveraging a script's use of .= with a long string.
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Analysis
by VulDB Data Team • 12/27/2024
The vulnerability identified as CVE-2017-8923 resides within the zend_string_extend function located in Zend/zend_string.h of the PHP interpreter. This flaw represents a critical buffer management issue that affects PHP versions through 7.1.5, creating a potential avenue for remote attackers to manipulate string objects in ways that can lead to system instability. The vulnerability specifically exploits the absence of proper validation mechanisms that should prevent modifications to string objects resulting in negative length values, which fundamentally violates the expected behavior of string handling operations within the PHP runtime environment.
The technical implementation of this vulnerability stems from insufficient input validation within the string extension mechanism. When PHP scripts utilize the concatenation assignment operator .= with extremely long strings, the zend_string_extend function fails to properly validate the resulting string length before allowing modifications to proceed. This oversight creates a scenario where string objects can be manipulated to contain negative length values, which subsequently triggers undefined behavior within the PHP interpreter's memory management subsystem. The flaw operates at the core level of PHP's string handling capabilities, making it particularly dangerous as it can be exploited through standard scripting operations that most web applications rely upon.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially encompass more severe consequences including arbitrary code execution or complete application compromise. Attackers can leverage this vulnerability by crafting malicious scripts that exploit the string concatenation behavior to force the interpreter into an unstable state. When the PHP interpreter encounters a string object with a negative length, it typically results in memory corruption that can cause segmentation faults, application crashes, or unpredictable behavior that may allow for further exploitation attempts. This vulnerability particularly affects web applications that process user input through string operations, making it a significant concern for any PHP-based web infrastructure.
Mitigation strategies for CVE-2017-8923 should prioritize immediate patching of affected PHP installations to versions that contain the necessary fixes for the zend_string_extend function. Organizations should implement comprehensive input validation measures that prevent excessively long string operations from being processed, particularly in applications that handle user-provided content. Network-level protections such as web application firewalls and rate limiting can help reduce the attack surface by monitoring and restricting potentially malicious string operations. The vulnerability aligns with CWE-129, which addresses improper validation of string length, and falls under ATT&CK technique T1499.004 for network denial of service, making it a critical target for both defensive and monitoring measures within cybersecurity frameworks. Security teams should also consider implementing runtime monitoring solutions that can detect anomalous string manipulation patterns and trigger alerts when negative length conditions are encountered during string operations.