CVE-2016-5094 in PHP
Summary
by MITRE
Integer overflow in the php_html_entities function in ext/standard/html.c in PHP before 5.5.36 and 5.6.x before 5.6.22 allows remote attackers to cause a denial of service or possibly have unspecified other impact by triggering a large output string from the htmlspecialchars function.
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Analysis
by VulDB Data Team • 09/13/2022
The vulnerability CVE-2016-5094 represents a critical integer overflow flaw within PHP's html entities processing functionality that affects versions prior to 5.5.36 and 5.6.22. This issue resides in the php_html_entities function located in the ext/standard/html.c source file, specifically within the htmlspecialchars implementation that transforms special characters into HTML entities. The vulnerability manifests when processing certain input strings that trigger an integer overflow condition during the calculation of output buffer sizes, creating a scenario where the system attempts to allocate memory for an extraordinarily large output string that far exceeds normal processing boundaries.
The technical exploitation of this vulnerability occurs through crafted input data that, when passed through the htmlspecialchars function, causes the internal integer arithmetic to overflow and wrap around to a small positive value or zero. This overflow condition results in the system attempting to allocate memory for an output string that is orders of magnitude larger than what was originally intended, leading to memory allocation failures or unpredictable behavior. The flaw stems from inadequate bounds checking and integer overflow protection mechanisms within the HTML entity conversion routine, making it particularly dangerous as it can be triggered through normal user input processing without requiring special privileges or complex attack vectors.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable more severe consequences including memory corruption and arbitrary code execution in certain circumstances. Remote attackers can leverage this vulnerability to exhaust system resources through massive memory allocation requests, causing service disruption and system instability. The vulnerability aligns with CWE-190, Integer Overflow or Wraparound, which specifically addresses the dangers of integer arithmetic overflow conditions that can lead to memory corruption and unpredictable system behavior. Additionally, this issue maps to ATT&CK technique T1499.004, Network Denial of Service, as it enables attackers to disrupt services through resource exhaustion attacks that can be executed remotely.
Mitigation strategies for CVE-2016-5094 require immediate patching of affected PHP installations to versions 5.5.36 or 5.6.22 and later, which contain the necessary fixes to prevent integer overflow conditions in the html entities processing functions. Organizations should also implement input validation and sanitization measures to limit the size and complexity of data processed through htmlspecialchars and related functions, reducing the attack surface. The vulnerability demonstrates the importance of proper integer overflow protection in security-critical code paths and highlights the need for comprehensive testing of boundary conditions in cryptographic and data transformation routines. System administrators should monitor for unusual memory consumption patterns and implement resource limits on PHP processes to minimize the impact of potential exploitation attempts. The fix implemented in the patched versions includes enhanced integer overflow detection and proper buffer size validation to prevent the allocation of excessively large output buffers during HTML entity conversion operations.