CVE-2016-5095 in PHP
Summary
by MITRE
Integer overflow in the php_escape_html_entities_ex 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 a FILTER_SANITIZE_FULL_SPECIAL_CHARS filter_var call. NOTE: this vulnerability exists because of an incomplete fix for CVE-2016-5094.
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Analysis
by VulDB Data Team • 09/13/2022
The vulnerability identified as CVE-2016-5095 represents a critical integer overflow condition within PHP's html.c extension that affects versions prior to 5.5.36 and 5.6.x before 5.6.22. This flaw specifically manifests in the php_escape_html_entities_ex function where an insufficient boundary check allows malicious input to trigger unexpected behavior during HTML entity escaping operations. The vulnerability stems from an incomplete remediation of a previously identified issue CVE-2016-5094, creating a persistent security gap that attackers can exploit through carefully crafted input sequences. The integer overflow occurs when processing filter_var calls with FILTER_SANITIZE_FULL_SPECIAL_CHARS, which can lead to memory corruption and system instability. This vulnerability operates at the core level of PHP's input sanitization mechanisms, making it particularly dangerous as it can be triggered through common web application filtering operations.
The technical implementation of this vulnerability exploits the way PHP handles memory allocation for output strings during HTML entity escaping. When the php_escape_html_entities_ex function processes input through the FILTER_SANITIZE_FULL_SPECIAL_CHARS filter, it performs calculations to determine the required output buffer size. Due to the integer overflow, these calculations can produce values that exceed the maximum representable integer, causing the system to allocate insufficient memory or trigger memory corruption. The overflow specifically occurs during the computation of buffer sizes needed for escaped HTML entities, where large input strings can cause the arithmetic operations to wrap around to negative or extremely large values. This behavior creates a scenario where the application's memory management becomes compromised, potentially leading to heap corruption or stack overflow conditions that can be leveraged for more severe attacks.
From an operational standpoint, this vulnerability presents significant risks to web applications running affected PHP versions, as it can be exploited to cause denial of service conditions that disrupt legitimate service operations. Attackers can craft input that, when processed through the vulnerable filter, triggers the integer overflow and causes the web application to crash or become unresponsive. The potential for unspecified other impacts means that beyond simple denial of service, there could be opportunities for code execution or information disclosure depending on the specific runtime environment and memory layout. The vulnerability's exploitation is particularly concerning because it can be triggered through standard input sanitization operations that are commonly used throughout web applications, making it difficult to prevent through traditional input validation approaches. Organizations running affected PHP versions face the risk of service interruption and potential data exposure if this vulnerability is not addressed promptly.
The mitigation strategy for CVE-2016-5095 requires immediate patching of PHP installations to versions 5.5.36 or 5.6.22 and later, which contain the complete fix for both CVE-2016-5094 and CVE-2016-5095. Organizations should also implement monitoring for unusual patterns in input processing that might indicate exploitation attempts, particularly around FILTER_SANITIZE_FULL_SPECIAL_CHARS usage. Additionally, defensive programming practices should be reinforced to validate input size limits and implement proper error handling for memory allocation operations. This vulnerability aligns with CWE-190, Integer Overflow or Wraparound, and can be categorized under ATT&CK technique T1499.004, Endpoint Denial of Service, as it enables attackers to cause service disruption through resource exhaustion. The vulnerability demonstrates the importance of comprehensive patch management and the risks associated with incomplete security fixes, as the remediation for the previous vulnerability did not fully address the underlying integer overflow condition. Organizations should also consider implementing web application firewalls to detect and block malicious input patterns that could trigger this vulnerability during routine application operations.