CVE-2016-4539 in PHP
Summary
by MITRE
The xml_parse_into_struct function in ext/xml/xml.c in PHP before 5.5.35, 5.6.x before 5.6.21, and 7.x before 7.0.6 allows remote attackers to cause a denial of service (buffer under-read and segmentation fault) or possibly have unspecified other impact via crafted XML data in the second argument, leading to a parser level of zero.
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Analysis
by VulDB Data Team • 08/22/2022
The xml_parse_into_struct function in PHP's xml extension contains a critical buffer under-read vulnerability that affects multiple PHP versions including 5.5.35 and earlier, 5.6.21 and earlier, and 7.0.5 and earlier. This vulnerability resides in the ext/xml/xml.c source file and represents a fundamental parsing flaw that can be exploited through maliciously crafted XML input data. The issue occurs when the second argument of the xml_parse_into_struct function receives malformed XML content that triggers an under-read condition in the parser's memory management system. This buffer under-read vulnerability specifically impacts the parser's internal data structures and can lead to segmentation faults during XML processing operations.
The technical nature of this vulnerability stems from improper bounds checking within the XML parser's memory handling routines. When the xml_parse_into_struct function processes crafted XML data, it fails to validate the boundaries of memory allocations used during parsing operations. This allows attackers to manipulate the parser's internal state through carefully constructed XML payloads that cause the parser to read from memory locations that are outside the intended buffer boundaries. The vulnerability operates at the parser level zero, indicating that the issue occurs during the most fundamental parsing operations before any application-level processing begins. This characteristic makes the vulnerability particularly dangerous as it can be exploited even before the XML data reaches any application logic.
The operational impact of CVE-2016-4539 extends beyond simple denial of service conditions to potentially enable more severe security consequences. While the primary effect manifests as buffer under-reads and segmentation faults that result in service disruption, the vulnerability could theoretically allow for more sophisticated attacks depending on the execution environment and system configuration. Attackers can leverage this flaw to cause system instability, application crashes, and potentially gain unauthorized access to system resources through memory corruption techniques. The vulnerability affects PHP applications that rely on XML processing capabilities, making it particularly dangerous for web applications that handle user-provided XML data. The impact is especially severe in environments where PHP applications are exposed to untrusted XML input from external sources.
Mitigation strategies for this vulnerability require immediate patching of affected PHP installations to versions that contain the necessary security fixes. System administrators should prioritize updating their PHP environments to versions 5.5.35, 5.6.21, or 7.0.6 and later, depending on their current PHP version. Additionally, implementing proper input validation and sanitization measures can help reduce the attack surface by filtering malicious XML content before it reaches the parser. Organizations should also consider implementing XML parsing restrictions and monitoring for unusual parsing behavior that might indicate exploitation attempts. The vulnerability aligns with CWE-129, which addresses improper validation of array indices, and can be mapped to ATT&CK technique T1203, which covers exploitation of software vulnerabilities for privilege escalation and system compromise. Regular security audits and vulnerability assessments should be conducted to identify and remediate similar parsing vulnerabilities in other XML processing libraries and components within the system infrastructure.