CVE-2015-7500 in iOS
Summary
by MITRE
The xmlParseMisc function in parser.c in libxml2 before 2.9.3 allows context-dependent attackers to cause a denial of service (out-of-bounds heap read) via unspecified vectors related to incorrect entities boundaries and start tags.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 07/11/2022
The vulnerability identified as CVE-2015-7500 resides within the libxml2 library's xmlParseMisc function in the parser.c file, affecting versions prior to 2.9.3. This flaw represents a critical security issue that demonstrates how improper handling of XML entities can lead to severe system instability. The vulnerability specifically manifests as an out-of-bounds heap read condition, which occurs when the parser encounters malformed XML content with incorrect entity boundaries and start tags. Such conditions can be exploited by context-dependent attackers who craft malicious XML payloads designed to trigger the vulnerable code path during parsing operations.
The technical implementation of this vulnerability stems from inadequate boundary checking within the XML parsing logic, particularly when processing entity declarations and their associated start tags. When libxml2 encounters XML content where entity boundaries are improperly defined or where start tags do not conform to expected patterns, the xmlParseMisc function fails to validate memory access properly, resulting in attempts to read data from heap memory locations that are outside the allocated buffer boundaries. This memory access violation occurs during the parsing phase when the library tries to process entity references that have malformed or inconsistent boundary markers. The flaw is categorized under CWE-125 as an out-of-bounds read, which is a common class of memory safety vulnerabilities that can lead to unpredictable behavior including denial of service conditions.
From an operational perspective, this vulnerability presents significant risk to systems that process untrusted XML input, as it can be exploited to cause denial of service attacks against applications relying on libxml2 for XML parsing. Attackers can craft specially formatted XML documents that trigger the out-of-bounds heap read condition, potentially causing application crashes, system instability, or resource exhaustion. The impact extends beyond simple service disruption as this vulnerability can be leveraged in broader attack chains where denial of service serves as a precursor to more sophisticated exploits. Systems that process XML data from external sources, including web applications, middleware, and enterprise integration platforms, are particularly vulnerable to this type of attack vector. The vulnerability's context-dependent nature means that exploitation requires specific conditions related to the XML content structure, making it more challenging to detect but no less dangerous in practice.
The recommended mitigation strategy involves upgrading to libxml2 version 2.9.3 or later, which contains the necessary patches to address the boundary checking issues in the xmlParseMisc function. Organizations should prioritize this upgrade across all systems that utilize libxml2 for XML processing, particularly in environments where untrusted input is processed. Additional defensive measures include implementing XML input validation and sanitization at application boundaries, employing XML parsers with better memory safety guarantees, and conducting regular security assessments of XML processing components. From a threat modeling perspective, this vulnerability aligns with ATT&CK technique T1210, which covers exploitation of remote services through malformed input, and represents a classic example of how memory safety issues in parsing libraries can be leveraged for denial of service attacks. System administrators should also consider implementing network-level controls and monitoring for unusual XML processing patterns that might indicate exploitation attempts, as the vulnerability's impact can be subtle and may not always result in immediate system crashes but rather gradual degradation of service availability.