CVE-2016-4447 in tvOS
Summary
by MITRE
The xmlParseElementDecl function in parser.c in libxml2 before 2.9.4 allows context-dependent attackers to cause a denial of service (heap-based buffer underread and application crash) via a crafted file, involving xmlParseName.
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Analysis
by VulDB Data Team • 09/06/2022
The vulnerability identified as CVE-2016-4447 represents a critical heap-based buffer underread condition within the libxml2 XML parsing library. This flaw exists in the xmlParseElementDecl function located in the parser.c file and affects all versions prior to 2.9.4. The vulnerability manifests when processing specially crafted XML files that exploit the xmlParseName parsing routine, creating a scenario where the application reads memory locations that have not been properly initialized or allocated. This type of buffer underread vulnerability falls under the CWE-126 category, which specifically addresses buffer over-read conditions that can lead to unpredictable behavior and system instability. The issue is particularly concerning because it operates within the core parsing functionality of a widely-used library that processes XML data across numerous applications and systems.
The technical exploitation of this vulnerability occurs when an attacker crafts an XML document containing malformed element declarations that trigger the buffer underread condition during parsing operations. The xmlParseElementDecl function attempts to parse element declarations within XML documents, but due to improper bounds checking when handling name parsing through xmlParseName, it accesses memory regions that have not been properly allocated or initialized. This memory access pattern can result in reading beyond the allocated buffer boundaries, leading to application crashes and potential information disclosure. The vulnerability is context-dependent, meaning that the malicious XML content must be specifically crafted to trigger the precise memory access pattern that causes the underread condition, making it somewhat more difficult to exploit compared to simpler buffer overflow scenarios.
The operational impact of CVE-2016-4447 extends beyond simple denial of service to potentially compromise system stability and availability across numerous applications that rely on libxml2 for XML processing. Systems utilizing vulnerable versions of libxml2 may experience unexpected application crashes when processing untrusted XML input, which can be particularly problematic in web applications, enterprise systems, and security tools that parse user-supplied or external XML data. The vulnerability can be leveraged by attackers to create persistent denial of service conditions, effectively preventing legitimate users from accessing services that depend on XML processing capabilities. From an attack framework perspective, this vulnerability aligns with techniques described in the ATT&CK matrix under the T1499.004 sub-technique for network denial of service attacks, where adversaries target application parsing functions to disrupt service availability.
Mitigation strategies for CVE-2016-4447 primarily focus on upgrading to libxml2 version 2.9.4 or later, which includes the necessary patches to address the buffer underread condition. Organizations should conduct comprehensive vulnerability assessments to identify all systems and applications that utilize vulnerable versions of libxml2, particularly those handling external XML input. Additionally, implementing proper input validation and sanitization measures can provide defense-in-depth protection against malformed XML content, though this approach does not address the underlying library vulnerability. Security teams should also consider implementing network-based intrusion detection systems that can identify and block suspicious XML content patterns that may attempt to exploit this vulnerability. The fix implemented in libxml2 2.9.4 specifically addresses the bounds checking issue in the xmlParseName function and ensures proper memory allocation handling during element declaration parsing, thereby preventing the heap-based buffer underread condition that previously enabled the denial of service attack.