CVE-2016-4449 in tvOS
Summary
by MITRE
XML external entity (XXE) vulnerability in the xmlStringLenDecodeEntities function in parser.c in libxml2 before 2.9.4, when not in validating mode, allows context-dependent attackers to read arbitrary files or cause a denial of service (resource consumption) via unspecified vectors.
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Analysis
by VulDB Data Team • 09/06/2022
The CVE-2016-4449 vulnerability represents a critical XML external entity processing flaw within the libxml2 library, specifically affecting versions prior to 2.9.4. This vulnerability resides in the xmlStringLenDecodeEntities function located within the parser.c file of the library. The flaw manifests when the library operates in non-validating mode, creating a dangerous attack surface that can be exploited by context-dependent adversaries. The vulnerability stems from insufficient input validation and improper handling of external entity references during XML parsing operations, making it particularly dangerous in applications that process untrusted XML data from various sources.
The technical exploitation of this XXE vulnerability occurs through manipulation of XML documents that contain external entity declarations. When libxml2 processes such malformed XML input without proper validation, it can be coerced into resolving external references and accessing local system resources. Attackers can leverage this capability to read arbitrary files from the system where the vulnerable library is deployed, potentially accessing sensitive configuration files, credentials, or other confidential data. Additionally, the vulnerability can be exploited for resource exhaustion attacks, where malicious XML documents contain recursive entity references that consume excessive system memory and processing power, leading to denial of service conditions that can effectively cripple applications relying on the affected library.
From an operational standpoint, this vulnerability poses significant risks to organizations using applications built on or dependent upon libxml2 for XML processing. The impact extends across numerous software systems including web applications, enterprise platforms, and middleware components that parse XML data from external sources. The vulnerability's context-dependent nature means that exploitation effectiveness varies based on the specific application environment and configuration, but the potential for both information disclosure and service disruption makes it particularly dangerous. Organizations may unknowingly expose themselves to this vulnerability if they are running outdated versions of libxml2, particularly in environments where XML processing is common.
Security practitioners should note that this vulnerability aligns with CWE-611, which specifically addresses improper restriction of XML external entity reference, and maps to ATT&CK technique T1213.002 for data from information repositories. The vulnerability demonstrates the critical importance of keeping third-party libraries updated and implementing proper XML validation controls. Mitigation strategies should include immediate upgrading to libxml2 version 2.9.4 or later, implementing strict XML schema validation, disabling external entity resolution in parsing configurations, and deploying web application firewalls that can detect and block suspicious XML content. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all systems using affected versions of libxml2 and ensure proper input sanitization measures are in place to prevent exploitation attempts.