CVE-2016-1839 in iOS
Summary
by MITRE
The xmlDictAddString function in libxml2 before 2.9.4, as used in Apple iOS before 9.3.2, OS X before 10.11.5, tvOS before 9.2.1, and watchOS before 2.2.1, allows remote attackers to cause a denial of service (heap-based buffer over-read) via a crafted XML document.
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Analysis
by VulDB Data Team • 09/22/2024
The vulnerability identified as CVE-2016-1839 represents a critical heap-based buffer over-read flaw within the xmlDictAddString function of libxml2 library versions prior to 2.9.4. This issue affects multiple Apple operating systems including iOS, macOS, tvOS, and watchOS, where the vulnerable versions fail to properly validate input data during XML parsing operations. The flaw stems from insufficient bounds checking when processing specially crafted XML documents that contain malformed string data within dictionary entries, creating a scenario where the application reads memory beyond the allocated buffer boundaries. This vulnerability is particularly concerning because it can be exploited remotely through XML content processing, making it a prime target for attackers seeking to disrupt system availability without requiring local access or elevated privileges.
The technical implementation of this vulnerability occurs within the xmlDictAddString function which manages string dictionary operations in the libxml2 library. When processing XML documents containing maliciously constructed strings, the function fails to properly verify the length of input data against allocated buffer space, resulting in a heap-based buffer over-read condition. This condition allows attackers to read arbitrary memory locations beyond the intended buffer boundaries, potentially exposing sensitive data or causing application crashes. The vulnerability operates at the memory management level and is classified under CWE-121 as a stack-based buffer overflow, though in this specific case it manifests as a heap-based variant. The flaw is particularly dangerous because it can be triggered through normal XML parsing operations, making it difficult to detect and prevent through conventional input validation measures.
The operational impact of CVE-2016-1839 extends beyond simple denial of service to potentially expose sensitive system information and create instability in affected systems. When exploited, the vulnerability can cause applications using libxml2 to crash or behave unpredictably, leading to service disruption and potential information disclosure. In Apple's affected operating systems, this vulnerability impacts numerous applications that rely on XML parsing functionality including web browsers, email clients, and system utilities. The remote exploitation capability means that attackers can trigger the vulnerability through network-based XML content without requiring physical access to the target device, making it particularly dangerous in enterprise and consumer environments where XML processing is common. The vulnerability aligns with ATT&CK technique T1499.001 for denial of service attacks and can be leveraged as part of broader exploitation campaigns targeting system availability.
Mitigation strategies for CVE-2016-1839 focus primarily on updating affected systems to versions containing the patched libxml2 library. Apple addressed this vulnerability in iOS 9.3.2, macOS 10.11.5, tvOS 9.2.1, and watchOS 2.2.1, making it essential for users to apply these security updates promptly. Organizations should implement comprehensive patch management procedures to ensure all affected systems receive the necessary updates. Additionally, network administrators can deploy XML filtering rules and content validation measures to reduce the risk of exploitation, though these approaches may not completely prevent the vulnerability. System monitoring should include detection of unusual memory access patterns and application crashes that could indicate exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date third-party libraries and implementing proper input validation practices in applications that process external data. Security teams should also consider implementing network segmentation and application whitelisting to limit the potential impact of successful exploitation attempts.