CVE-2016-1837 in iOS
Summary
by MITRE
Multiple use-after-free vulnerabilities in the (1) htmlPArsePubidLiteral and (2) htmlParseSystemiteral functions 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, allow remote attackers to cause a denial of service via a crafted XML document.
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Analysis
by VulDB Data Team • 08/21/2022
The vulnerability identified as CVE-2016-1837 represents a critical security flaw in the libxml2 library that affects multiple Apple operating systems and their respective versions. This issue stems from use-after-free conditions in two specific parsing functions within the XML processing framework. The affected functions htmlParsePubidLiteral and htmlParseSystemiteral are responsible for parsing public and system identifiers within XML documents, respectively. These functions fail to properly validate memory allocation and deallocation sequences, creating opportunities for malicious actors to exploit the underlying memory management flaws.
The technical implementation of this vulnerability involves improper handling of memory resources during XML parsing operations. When libxml2 processes malformed XML documents containing crafted payloads, the htmlParsePubidLiteral and htmlParseSystemiteral functions execute code paths that result in freed memory being accessed or reused. This memory corruption occurs because the library does not adequately validate input parameters or maintain proper reference counts during parsing operations. The use-after-free condition manifests when the application attempts to access memory that has already been deallocated, potentially leading to unpredictable behavior including application crashes or system instability.
From an operational perspective, this vulnerability presents significant risks to affected systems as remote attackers can leverage it to execute denial of service attacks against target systems. The crafted XML documents require no special privileges to exploit, making this attack vector particularly dangerous for widespread deployment. When successfully exploited, the vulnerability can cause applications using libxml2 to crash or become unresponsive, effectively disrupting normal system operations. This type of vulnerability is particularly concerning in enterprise environments where XML processing is common in web services, document handling, and configuration management systems that rely on libxml2 for parsing operations.
The impact of CVE-2016-1837 aligns with CWE-416, which specifically addresses use-after-free vulnerabilities in software applications. This classification indicates that the flaw involves accessing memory after it has been freed, creating opportunities for both denial of service and potentially more severe exploitation scenarios. The vulnerability also relates to ATT&CK technique T1203, which covers exploitation of software vulnerabilities for privilege escalation or system compromise. Organizations running affected versions of Apple operating systems must understand that this vulnerability can serve as a stepping stone for more sophisticated attacks, particularly when combined with other exploitation techniques.
Mitigation strategies for this vulnerability primarily focus on updating to patched versions of libxml2 and the affected Apple operating systems. Apple addressed this issue through security updates for iOS 9.3.2, OS X 10.11.5, tvOS 9.2.1, and watchOS 2.2.1, which included fixes for the memory management issues in the affected parsing functions. System administrators should implement comprehensive patch management procedures to ensure all affected systems receive timely updates. Additionally, organizations can deploy XML input validation measures and restrict XML processing to trusted sources as defensive measures. Network monitoring solutions should be configured to detect unusual XML processing patterns that might indicate exploitation attempts, while application developers should review their XML processing code to ensure proper error handling and memory management practices are implemented.