CVE-2016-1833 in iOS
Summary
by MITRE
The htmlCurrentChar 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 • 08/21/2022
The vulnerability identified as CVE-2016-1833 represents a critical heap-based buffer over-read flaw within the libxml2 library version 2.9.3 and earlier. This issue affects multiple Apple operating systems including iOS, macOS, tvOS, and watchOS, where the vulnerable libxml2 version is embedded within the system libraries. The vulnerability stems from improper bounds checking within the htmlCurrentChar function, which processes HTML character sequences during XML parsing operations. When processing malformed XML documents, the function fails to validate buffer boundaries, allowing attackers to craft specially designed XML content that triggers memory access violations. The flaw specifically manifests as a heap-based buffer over-read condition, where the parser attempts to read memory locations beyond the allocated buffer boundaries, potentially exposing sensitive data or causing application crashes.
The technical implementation of this vulnerability involves the htmlCurrentChar function in libxml2 which handles character processing in HTML documents. During XML parsing, when the function encounters certain malformed character sequences, it does not properly validate the buffer limits before accessing memory locations. This condition occurs because the function assumes that character data will always fit within expected buffer boundaries without proper bounds checking. The vulnerability is particularly dangerous because it can be triggered through standard XML parsing operations, making it accessible to remote attackers who can craft malicious XML documents. The heap-based nature of the over-read means that the parser accesses memory regions that may contain sensitive information from other parts of the application's memory space, potentially leading to information disclosure or system instability.
The operational impact of CVE-2016-1833 extends beyond simple denial of service conditions, as it represents a fundamental memory safety issue that can be exploited to compromise system integrity. Remote attackers can leverage this vulnerability to cause unpredictable behavior in applications that utilize libxml2 for XML processing, including web browsers, email clients, and other applications that parse XML content. The vulnerability affects a wide range of Apple products and services, making it particularly concerning from a security perspective. When exploited, the buffer over-read can result in application crashes, system instability, and potentially information disclosure through memory exposure. The vulnerability's remote exploitability means that attackers do not need physical access to the device or local network privileges to trigger the condition, making it a significant threat vector for mobile and desktop environments.
Mitigation strategies for CVE-2016-1833 focus primarily on updating to patched versions of libxml2 and the affected Apple operating systems. The most effective remediation involves upgrading to libxml2 version 2.9.4 or later, which includes proper bounds checking and buffer validation mechanisms. Apple addressed this vulnerability through security updates for iOS 9.3.2, macOS 10.11.5, tvOS 9.2.1, and watchOS 2.2.1, making it essential for organizations to deploy these updates promptly. System administrators should also implement additional protective measures such as XML input validation, sandboxing of XML processing components, and network monitoring to detect potential exploitation attempts. From a cybersecurity perspective, this vulnerability aligns with CWE-125, which describes out-of-bounds read conditions, and represents a typical example of memory safety issues that can be addressed through proper bounds checking and input validation practices. The ATT&CK framework categorizes this vulnerability under privilege escalation and denial of service tactics, as it can be used to disrupt system operations and potentially gain unauthorized access to system resources through information disclosure. Organizations should also consider implementing automated patch management systems to ensure timely deployment of security updates and reduce the window of exposure to known vulnerabilities.