CVE-2016-1834 in iOS
Summary
by MITRE
Heap-based buffer overflow in the xmlStrncat 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 execute arbitrary code or cause a denial of service (memory corruption) via a crafted XML document.
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Analysis
by VulDB Data Team • 12/04/2025
The heap-based buffer overflow vulnerability in libxml2's xmlStrncat function represents a critical security flaw that affected multiple Apple operating systems and their underlying XML processing libraries. This vulnerability exists within the libxml2 library version 2.9.3 and earlier, which was widely deployed across Apple's ecosystem including iOS, macOS, tvOS, and watchOS. The flaw specifically manifests in the xmlStrncat function, which is responsible for concatenating strings while enforcing a maximum length constraint, making it a prime target for memory corruption attacks that can lead to arbitrary code execution or system crashes.
The technical implementation of this vulnerability stems from improper bounds checking within the xmlStrncat function, which fails to adequately validate the length of concatenated strings before writing data to heap-allocated memory regions. When processing maliciously crafted XML documents, the function does not properly enforce the maximum length parameter, allowing attackers to write beyond the allocated buffer boundaries and overwrite adjacent memory locations. This memory corruption can result in unpredictable behavior including program crashes, memory corruption, or more critically, the execution of arbitrary code within the context of the vulnerable application. The vulnerability is particularly dangerous because it can be triggered remotely through XML processing, making it exploitable via web browsers, email clients, or any application that parses XML content using the affected libxml2 library version.
The operational impact of this vulnerability extends across Apple's entire product portfolio that relied on the vulnerable libxml2 version, affecting iOS devices before 9.3.2, macOS before 10.11.5, tvOS before 9.2.1, and watchOS before 2.2.1. Attackers could exploit this weakness by crafting specially designed XML documents that, when processed by vulnerable applications, would trigger the buffer overflow condition. The implications include potential privilege escalation, persistent system compromise, and denial of service conditions that could render devices unusable. This vulnerability aligns with CWE-121, heap-based buffer overflow, and maps to attack patterns in the MITRE ATT&CK framework under T1059 for command and control execution and T1499 for endpoint denial of service, demonstrating how memory corruption flaws can be leveraged for both system compromise and availability attacks.
The remediation strategy for this vulnerability required immediate patching of the libxml2 library to version 2.9.4 or later, which included proper bounds checking and memory validation within the xmlStrncat function. Apple addressed this issue through system updates that replaced the vulnerable library components across all affected operating systems, requiring users to install the latest security patches to mitigate the risk. Organizations should implement comprehensive patch management procedures to ensure all systems running vulnerable versions of libxml2 receive updates promptly, as the vulnerability could be exploited in the wild without user interaction. Additionally, input validation and sandboxing measures should be implemented at application layers to reduce the attack surface, particularly for XML processing components that may be exposed to untrusted input from external sources.