CVE-2016-1836 in tvOS
Summary
by MITRE
Use-after-free vulnerability in the xmlDictComputeFastKey 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 via a crafted XML document.
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Analysis
by VulDB Data Team • 09/06/2022
The vulnerability identified as CVE-2016-1836 represents a critical use-after-free flaw within the xmlDictComputeFastKey function of libxml2 library versions prior to 2.9.4. This vulnerability affects multiple Apple operating systems including iOS, macOS, tvOS, and watchOS, creating a widespread security concern across Apple's ecosystem. The flaw resides in the memory management handling of XML dictionary computations, where improper memory deallocation followed by subsequent access creates exploitable conditions that can be leveraged by remote attackers.
The technical implementation of this vulnerability stems from the xmlDictComputeFastKey function's failure to properly manage memory references when processing XML documents. When a crafted XML document is processed, the function computes a fast key for dictionary entries but does not adequately validate or manage the memory lifecycle of these entries. This creates a scenario where memory allocated for dictionary entries can be freed while still being referenced elsewhere in the code execution path, leading to undefined behavior and potential system instability. The vulnerability operates at the intersection of memory safety issues and XML parsing, making it particularly dangerous in environments where XML processing is common.
The operational impact of CVE-2016-1836 manifests primarily as a denial of service condition that can be remotely triggered through crafted XML content. Attackers can construct malicious XML documents that, when processed by vulnerable systems, cause the xmlDictComputeFastKey function to execute the use-after-free pattern. This results in system crashes, application hangs, or complete service unavailability, effectively rendering affected systems unusable. The vulnerability's remote exploitability means that adversaries can trigger the condition without physical access to the target device, making it particularly concerning for networked environments where XML processing occurs.
From a cybersecurity perspective, this vulnerability aligns with CWE-416, which specifically addresses use-after-free conditions in memory management. The ATT&CK framework categorizes this as a memory corruption technique that can be leveraged for denial of service attacks, potentially serving as a precursor to more sophisticated exploitation techniques. The widespread adoption of libxml2 across multiple platforms and applications amplifies the vulnerability's impact, as it affects not just Apple's proprietary software but also numerous third-party applications that rely on this library for XML processing. Organizations should prioritize patching affected systems to prevent exploitation, as the vulnerability demonstrates the critical importance of proper memory management in security-critical libraries. The remediation process requires updating to libxml2 version 2.9.4 or later, which includes proper memory handling mechanisms to prevent the use-after-free condition from occurring during XML dictionary processing operations.