CVE-2015-6789 in Chrome
Summary
by MITRE
Race condition in the MutationObserver implementation in Blink, as used in Google Chrome before 47.0.2526.80, allows remote attackers to cause a denial of service (use-after-free) or possibly have unspecified other impact by leveraging unanticipated object deletion.
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability CVE-2015-6789 represents a critical race condition within the MutationObserver implementation of Blink engine, which serves as the core rendering engine for Google Chrome browser. This flaw exists in versions prior to 47.0.2526.80 and demonstrates a fundamental issue in how the browser handles asynchronous DOM mutations and object lifecycle management. The vulnerability specifically targets the interaction between MutationObserver callbacks and the garbage collection process, creating a window where objects can be prematurely deallocated while still being referenced by active observer callbacks.
The technical exploitation of this vulnerability occurs through a use-after-free condition that arises when the MutationObserver API processes DOM changes concurrently with object deallocation. When a web page triggers multiple DOM mutations in rapid succession, the observer callbacks may attempt to access objects that have already been freed by the garbage collector, leading to unpredictable behavior. This race condition manifests when the observer's callback execution overlaps with the memory deallocation process, creating a scenario where freed memory locations are accessed, potentially causing crashes or enabling arbitrary code execution. The vulnerability falls under CWE-367 which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) flaws, though this instance is more precisely categorized as a use-after-free condition.
The operational impact of CVE-2015-6789 extends beyond simple denial of service to potentially enable more sophisticated attacks. Remote attackers can leverage this vulnerability to crash the browser application, leading to denial of service for legitimate users, or in more severe cases, execute arbitrary code within the browser sandbox. The attack vector requires a malicious website to be loaded, where the attacker crafts DOM mutations that trigger the race condition during observer callback execution. This vulnerability is particularly dangerous because it operates within the browser's core rendering engine, making it difficult to detect and exploit through traditional security measures. The impact aligns with ATT&CK technique T1059.001 for command and scripting interpreter, as successful exploitation could lead to code execution within the browser context.
Mitigation strategies for this vulnerability primarily involve updating to Chrome version 47.0.2526.80 or later, where Google implemented fixes to synchronize observer callback execution with object lifecycle management. Browser vendors should also consider implementing additional memory safety mechanisms such as heap sanitization and enhanced garbage collection synchronization. The fix typically involves ensuring that observer callbacks are properly synchronized with object deallocation, preventing the scenario where freed objects are accessed during callback execution. Organizations should prioritize patch management and ensure all browser installations are kept current with security updates. Additionally, implementing content security policies and sandboxing mechanisms can provide defense-in-depth against exploitation attempts, though the primary mitigation remains the timely application of the vendor-provided security patch.