CVE-2019-11716 in Firefox
Summary
by MITRE
Until explicitly accessed by script, window.globalThis is not enumerable and, as a result, is not visible to code such as Object.getOwnPropertyNames(window). Sites that deploy a sandboxing that depends on enumerating and freezing access to the window object may miss this, allowing their sandboxes to be bypassed. This vulnerability affects Firefox < 68.
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Analysis
by VulDB Data Team • 07/10/2020
The vulnerability described in CVE-2019-11716 represents a critical sandboxing bypass issue within the Firefox browser that stems from improper handling of the globalThis property. This flaw specifically affects Firefox versions prior to 68, where the window.globalThis object exhibits non-enumerable behavior until explicitly accessed by script code. The technical implementation of this vulnerability exploits the discrepancy between how JavaScript engines handle property enumeration and how security-conscious applications implement sandboxing mechanisms. When developers create sandboxed environments by enumerating window object properties to identify and restrict access to potentially dangerous APIs, they inadvertently overlook the globalThis property due to its non-enumerable nature. This oversight creates a potential attack vector where malicious code could bypass security restrictions that rely on comprehensive property enumeration.
The core technical flaw lies in the inconsistency between the ECMAScript specification and Firefox's implementation of the globalThis property. According to ECMAScript standards, globalThis should be accessible and enumerable as part of the global object, yet Firefox's pre-68 implementation treated it as non-enumerable. This behavior creates a security gap that aligns with CWE-254 vulnerability category, which addresses security weaknesses related to improper handling of object properties and their accessibility. The vulnerability demonstrates how seemingly minor implementation details in JavaScript engines can have significant security implications, particularly when applications depend on comprehensive property enumeration for security purposes. The non-enumerable nature of window.globalThis means that standard sandboxing techniques that rely on Object.getOwnPropertyNames() or similar enumeration methods fail to detect this property, leaving it accessible to potentially malicious code within sandboxed contexts.
The operational impact of this vulnerability extends beyond simple browser security concerns to encompass broader sandboxing and isolation mechanisms that depend on comprehensive object inspection. Attackers could leverage this weakness to access restricted APIs or execute code that should normally be prevented by sandboxing policies. This vulnerability particularly affects web applications that implement custom security models or content security policies that rely on enumerating the global window object to identify and restrict access to sensitive functionality. The attack surface is amplified in environments where developers implement defensive programming practices that depend on thorough property inspection, such as in web application frameworks or security-focused browser extensions. This issue demonstrates the importance of adhering to standardized JavaScript behavior across browser implementations and highlights the potential for implementation-specific security gaps that can be exploited by attackers.
Organizations and developers should immediately update to Firefox 68 or later versions to address this vulnerability, as the fix involves proper implementation of the globalThis property according to ECMAScript specifications. The recommended mitigation strategy includes not only updating the browser but also reviewing existing sandboxing implementations to ensure they do not rely solely on property enumeration techniques. Security teams should implement monitoring for unusual access patterns that might indicate exploitation attempts, particularly in environments where custom security policies are in place. This vulnerability also emphasizes the importance of comprehensive testing of security mechanisms against edge cases in JavaScript engine behavior, particularly when dealing with newer language features that may have implementation-specific quirks. The fix implemented in Firefox 68 ensures that globalThis behaves consistently with the ECMAScript specification, making it properly enumerable and accessible to security mechanisms that depend on property enumeration for sandbox enforcement. This remediation addresses the underlying ATT&CK technique of privilege escalation through exploitation of implementation flaws in browser security models.