CVE-2017-7820 in Firefox
Summary
by MITRE
The "instanceof" operator can bypass the Xray wrapper mechanism. When called on web content from the browser itself or an extension the web content can provide its own result for that operator, possibly tricking the browser or extension into mishandling the element. This vulnerability affects Firefox < 56.
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Analysis
by VulDB Data Team • 01/15/2021
The vulnerability described in CVE-2017-7820 represents a significant bypass of security mechanisms within the Firefox browser's Xray wrapper system. This flaw specifically targets the instanceof operator implementation and demonstrates how web content can manipulate the behavior of core JavaScript operators to circumvent security protections. The Xray wrapper mechanism was designed to provide a secure isolation layer between privileged browser code and untrusted web content, ensuring that malicious scripts cannot directly access or manipulate internal browser objects and their properties. When the instanceof operator is invoked on objects from the browser environment or extensions, the vulnerability allows web content to provide its own custom result rather than adhering to the expected browser behavior.
The technical exploitation of this vulnerability occurs through the manipulation of the instanceof operator's behavior within the browser's security context. This operator normally performs type checking by traversing the prototype chain of objects to determine their inheritance relationships. However, the flaw allows malicious web content to override or manipulate this behavior, potentially causing the browser or extension to incorrectly identify object types. This bypass mechanism can be particularly dangerous because it operates at the fundamental level of JavaScript type checking, which is used extensively throughout browser security systems to validate object access and prevent unauthorized operations. The vulnerability specifically affects Firefox versions prior to 56, indicating that it was a targeted issue within a specific release cycle where the security boundaries were not properly enforced.
The operational impact of this vulnerability extends beyond simple type checking bypasses and can potentially enable more sophisticated attacks within the browser environment. Attackers could leverage this weakness to manipulate how browser security systems interpret object relationships, possibly allowing them to access privileged objects or methods that should be restricted to trusted browser code. This type of vulnerability falls under CWE-254, which addresses security weaknesses related to improper handling of security-sensitive operations. The attack surface is particularly concerning because it affects the core security infrastructure that protects browser internals from malicious web content. The vulnerability could enable attackers to escalate privileges or bypass security controls that depend on proper instanceof behavior, creating potential pathways for further exploitation within the browser sandbox.
This vulnerability demonstrates the complexity of implementing secure object isolation mechanisms in web browsers and highlights the importance of proper operator behavior enforcement in security-critical contexts. The Xray wrapper system's failure to properly restrict instanceof operations represents a fundamental breakdown in the security model that separates browser internals from user content. From an ATT&CK perspective, this vulnerability could be categorized under privilege escalation techniques, as it allows attackers to bypass security boundaries that should prevent unauthorized access to privileged browser objects. The mitigation for this vulnerability required Firefox developers to strengthen the Xray wrapper implementation and ensure that the instanceof operator maintains its security properties even when invoked from untrusted contexts. Browser vendors must continuously evaluate and enhance their security boundaries to prevent such bypasses that can undermine the fundamental security model of web browsers. The fix implemented in Firefox 56 likely involved tightening the restrictions on how instanceof operations are handled when crossing security boundaries between browser code and web content, ensuring that the operator behaves consistently regardless of the calling context.