CVE-2018-5136 in Firefox
Summary
by MITRE
A shared worker created from a "data:" URL in one tab can be shared by another tab with a different origin, bypassing the same-origin policy. This vulnerability affects Firefox < 59.
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Analysis
by VulDB Data Team • 02/18/2020
This vulnerability represents a critical bypass of the fundamental same-origin policy mechanism that governs web browser security models. The issue resides in how Firefox handled shared workers when initialized from data URLs, creating a scenario where tabs with different origins could inadvertently share the same worker instance. This flaw directly contravenes the core security principle that isolates execution contexts between different origins to prevent cross-site scripting attacks and data leakage. The vulnerability specifically impacted Firefox versions prior to 59, where the browser's implementation failed to properly enforce origin boundaries when establishing shared worker connections through data URL schemes.
The technical implementation of this vulnerability stems from Firefox's inadequate validation of origin constraints during shared worker initialization. When a shared worker was created from a "data:" URL in one tab, the browser failed to properly associate this worker with the originating tab's security context. This oversight allowed a second tab from a different origin to access and utilize the same worker instance, effectively creating a communication channel that bypassed normal cross-origin restrictions. The flaw exploited the fact that data URLs are typically treated as having unique origins, but Firefox's shared worker implementation did not properly enforce this isolation when the worker was instantiated from such URLs.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it could enable sophisticated attacks targeting sensitive data exposure and cross-site communication. An attacker could potentially use this vulnerability to extract information from one tab and relay it to another tab with different privileges or origins, creating a vector for data exfiltration and privilege escalation attacks. The vulnerability particularly affects scenarios where users navigate between trusted and untrusted sites, as it could allow malicious sites to establish persistent communication channels through shared workers. This issue aligns with attack patterns described in the attack tree methodology where bypassing same-origin policies creates pathways for advanced persistent threats.
Security researchers classified this vulnerability under the broader category of web browser security flaws that compromise fundamental isolation mechanisms. The flaw demonstrates a failure in Firefox's implementation of the web platform's worker APIs, specifically affecting the shared worker interface that should enforce strict origin-based access controls. Organizations implementing security controls should consider this vulnerability as part of their browser security posture assessment, particularly in environments where users access multiple domains simultaneously. The vulnerability also highlights the importance of proper origin validation in web platform APIs, as outlined in various security standards including those addressing browser security model implementation and cross-origin resource sharing.
Mitigation strategies for this vulnerability require immediate patching of affected Firefox versions to 59 or later, where the implementation correctly enforces shared worker origin boundaries. System administrators should also implement browser hardening measures including disabling shared worker functionality when not required, though this approach may impact legitimate web applications. Organizations should conduct comprehensive security assessments of their browser environments to identify potential exploitation vectors, particularly in scenarios involving multi-tab browsing or applications that utilize shared workers. The vulnerability underscores the necessity of continuous security testing of browser implementations against established security frameworks and standards that govern web application security.