CVE-2026-100809 in Firefox
Summary
by MITRE • 09/29/2026
Same-origin policy bypass in the DevTools component. This vulnerability was fixed in Firefox ESR 153.4 and Firefox 157.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical failure in the Same-Origin Policy enforcement mechanisms within the browser's Developer Tools subsystem, specifically affecting versions prior to Firefox ESR 153.4 and Firefox 157. The Same-Origin Policy is a fundamental security concept for web browsers that restricts how a document or script loaded from one origin can interact with resources from another origin. This isolation mechanism prevents malicious scripts on one page from obtaining access to sensitive data on another page through the Document Object Model, thereby mitigating cross-site scripting attacks and unauthorized data exfiltration. When this policy is bypassed within the DevTools component, it creates a significant security gap because Developer Tools are granted elevated privileges by design, allowing them to inspect and manipulate web content more deeply than standard scripts.
The technical flaw likely stems from an improper validation of origin contexts when handling cross-origin resources through the debugging interface. In many browser architectures, the DevTools panel operates in a separate process or context that requires careful boundary checks to ensure it does not inadvertently expose sensitive data across origins. If the implementation fails to correctly isolate these contexts during specific interactions, such as inspecting network requests, examining DOM elements from cross-origin iframes, or debugging scripts loaded via different protocols, an attacker could exploit this inconsistency. By crafting a malicious webpage that leverages the DevTools interface in conjunction with other browser features, it is possible to trick the engine into revealing information belonging to a different origin than intended by the security model.
From an operational impact perspective, this vulnerability allows for potential data leakage and unauthorized access to sensitive web application states. An attacker could use this bypass to read cookies, local storage contents, or DOM structures from high-value targets such as banking portals, email services, or corporate intranets if a user is tricked into visiting the malicious site while having DevTools open or under specific conditions that trigger the flaw. This undermines the trust model of the web platform and can lead to account takeover, session hijacking, or sensitive information disclosure. The severity is heightened by the fact that Developer Tools are often used during development and debugging phases where security boundaries might be relaxed or misconfigured, making this a high-risk vector for targeted attacks against developers or users who frequently utilize these tools.
This vulnerability aligns with CWE-200, which classifies it as an Information Exposure Vulnerability, specifically detailing how sensitive information is disclosed to unauthorized actors due to improper control of security-relevant data flow. In terms of the MITRE ATT&CK framework, this behavior maps to T1539, Steal Web Session Cookie, and potentially T1078, Valid Accounts, if the bypass allows for deeper session manipulation or credential theft through DOM access. The exploitation technique would likely fall under T1204, User Execution, requiring a user to interact with a malicious site that triggers the DevTools context mismatch.
Mitigation strategies primarily involve applying the vendor-provided patches available in Firefox ESR 153.4 and Firefox 157 immediately across all affected systems. For organizations managing large fleets of browsers, automated deployment tools should be utilized to ensure rapid patching. In environments where immediate patching is not feasible, network-level controls such as strict Content Security Policy headers can help mitigate some downstream effects by restricting resource loading origins, although this does not fix the underlying browser flaw. Additionally, security awareness training for developers and IT staff regarding the risks of using Developer Tools in untrusted or public computing environments can reduce the attack surface. Regular vulnerability scanning and monitoring for exploitation attempts related to DevTools anomalies are recommended as part of a comprehensive defense-in-depth strategy.