CVE-2026-100816 in Firefox
Summary
by MITRE • 09/29/2026
Site isolation issue in the DOM: Networking 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 disclosed vulnerability represents a critical failure in the site isolation mechanisms within the Document Object Model networking subsystem of Mozilla Firefox browsers, specifically affecting versions prior to ESR 153.4 and version 157. Site isolation is a fundamental security architecture designed to prevent cross-site scripting attacks by ensuring that resources from different origins are processed in separate processes or contexts. When this mechanism fails within the networking component, it creates an environment where malicious actors can potentially bypass these boundaries, leading to unauthorized data access across domain lines. This flaw undermines one of the primary defenses against modern web-based threats, exposing users to sophisticated attacks that rely on leaking sensitive information from isolated browsing sessions.
From a technical perspective, the defect likely involves improper handling of network requests or responses within the DOM context, allowing scripts originating from an untrusted site to interact with resources belonging to another origin. This misconfiguration may stem from race conditions in process separation logic or insufficient validation of cross-origin resource sharing policies during dynamic content loading. Such vulnerabilities are particularly dangerous because they operate at a low level within the browser engine, making them difficult for standard web application firewalls or client-side security scripts to detect and mitigate. The exploitation typically requires user interaction, such as visiting a maliciously crafted webpage that triggers the flawed networking behavior, but once triggered, it can lead to significant data exfiltration without raising immediate alarms in typical browsing environments.
The operational impact of this vulnerability is severe, primarily centering on cross-site information leakage and potential session hijacking. Attackers could exploit this flaw to read sensitive content from other open tabs or windows that belong to different domains, such as banking portals, email services, or corporate intranets. This capability effectively neutralizes the privacy protections offered by modern browsers, allowing for detailed profiling of user behavior and credentials theft through side-channel attacks or direct DOM manipulation. In enterprise environments where employees access multiple sensitive applications simultaneously, this vulnerability poses a substantial risk to organizational data security, potentially leading to compliance violations under standards like GDPR or HIPAA if protected health information or personally identifiable information is compromised.
To mitigate the risks associated with this site isolation failure, immediate action must be taken to update all affected Firefox installations to version 153.4 for Extended Support Release users and version 157 for standard release users. Organizations should enforce automated patch management policies to ensure that browser updates are deployed promptly across all endpoints. Additionally, implementing strict Content Security Policy headers on web applications can provide an additional layer of defense by restricting the sources from which content can be loaded, thereby reducing the attack surface even if a browser vulnerability is present. Security teams should also monitor for indicators of compromise related to unusual network traffic patterns or unexpected DOM access events that might signal exploitation attempts in progress.
This issue aligns with Common Weakness Enumeration category CWE-200, which covers exposure of sensitive information to unauthorized actors, and falls under the MITRE ATT&CK technique T1537, involving communication through a compromised component. The failure of site isolation mechanisms is increasingly recognized as a high-severity flaw in browser security architectures, reflecting the ongoing arms race between web developers implementing robust sandboxing techniques and attackers finding novel ways to escape these boundaries. Continuous monitoring of vendor advisories and adherence to recommended update schedules remain the most effective strategies for maintaining resilience against such sophisticated vulnerabilities that target core browsing functionalities.