CVE-2026-16387 in Firefox
Summary
by MITRE • 07/21/2026
Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153 and Firefox ESR 140.13.
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Analysis
by VulDB Data Team • 07/21/2026
The site isolation vulnerability within the networking component represents a critical security flaw that undermines the fundamental security boundaries established by modern web browsers. This issue stems from insufficient compartmentalization of network resources between different websites, creating potential pathways for malicious actors to exploit cross-site communication channels. The vulnerability allows for unauthorized data leakage and information disclosure between isolated browsing contexts, effectively breaking the security model that separates user sessions and protects against malicious interactions between different origins.
The technical implementation flaw manifests in how the networking component manages resource allocation and memory handling across different site contexts. When site isolation mechanisms fail to properly enforce boundaries between origins, network requests and responses can be inadvertently shared or accessed by unauthorized parties. This weakness particularly affects the handling of HTTP headers, cookies, and other network-level data that should remain isolated between different domains. The vulnerability is classified under CWE-284, which addresses improper access control in network security implementations, specifically targeting weaknesses in privilege management and resource isolation.
The operational impact of this vulnerability extends beyond simple data exposure to encompass potential full system compromise scenarios. Attackers could leverage the site isolation failure to perform cross-site request forgery attacks, steal session tokens, or access sensitive user information from other domains within the same browser instance. The implications are particularly severe for users engaging in high-security activities such as online banking, corporate network access, or handling confidential data. This vulnerability directly relates to ATT&CK technique T1071.004 which covers application layer protocol: dns, where compromised isolation could enable attackers to manipulate DNS resolution and network traffic between different sites.
Mitigation strategies for this vulnerability require immediate deployment of the patched versions mentioned in the advisory, specifically Firefox 153 and Firefox ESR 140.13, which contain the necessary code modifications to restore proper site isolation boundaries. Organizations should implement comprehensive browser update policies ensuring all users receive these security patches promptly. Additional protective measures include monitoring network traffic for anomalous patterns that might indicate exploitation attempts, implementing strict content security policies, and maintaining regular security assessments of browser configurations. The fix addresses core networking stack components and reinforces the essential principle of process isolation that prevents malicious code from accessing resources belonging to other sites or applications within the same browsing environment.