CVE-2026-74981 in Firefox
Summary
by MITRE • 08/18/2026
Site isolation issue in the Audio/Video: Web Codecs component. This vulnerability was fixed in Firefox 154 and Firefox ESR 153.1.
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Analysis
by VulDB Data Team • 08/18/2026
The identified security flaw resides within the Audio Video Web Codecs subsystem of Mozilla Firefox, specifically concerning a failure to properly enforce site isolation boundaries. Site isolation is a critical browser security architecture designed to prevent cross-site data leakage by ensuring that resources from different origins are processed in separate processes with isolated memory spaces. When this mechanism fails, it creates an environment where maliciously crafted web content can potentially access or infer sensitive information belonging to other sites loaded within the same browser instance. This vulnerability was addressed and patched in Firefox version 154 as well as the extended support release branch at version 153.1, indicating that users running older versions remain exposed to potential exploitation vectors related to cross-origin data exposure through media decoding routines.
From a technical perspective, Web Codecs provide low-level APIs for encoding and decoding audio and video streams directly within the browser environment. These components often handle complex binary data structures and interact closely with underlying system resources. The specific defect likely involves improper validation or separation of state between different origin contexts during the processing of media payloads. An attacker could construct a malicious webpage containing specially crafted audio or video files that exploit this lack of isolation. By leveraging the Web Codecs API, such an attack might allow the reading of memory contents from another tab or frame belonging to a trusted site, potentially leading to the exfiltration of session cookies, authentication tokens, or other sensitive user data stored in local storage or indexed databases associated with those sites.
The operational impact of this vulnerability is significant for users who engage in multi-tab browsing scenarios involving both untrusted and highly sensitive websites simultaneously. If successfully exploited, an attacker could perform cross-site information leakage attacks without requiring traditional phishing techniques to trick the user into visiting a malicious site alone. Instead, the attack relies on the victim having another tab open with a target organization or service while interacting with the attacker's content in a separate context. This undermines the fundamental trust model of web browsers and compromises confidentiality guarantees provided by modern security architectures. The severity is further amplified because media processing often runs at high privilege levels to ensure performance, meaning that any boundary breach could have broader implications for system stability and data integrity beyond simple information disclosure.
Mitigation strategies primarily involve ensuring that all users update their Firefox installations to version 154 or later, including the ESR branch updates to 153.1 where applicable. For organizations deploying Firefox across enterprise environments, it is essential to configure automatic update policies to guarantee timely patching of such vulnerabilities as they are disclosed. Additionally, administrators should enforce strict content security policies and consider enabling enhanced tracking protection features which can limit the ability of third-party scripts and embedded media from different origins to interact with each other. While technical mitigations within the browser address the root cause, users should also practice safe browsing habits by avoiding loading untrusted media sources in tabs that contain sensitive session data whenever possible.
This vulnerability aligns with common classification standards such as CWE-200 which covers exposure of sensitive information to an unauthorized actor and CWE-1021 which relates to improper restriction of renderers for cross-origin sites. In terms of the MITRE ATT&CK framework, this flaw facilitates techniques associated with Collection via Compromise or potentially Taint and Probe if used in conjunction with other exploits to map out internal network structures through side-channel analysis. The fix represents a critical step in maintaining the integrity of site isolation guarantees which are foundational to modern web security models like Same-Origin Policy enforcement at the process level rather than just script level.