The vulnerability identified as CVE-2026-100803 affects the WebExtensions component of Mozilla Firefox, enabling a bypass of the same-origin policy. This policy is fundamental to web security, restricting how documents or scripts loaded from one origin can interact with resources from another origin. The flaw was addressed and fixed in specific Firefox ESR and standard releases: ESR 153.4, Firefox 157, ESR 115.42, and ESR 140.17. The vendor advisory confirms the fix and categorizes the impact as high. There are no reports of active exploitation in the wild.

Bypassing the same-origin policy can allow malicious web content to access or manipulate data from other origins, potentially leading to unauthorized data access or actions within the browser context. This undermines a core security boundary in web browsers, increasing the risk of cross-origin attacks. However, no active exploitation has been observed.

Users and administrators should update Firefox to versions ESR 153.4, 157, ESR 115.42, ESR 140.17, or later to remediate this vulnerability. The vendor advisory confirms that these versions contain the official fix. No additional mitigation steps are required beyond applying the update.