CVE-2025-5266 in Firefox
Summary
by MITRE • 05/27/2025
Script elements loading cross-origin resources generated load and error events which leaked information enabling XS-Leaks attacks. This vulnerability affects Firefox < 139 and Firefox ESR < 128.11.
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Analysis
by VulDB Data Team • 08/17/2025
This vulnerability in Firefox represents a significant information disclosure flaw that enables cross-site leaks attacks through improper handling of script element events. The issue specifically manifests when script elements load cross-origin resources and generate load and error events that inadvertently expose sensitive information to malicious actors. The vulnerability stems from the browser's insufficient isolation mechanisms between different origin contexts, allowing attackers to infer information about cross-origin resources through timing analysis and event propagation patterns.
The technical flaw lies in how Firefox processes load and error events generated by script elements that reference resources from different origins. When these cross-origin script elements trigger events, the browser's event handling system does not properly sanitize the information flow, creating observable differences in timing and event processing that can be exploited by attackers. This behavior violates fundamental security principles of origin isolation and cross-origin resource protection that are essential for maintaining web application security boundaries.
The operational impact of this vulnerability is substantial as it enables sophisticated XS-Leaks attacks that can be used to track users across different websites, infer their browsing behavior, and potentially extract sensitive information from other origins. Attackers can leverage this vulnerability to perform user fingerprinting, track user sessions, and even determine if specific resources are accessible to users, which could reveal personal information or security credentials. The vulnerability affects a wide range of Firefox versions including those below 139 and Firefox ESR versions below 128.11, representing a significant portion of deployed browser installations.
Security researchers have classified this vulnerability under the CWE-200 category for exposure of sensitive information and potentially related to CWE-1247 for improper handling of cross-origin resources. The attack vector aligns with ATT&CK technique T1566.002 for credential access through web application attacks, and T1589.002 for reconnaissance through web application fingerprinting. The vulnerability demonstrates the ongoing challenges in implementing proper cross-origin isolation mechanisms in modern browsers where event handling and resource loading interactions can create covert channels for information leakage.
Organizations should immediately update their Firefox installations to versions 139 or later for regular Firefox releases and 128.11 or later for Firefox ESR to mitigate this vulnerability. System administrators should also implement network-level monitoring to detect potential exploitation attempts and consider deploying additional security controls such as Content Security Policy headers that restrict script execution from untrusted origins. The vulnerability highlights the importance of continuous security assessment of browser event handling mechanisms and proper implementation of cross-origin resource sharing policies that prevent information leakage through seemingly benign event processing.