CVE-2025-6429 in Firefox
Summary
by MITRE • 06/24/2025
Firefox could have incorrectly parsed a URL and rewritten it to the youtube.com domain when parsing the URL specified in an `embed` tag. This could have bypassed website security checks that restricted which domains users were allowed to embed. This vulnerability affects Firefox < 140 and Firefox ESR < 128.12.
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Analysis
by VulDB Data Team • 08/17/2025
This vulnerability represents a critical URL parsing flaw in Mozilla Firefox that could enable malicious actors to bypass domain restriction controls implemented by websites. The issue specifically affects how Firefox processes URLs within embed tags, where the browser incorrectly rewrites certain malformed or specially crafted URLs to point to youtube.com domain. This behavior occurs during the URL parsing phase when Firefox encounters embed elements in web content, creating a potential pathway for unauthorized content embedding that circumvents security policies.
The technical implementation of this vulnerability stems from Firefox's insufficient validation of URL schemes and domains when processing embedded content. When parsing an embed tag containing a URL, the browser's URL parser fails to properly validate the target domain against the whitelist or security policies established by the hosting website. This parsing error allows crafted URLs to be rewritten in a manner that redirects the embedding operation to youtube.com, effectively bypassing the intended security boundaries. The flaw demonstrates a classic case of improper input validation and URL normalization that could be exploited by attackers who control content served to vulnerable Firefox users.
The operational impact of this vulnerability extends beyond simple content embedding bypass, as it could enable sophisticated phishing attacks, cross-site scripting scenarios, and unauthorized data exfiltration. Websites that rely on domain-specific embedding restrictions for security purposes would be vulnerable to this attack vector, potentially allowing malicious actors to inject YouTube content from unauthorized domains or manipulate the embedding behavior to redirect users to unintended destinations. This represents a significant weakening of the browser's security model for handling embedded content, particularly affecting users of older Firefox versions where the fix has not yet been deployed.
Organizations and users should immediately update to Firefox version 140 or Firefox ESR version 128.12 to remediate this vulnerability, as the affected versions lack proper URL validation mechanisms. Security teams should review their content security policies to ensure that embedded content restrictions are not solely dependent on browser-side validation, and implement additional server-side controls to prevent unauthorized embedding. The vulnerability aligns with CWE-20 Improper Input Validation and could be categorized under ATT&CK technique T1566.001 Phishing via Social Media, particularly when used in conjunction with malicious embedding to deceive users into interacting with unauthorized content. Organizations should also consider implementing network-level controls and monitoring for suspicious embedding patterns that could indicate exploitation attempts.