CVE-2025-6434

The exception page for the HTTPS-Only feature, displayed when a website is opened via HTTP, lacked an anti-clickjacking delay, potentially allowing an attacker to trick a user into granting an exception and loading a webpage over HTTP. This vulnerability affects Firefox < 140 and Thunderbird < 140.

Weakness

The web application does not restrict or incorrectly restricts frame objects or UI layers that belong to another application or domain, which can lead to user confusion about which interface the user is interacting with.

Affected Software

Potential Mitigations

• The use of X-Frame-Options allows developers of web content to restrict the usage of their application within the form of overlays, frames, or iFrames. The developer can indicate from which domains can frame the content.
• The concept of X-Frame-Options is well documented, but implementation of this protection mechanism is in development to cover gaps. There is a need for allowing frames from multiple domains.
• A developer can use a “frame-breaker” script in each page that should not be framed. This is very helpful for legacy browsers that do not support X-Frame-Options security feature previously mentioned.
• It is also important to note that this tactic has been circumvented or bypassed. Improper usage of frames can persist in the web application through nested frames. The “frame-breaking” script does not intuitively account for multiple nested frames that can be presented to the user.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/103.html
• https://cwe.mitre.org/data/definitions/181.html
• https://cwe.mitre.org/data/definitions/222.html
• https://cwe.mitre.org/data/definitions/504.html
• https://cwe.mitre.org/data/definitions/506.html
• https://cwe.mitre.org/data/definitions/587.html
• https://cwe.mitre.org/data/definitions/654.html

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1955182
• https://www.mozilla.org/security/advisories/mfsa2025-51/
• https://www.mozilla.org/security/advisories/mfsa2025-54/