CVE-2025-6429: firefox Improper Encoding — Fix

Description

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 was fixed in Firefox 140, Firefox ESR 128.12, Thunderbird 140, and Thunderbird 128.12.

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CVSS v3.1 Score

6.5

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

Weakness Type (CWE)

CWE-116 CWE-116

Affected Products

Vendor	Product	Versions
mozilla	firefox	< 128.12.0
mozilla	firefox	< 140.0

References

Advisories & Patches

https://www.mozilla.org/security/advisories/mfsa2025-51/ https://www.mozilla.org/security/advisories/mfsa2025-53/

Other References

https://bugzilla.mozilla.org/show_bug.cgi?id=1970658 https://www.mozilla.org/security/advisories/mfsa2025-54/ https://www.mozilla.org/security/advisories/mfsa2025-55/ https://lists.debian.org/debian-lts-announce/2025/06/msg00029.html https://lists.debian.org/debian-lts-announce/2025/07/msg00002.html

Frequently Asked Questions

What is CVE-2025-6429? +

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 was fixed in Firefox 140, Firefox ESR 128.12, Thunderbird 140, and Thunderbird 128.12. It has a CVSS v3.1 base score of 6.5 (MEDIUM).

How severe is CVE-2025-6429? +

CVE-2025-6429 has a CVSS v3.1 score of 6.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

What products are affected by CVE-2025-6429? +

CVE-2025-6429 affects products from mozilla, specifically: firefox. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-6429? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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