Adding %00 and a fake extension to a jar: URL changed the interpretation of the contents

Vulnerability Description

jar: URLs retrieve local file content packaged in a ZIP archive. The null and everything after it was ignored when retrieving the content from the archive, but the fake extension after the null was used to determine the type of content. This could have been used to hide code in a web extension disguised as something else like an image. This vulnerability was fixed in Firefox 136, Firefox ESR 128.8, Thunderbird 136, and Thunderbird 128.8.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1940027
• https://www.mozilla.org/security/advisories/mfsa2025-14/
• https://www.mozilla.org/security/advisories/mfsa2025-16/
• https://www.mozilla.org/security/advisories/mfsa2025-17/
• https://www.mozilla.org/security/advisories/mfsa2025-18/