Medium (5.3). EPSS 1.8% chance of exploitation in the next 30 days.
On pages containing an iframe, the "data:" protocol can be used to create a modal alert that will render over arbitrary domains following page navigation, spoofing of the origin of the modal alert from the iframe content. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
Severity
CVSS 3.0
5.3 medium
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
EPSS
1.8% chance of exploitation in the next 30 days; higher than 78% of CVEs (scores of 2026-11-01)