CVE-2017-7820: Mozilla Firefox

Medium severity, CVSS 5.3. EPSS: 1.2% chance of exploitation in the next 30 days.

The "instanceof" operator can bypass the Xray wrapper mechanism. When called on web content from the browser itself or an extension the web content can provide its own result for that operator, possibly tricking the browser or extension into mishandling the element. This vulnerability affects Firefox < 56.

Affected products

• Mozilla Firefox: up to and including 55.0.3

Published 2018-06-11. Last modified 2026-06-17.