Summary The platform does not filter input and encode output in Quick Post validation error message, which can deliver an XSS payload.

Details Old CVE fixed the XSS in label HTML but didn’t fix it when clicking save.

PoC 1. Login at admin 2. Go to setting 3. Create a Section 4. On Entry page, click Edit label 5. Inject the XSS payload into the label and save 6. On the admin dashboard choose new widget -> Quick Post 7. In Quick Post, click save with blank slug; The XSS will be executed

"errors":{"title":["<script>alert('nono')</script> cannot be blank."],"slug":["Slug cannot be blank."]

Fixed in https://github.com/craftcms/cms/commit/9d0cd0bda7c8a830a3373f8c0f06943e519ac888