DOMPurify allows tampering by prototype pollution

It has been discovered that malicious HTML using special nesting techniques can bypass the depth checking added to DOMPurify in recent releases. It was also possible to use Prototype Pollution to weaken the depth check.

This renders dompurify unable to avoid XSS attack.

Fixed by https://github.com/cure53/DOMPurify/commit/1e520262bf4c66b5efda49e2316d6d1246ca7b21 (3.x branch) and https://github.com/cure53/DOMPurify/commit/26e1d69ca7f769f5c558619d644d90dd8bf26ebc (2.x branch).