DOMPurify through 3.3.3 fails to sanitize DOM elements passed via IN_PLACE mode when the element originates from a different window/realm (e.g., an iframe's contentDocument). A cross-realm instanceof check in the private _isNode() function returns false for foreign-realm nodes, causing DOMPurify to stringify the element (yielding '[object HTMLDivElement]'), silently reset IN_PLACE to false, and return the unsanitized element unchanged with any XSS payloads intact.
Exploitability
AV:NAC:HAT:PPR:NUI:PVulnerable System
VC:NVI:NVA:NSubsequent System
SC:LSI:LSA:NCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N