CSSOM access to a stylesheet’s rules is gated by CSSStyleSheet::canAccessRules(), which must return false for a cross-origin (not origin-clean) sheet so script cannot read or mutate cross-origin CSS. Two defects made that gate leaky. First, canAccessRules() returned true when ownerDocument() was null: Document* document = ownerDocument(); if (!document) return true;. A CSSStyleSheet wrapper can outlive its owner node / be detached from the document (e.g. the owning element is removed), at which point ownerDocument() is null and the check fell open, granting access to a sheet whose origin can no longer be verified. Second, the origin-clean flag was not propagated correctly when constructing CSSStyleSheet CSSOM wrappers for imported/linked sheets: CSSImportRule::styleSheet(), ProcessingInstruction::setCSSStyleSheet() and HTMLLinkElement::initializeStyleSheet() created wrappers without threading the real CachedCSSStyleSheet::isCORSSameOrigin() value (HTMLLinkElement even had a FIXME and only set originClean when fetch mode was CORS), so a cross-origin sheet could be treated as origin-clean.

The fix (a) makes canAccessRules() return false when there is no owner document, (b) adds explicit canAccessRules() checks to insertRule()/deleteRule() that throw SecurityError for cross-origin sheets, and (c) plumbs the true isCORSSameOrigin/origin-clean flag into CSSStyleSheet::create for import rules, processing instructions and link elements.

The restored invariant is that rule read/write on a stylesheet is permitted only when the sheet is provably same-origin (or CORS-clean), including after it is detached from its document. The added layout test reads a cross-origin sheet after removing it from the document.

A same-origin-policy bypass allowing script to read (and modify) the rules of a cross-origin stylesheet, particularly after detaching it from the document. This is an information-disclosure primitive — cross-origin CSS can encode user-specific state — with no direct memory-safety component; it executes in the WebContent process. Rated medium.