fix(security): fail closed on hostile clipboard throw values

src/extensions/SafeClipboard.ts

@@ -58,6 +58,8 @@ const ERROR_MESSAGES: Readonly<Record<ClipboardSanitizationErrorCode, string>> =
     invalid_html: 'Rich clipboard HTML could not be sanitized.',
   });
 
+const CLIPBOARD_SANITIZATION_ERRORS = new WeakSet<object>();
+
 /** Error whose stable code and message never disclose clipboard content. */
 export class ClipboardSanitizationError extends Error {
   /** Machine-readable rejection category safe for host telemetry. */
@@ -68,9 +70,17 @@ export class ClipboardSanitizationError extends Error {
     super(ERROR_MESSAGES[code]);
     this.name = 'ClipboardSanitizationError';
     this.code = code;
+    CLIPBOARD_SANITIZATION_ERRORS.add(this);
   }
 }
 
+/** Return whether an unknown value is a genuine module-created sanitizer error. */
+export function isClipboardSanitizationError(
+  value: unknown,
+): value is ClipboardSanitizationError {
+  return CLIPBOARD_SANITIZATION_ERRORS.has(value as object);
+}
+
 interface ResolvedClipboardConfig {
   readonly maxHtmlBytes: number;
   readonly maxNodes: number;
@@ -241,7 +251,7 @@ function resolveClipboardConfig(
     });
   } catch (error) {
     if (
-      error instanceof ClipboardSanitizationError &&
+      isClipboardSanitizationError(error) &&
       error.code === 'invalid_configuration'
     ) {
       throw error;
@@ -577,7 +587,7 @@ export function sanitizeRichClipboardHtml(
     }
     return outputContainer.innerHTML;
   } catch (error) {
-    if (error instanceof ClipboardSanitizationError) throw error;
+    if (isClipboardSanitizationError(error)) throw error;
     throw new ClipboardSanitizationError('invalid_html');
   }
 }
@@ -620,10 +630,9 @@ export const SafeClipboard = Extension.create<SafeClipboardOptions>({
           : this.options.config;
       return sanitizeRichClipboardHtml(html, config, this.options.document);
     } catch (error) {
-      const clipboardError =
-        error instanceof ClipboardSanitizationError
-          ? error
-          : new ClipboardSanitizationError('invalid_html');
+      const clipboardError = isClipboardSanitizationError(error)
+        ? error
+        : new ClipboardSanitizationError('invalid_html');
       try {
         this.options.onError?.(clipboardError);
       } catch {
@@ -634,4 +643,4 @@ export const SafeClipboard = Extension.create<SafeClipboardOptions>({
   },
 });
 
-export default SafeClipboard;
+export default SafeClipboard;

src/extensions/SafeClipboardExtension.hostileThrow.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it, vi } from 'vitest';
+import {
+  DEFAULT_CLIPBOARD_HTML_BYTES,
+  DEFAULT_CLIPBOARD_MAX_DEPTH,
+  DEFAULT_CLIPBOARD_MAX_NODES,
+  type ClipboardSanitizationError,
+} from './SafeClipboard.js';
+import {
+  SafeClipboard,
+  type SafeClipboardOptions,
+} from './SafeClipboardExtension.js';
+
+/**
+ * Exercise the real ProseMirror paste transform with a hostile value thrown by
+ * host option access. The thrown proxy must never be inspected by Inkspan.
+ */
+describe('SafeClipboard hostile thrown-value containment', () => {
+  it('fails closed without prototype inspection when a config getter throws a proxy', () => {
+    const privateSentinel = new Error('private prototype sentinel');
+    const hostileThrownValue = new Proxy(Object.create(null) as object, {
+      getPrototypeOf() {
+        throw privateSentinel;
+      },
+    });
+    const onError = vi.fn((_error: ClipboardSanitizationError) => undefined);
+    const hostileOptions = {
+      get config(): never {
+        throw hostileThrownValue;
+      },
+      maxHtmlBytes: DEFAULT_CLIPBOARD_HTML_BYTES,
+      maxNodes: DEFAULT_CLIPBOARD_MAX_NODES,
+      maxDepth: DEFAULT_CLIPBOARD_MAX_DEPTH,
+      onError,
+      document,
+    } as SafeClipboardOptions;
+
+    const addPlugins = SafeClipboard.config.addProseMirrorPlugins;
+    if (!addPlugins) throw new Error('SafeClipboard plugin factory is unavailable');
+    const plugins = addPlugins.call({ options: hostileOptions } as never);
+    const plugin = plugins[0];
+    const transform = plugin?.props.transformPastedHTML;
+    if (!plugin || !transform) {
+      throw new Error('SafeClipboard paste transform is unavailable');
+    }
+
+    let transformed: string | undefined;
+    expect(() => {
+      transformed = transform.call(plugin, '<p>private source</p>', {} as never);
+    }).not.toThrow();
+
+    expect(transformed).toBe('');
+    expect(onError).toHaveBeenCalledTimes(1);
+    expect(onError).toHaveBeenCalledWith(
+      expect.objectContaining({
+        code: 'invalid_html',
+        message: 'Rich clipboard HTML could not be sanitized.',
+      }),
+    );
+  });
+});

src/extensions/SafeClipboardExtension.ts

@@ -9,6 +9,7 @@ import {
   DEFAULT_CLIPBOARD_HTML_BYTES,
   DEFAULT_CLIPBOARD_MAX_DEPTH,
   DEFAULT_CLIPBOARD_MAX_NODES,
+  isClipboardSanitizationError,
   sanitizeRichClipboardHtml,
   type ClipboardConfig,
 } from './SafeClipboard.js';
@@ -55,10 +56,9 @@ function transformPastedClipboardHtml(
         : options.config;
     return sanitizeRichClipboardHtml(html, config, options.document);
   } catch (error) {
-    const clipboardError =
-      error instanceof ClipboardSanitizationError
-        ? error
-        : new ClipboardSanitizationError('invalid_html');
+    const clipboardError = isClipboardSanitizationError(error)
+      ? error
+      : new ClipboardSanitizationError('invalid_html');
     try {
       options.onError?.(clipboardError);
     } catch {