Merge pull request #250 from microsoft/user/gnjoseph/pr-248-ocr-xss-fix

fix(ocr): sanitize Graph validationToken echo to prevent reflected XSS

Author: dluces

AI/ocr/server/onReceiptAdded.ts

@@ -6,10 +6,23 @@ import { ReceiptProcessor } from "./ReceiptProcessor";
 
 require('isomorphic-fetch');
 
+  // Microsoft Graph sends an opaque, URL-safe validationToken when a subscription is
+  // created and expects it echoed back verbatim as text/plain. The value is attacker
+  // influenceable, so strip anything outside the token's known-safe character set
+  // before reflecting it. This is a no-op for legitimate tokens (base64url/base64)
+  // while removing the characters needed for reflected cross-site scripting.
+  const sanitizeValidationToken = (value: unknown): string =>
+    String(value).replace(/[^A-Za-z0-9._~+/=\- ]/g, '');
+
   export const onReceiptAdded = async (req: Request, res: Response) => {
     const validationToken = req.query['validationToken'];
     if (validationToken) {
-      res.status(200).type('text/plain').send(String(validationToken));
+      const safeValidationToken = sanitizeValidationToken(validationToken);
+      res
+        .status(200)
+        .type('text/plain')
+        .set('X-Content-Type-Options', 'nosniff')
+        .send(safeValidationToken);
         return;
     }