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fix(ocr): sanitize Graph validationToken echo to prevent reflected XSS #250

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fix(ocr): sanitize Graph validationToken echo to prevent reflected XSS #250

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15 changes: 14 additions & 1 deletion AI/ocr/server/onReceiptAdded.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
}

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