invoices: don't let a supplied id overwrite another user's invoice

generate() writes invoice:<id> wholesale with the caller's uid, and the old
bolt11 pointer keeps resolving to that id, so passing someone else's invoice
id re-pointed their invoice at the caller's account. Invoice ids aren't
secret (LUD-21 verify URLs, invoice pages, receipts), so refuse an existing
id that belongs to a different user, both in generate() and in POST /invoice
for anonymous pay-page callers. Both fail with "invoice not found" so ids
can't be probed for existence.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

1 files changed, 10 insertions, 0 deletions

diff --git a/routes/invoices.ts b/routes/invoices.ts
index 9ba45af6..b8d06bba 100644
--- a/routes/invoices.ts
+++ b/routes/invoices.ts
@@ -36,6 +36,16 @@ export default {
     else invoice.own = false;
 
     try {
+      // A supplied id that already exists is an UPDATE of that record —
+      // generate() rewrites it wholesale. Only the authenticated owner may do
+      // that. Without this, an anonymous pay-page session passing
+      // body.user = merchant could rewrite the amount/memo of a merchant's
+      // live invoice. generate() separately refuses to move an invoice to a
+      // different uid.
+      if (invoice?.id) {
+        const existing = await getInvoice(invoice.id);
+        if (existing && existing.uid !== req.user.id) fail("invoice not found");
+      }
       const result = await generate({ invoice, user });
       res.send(result);
     } catch (e) {