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, 11 insertions, 0 deletions

diff --git a/lib/invoices.ts b/lib/invoices.ts
index ba55b77c..7f25f4ba 100644
--- a/lib/invoices.ts
+++ b/lib/invoices.ts
@@ -84,6 +84,17 @@ export const generate = async ({ invoice, user }) => {
   if (memo && memo.length > 5000) fail("memo too long");
 
   if (!id) id = v4();
+  else {
+    // A caller-supplied id re-uses an existing record (tip/webhook update,
+    // repeated bolt12 offer create). It must be the caller's own: the write
+    // below replaces `invoice:<id>` wholesale with uid = this user, and the
+    // old bolt11 pointer keeps resolving to the same id, so overwriting
+    // another user's invoice re-routes whatever their payer sends into this
+    // account. Same error as a missing invoice so ids can't be probed for
+    // existence.
+    const existing = await getInvoice(id);
+    if (existing && existing.uid !== user.id) fail("invoice not found");
+  }
 
   let hash;
   let text;