Daily Hack
A fresh 60-second challenge every day. Solve it, earn XP, and keep your streak alive.
Daily Hack #68
Replay the withdrawal
This custodial wallet API signs each withdrawal with a one-time nonce so a request cannot be used twice. Except it never records which nonces it has already spent. Send your fresh withdrawal first to see it succeed, then resend a nonce from an earlier signed withdrawal and watch the same money leave twice. Read the flag the double-spend leaks.
Play freely - sign up to submit your answer and earn XP.
Solved!
+10 XP
First blood! You were first to crack today's hack.
How it works