README.md

vulnerable/replay_attack

Vulnerability: Replay Attack (Missing Nonce)

Severity: High

Description

The contract processes signed messages or actions without invalidating them after use. An attacker can capture a valid signed message and replay it multiple times, executing the same action repeatedly.

Exploit Scenario

1. Alice signs a withdrawal request for 100 tokens.
2. The contract processes the request and pays Alice.
3. Attacker replays the same signed message; the contract pays out again.
4. Attacker repeats until the pool is drained.

Vulnerable Code

pub fn withdraw(env: Env, signature: BytesN<32>, amount: i128) {
    verify_signature(&env, &signature, amount);
    // ❌ Missing: mark signature as used
    do_withdraw(&env, amount);
}

Secure Fix

assert!(!is_used(&env, &signature), "signature already used"); // ✅
mark_used(&env, &signature);
do_withdraw(&env, amount);

See the inline secure.rs module inside this crate for the full corrected implementation.

References

• docs/vulnerabilities.md
• docs/threat_model.md