Lab: pass every value cap and lose everything

The signing lesson listed unlimited ERC-20 approvals among the things a value cap does not catch. This is that in fifteen lines, and the reason it deserves a lab is that the dangerous transaction looks like the safest one you will sign all day.

1. 1

   Model an allowance and a guardrail that caps the value moved by any single transaction.

   bash Copy

   cat > /tmp/appr.py <<'EOF'
   MAX_UINT = 2**256 - 1
   allowance = {}
   MAX_TX_VALUE = 100          # the guardrail
   
   def submit(name, value, effect):
       if value > MAX_TX_VALUE:
           return f"  {name}: DENIED (value {value} over cap {MAX_TX_VALUE})"
       effect()
       return f"  {name}: ALLOWED (value {value})"
   
   print(submit("approve(0xDex, MAX_UINT)", 0,
                lambda: allowance.__setitem__("0xDex", MAX_UINT)))
   
   def transfer_from(spender, amount):
       if allowance.get(spender, 0) < amount:
           return f"  DENIED (allowance {allowance.get(spender, 0)})"
       allowance[spender] -= amount
       return f"  moved {amount:,}"
   
   print("\nlater, with no further approval from you and no new signature:")
   print(transfer_from("0xDex", 1_000_000))
   print(transfer_from("0xDex", 5_000_000_000))
   EOF
   python3 /tmp/appr.py

2. 2

   The approval carried zero value, so every value-based control waved it through.

   text Copy

     approve(0xDex, MAX_UINT): ALLOWED (value 0)
   
   later, with no further approval from you and no new signature:
     moved 1,000,000
     moved 5,000,000,000

3. 3

   Clean up.

   bash Copy

   rm /tmp/appr.py

A per-transaction value cap asks how much this transaction moves. An approval moves nothing; it grants standing authority for someone else to move funds later, without another signature from you and without another transaction for your guardrails to inspect.

• The dangerous parameter is the allowance amount, which is data inside the call rather than the transaction's value. A guardrail that only reads value will never see it.
• MAX_UINT is the default in a great deal of tooling, because it saves users a second approval later. Convenience is why it is everywhere.
• Revoking means sending another transaction to set the allowance to zero, and until you do, the grant survives the agent being deleted, its keys being rotated and its policies being revoked.