Vulnerability in transferFrom method

[beillahi]

Hello,

I just want to report what I consider a vulnerability in your ERC20 implementation with regard to the reference specification of ERC20 described here

Description

The methodtransferFrom should have a guard to check that the balance of the address From is bigger or equal to the _value.
Otherwise, a strange behavior is allowed such as described in the following trace:

Initial State : balances[0x2] = balances[0x1] = 0 and allowed[0x2][0x1] = 0

0x2:approve(0x1,1);

State :  balances[0x2] = balances[0x1] = 0 and allowed[0x2][0x1] = 1

0x1:transferFrom(0x2,0x2,1);

State :  balances[0x2] = balances[0x1] = 0 and allowed[0x2][0x1] = 0

The last method invocation should have failed if the method has a guard or if the instructions in lines 30 and 31 are swapped.