Improper Input Validation

Summary

Vyper is a pythonic Smart Contract Language for the Ethereum virtual machine. In Vyper , using the `slice` builtin can result in a double eval vulnerability when the buffer argument is either `msg.data`, `self.code` or `<address>.code` and either the `start` or `length` arguments have side-effects. It can be easily triggered only with the versions `<0.3.4` as `0.3.4` introduced the unique symbol fence. No vulnerable production contracts were found. Additionally, double evaluation of side-effects should be easily discoverable in client tests. As such, the impact is low. As of the time of publication, no fixed versions are available. This issue affects versions prior to 0.4.0rc6.