Always-Incorrect Control Flow Implementation

Summary

Vyper is a Pythonic smart Contract Language for the EVM. When the Vyper Compiler uses the precompiles EcRecover (0x1) and Identity (0x4), the success flag of the call is not checked. As a consequence, an attacker can provide a specific amount of gas to make these calls fail but let the overall execution continue. Then the execution result can be incorrect. Based on EVM's rules, after the failed precompile, the remaining code has only 1/64 of the pre-call-gas left (as 63/64 were forwarded and spent). Hence, only fairly simple executions can follow the failed precompile calls. Therefore, we found no significant impact on real-world contracts. Nonetheless, an advisory has been made out of an abundance of caution. This issue affects versions prior to 0.4.1rc1.