Zeppelin Solutions found critical vulnerability affected the production REP token. In a nutshell, an out-of-bounds write on the token’s reputation array allowed an attacker to modify the creation timestamp, making it believe the crowdsale was still ongoing and disabling all token transfers. This critical severity vulnerability, if exploited, could have halted the whole REP economy, worth over $200 million. First, Serpent contracts can overwrite storage locations when accessing arrays out of bounds. This means that if a Serpent contract attempts to access an array at a position greater than the array’s length, Serpent won’t stop it. Second, the Serpent language is untyped. It allows any operation to be performed on any data. Every value is a 256-bit sequence which can be used as an address, a contract, an integer, or an array. Moreover, it performs no checks on the data sent by a user on a transaction.