[jibal]

There's no security issue here. Certainly the OP hasn't explained why there is one.

[jmclnx]

There could be. Say you have '#!sh', someone could have put a broken 'sh' somewhere, that could be the first one the '#!' sees.

[jibal]

No, this is about relative paths, not searching $PATH ..., which it doesn't do. (That happens when people do '#!/usr/bin/env sh' ... the security issues with that are a separate discussion.)