Описание
vyper performs double eval of the slice start/length args in certain cases
Summary
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.
A contract search was performed and no vulnerable contracts were found in production. Having side-effects in the start and length patterns is also an unusual pattern which is not that likely to show up in user code. It is also much harder (but not impossible!) to trigger the bug since 0.3.4 since the unique symbol fence was introduced (https://github.com/vyperlang/vyper/pull/2914).
Details
It can be seen that the _build_adhoc_slice_node function of the slice builtin doesn't cache the mentioned arguments to the stack: https://github.com/vyperlang/vyper/blob/4595938734d9988f8e46e8df38049ae0559abedb/vyper/builtins/functions.py#L244
As such, they can be evaluated multiple times (instead of retrieving the value from the stack).
PoC
with Vyper version 0.3.3+commit.48e326f the call to foo passes the asserts:
Patches
Patched in https://github.com/vyperlang/vyper/pull/3976.
Impact
No vulnerable production contracts were found.
Пакеты
vyper
< 0.4.0
0.4.0
Связанные уязвимости
Vyper is a pythonic Smart Contract Language for the Ethereum virtual machine. In versions 0.3.10 and prior, 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 time of publication, no fixed versions are available.