Vyper negative array index bounds checks

Summary

Arrays can be keyed by a signed integer, while they are defined for unsigned integers only. The typechecker doesn't throw when spotting the usage of an int as an index for an array. Typically, negative integers are filtered out at runtime by the bounds checker, but small enough (i.e. large in magnitude, ex. -2**255 + 5) quantities combined with large enough arrays (at least 2**255 in length) can pass the bounds checker, resulting in unexpected behavior.

A contract search was performed, and no production contracts were found to be impacted.

Details

The typechecker allows the usage of signed integers to be used as indexes to arrays. The vulnerability is present in different forms in all versions. Here is an example from 0.3.10: https://github.com/vyperlang/vyper/blob/c150fc49ee9375a930d177044559b83cb95f7963/vyper/semantics/types/subscriptable.py#L127-L137

As can be seen, the validation is performed against IntegerT.any().

PoC

If the array is sufficiently large, it can be indexed with a negative value:

arr: public(uint256[MAX_UINT256])

@external
def set(idx: int256, num: uint256):
    self.arr[idx] = num

For signed integers, the 2's complement representation is used. Because the array was declared very large, the bounds checking will pass (negative values will simply be represented as very large numbers): https://github.com/vyperlang/vyper/blob/a1fd228cb9936c3e4bbca6f3ee3fb4426ef45490/vyper/codegen/core.py#L534-L541

Patches

Patched in https://github.com/vyperlang/vyper/pull/3817.

Impact

There are two potential vulnerability classes: unpredictable behavior and accessing inaccessible elements.

1. If it is possible to index an array with a negative integer without reverting, this is most likely not anticipated by the developer and such accesses can cause unpredictable behavior for the contract.

2. If a contract has an invariant in the form assert index < x where both index and x are signed integers, the developer might suppose that no elements on indexes y | y >= x are accessible. However, by using negative indexes this can be bypassed.

The contract search found no production contracts impacted by these two classes of issues.