GHSA-4w26-8p97-f4jp: AugAssign evaluation order causing OOB write within the object in Vyper

Vyper handles AugAssign statements by first caching the target location to avoid double evaluation. However, in the case when target is an access to a DynArray and the rhs modifies the array, the cached target will evaluate first, and the bounds check will not be re-evaluated during the write portion of the statement. In other words, the following code

def poc():
    a: DynArray[uint256, 2] = [1, 2]
    a[1] += a.pop()

is equivalent to:

def poc():
    a: DynArray[uint256, 2] = [1, 2]
    a[1] += a[len(a) - 1]
    a.pop()

rather than:

def poc():
    a: DynArray[uint256, 2] = [1, 2]
    s: uint256 = a[1]
    t: uint256 = a.pop()
    a[1] = s + t  # reverts due to oob access