GHSA-4w26-8p97-f4jp
AugAssign evaluation order causing OOB write within the object in Vyper
Packages
Details
As written in the advisory (Markdown source, shown as plain text).
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
```vyper
def poc():
a: DynArray[uint256, 2] = [1, 2]
a[1] += a.pop()
```
is equivalent to:
```vyper
def poc():
a: DynArray[uint256, 2] = [1, 2]
a[1] += a[len(a) - 1]
a.pop()
```
rather than:
```vyper
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
```
References
External links from the advisory. They lead to third-party sites.