CVE-2025-27104

Multiple evaluation of a single expression is possible in the iterator target of a for loop. While the iterator expression cannot produce multiple writes, it can consume side effects produced in the loop body (e.g. read a storage variable updated in the loop body) and thus lead to unexpected program behavior. Specifically, reads in iterators which contain an ifexp (e.g. `for s: uint256 in ([read(), read()] if True else [])`) may interleave reads with writes in the loop body. The fix is tracked in https://github.com/vyperlang/vyper/pull/4488. ### Vulnerability Details Vyper for loops allow two kinds of iterator targets, namely the `range()` builtin and an iterable type, like SArray and DArray. During codegen, iterable lists are required to not produce any side-effects (in the following code, `range_scope` forces `iter_list` to be parsed in a constant context, which is checked against `is_constant`). ```python def _parse_For_list(self): with self.context.range_scope(): iter_list = Expr(self.stmt.iter, self.context).ir_node ... def range_scope(self): prev_value = self.in_range_expr self.in_range_expr = True yield self.in_range_expr = prev_value def is_constant(self): return self.constancy is Constancy.Constant or self.in_range_expr ``` However, this does not prevent the iterator from consuming side effects provided by the body of the loop. For dynamic arrays, the compiler simply panics: ```vyper x: DynArray[uint256, 3] @external def test(): for i: uint256 in (self.usesideeffect() if True else self.usesideeffect()): pass @view def usesideeffect() -> DynArray[uint256, 3]: return self.x ``` For SArrays on the other hand, `iter_list` is instantiated in the body of a `repeat` ir, so it can be evaluated several times. Here are three illustrating examples. In the first example, the following test case pre-evaluates the iter list and stores the result to a temporary list in memory. So the list is only evaluated once, before entry into the loop body, and the log output will be 0, 0, 0. ```vyper event I: i: uint256 x: uint256 @deploy def __init__(): self.x = 0 @external def test(): for i: uint256 in [self.usesideeffect(), self.usesideeffect(), self.usesideeffect()]: self.x += 1 log I(i) @view def usesideeffect() -> uint256: return self.x ``` However, in the next two examples, because the iterator target is not a list literal, it will be evaluated in the loop body. In the second example, `iter_list` is an ifexp, thus it will be evaluated lazily in the loop body. The log output will be 0, 1, 2 due to consumption of side effects. ```vyper event I: i: uint256 x: uint256 @deploy def __init__(): self.x = 0 @external def test(): for i: uint256 in ([self.usesideeffect(), self.usesideeffect(), self.usesideeffect()] if True else self.otherclause()): self.x += 1 log I(i) @view def usesideeffect() -> uint256: return self.x @view def otherclause() -> uint256[3]: return [0, 0, 0] ``` In the third example, `iter_list` is also an ifexp, thus it will only be evaluated in the loop body. The log output will be 0, 1, 2 due to consumption of side effects. ```vyper event I: i: uint256 x: uint256[3] @deploy def __init__(): self.x = [0, 0, 0] @external def test(): for i: uint256 in (self.usesideeffect() if True else self.otherclause()): self.x[0] += 1 self.x[1] += 1 self.x[2] += 1 log I(i) @view def usesideeffect() -> uint256[3]: return self.x @view def otherclause() -> uint256[3]: return [0, 0, 0] ```

vyper is a Pythonic Smart Contract Language for the EVM. Multiple evaluation of a single expression is possible in the iterator target of a for loop. While the iterator expression cannot produce multiple writes, it can consume side effects produced in the loop body (e.g. read a storage variable updated in the loop body) and thus lead to unexpected program behavior. Specifically, reads in iterators which contain an ifexp (e.g. `for s: uint256 in ([read(), read()] if True else [])`) may interleave reads with writes in the loop body. Vyper for loops allow two kinds of iterator targets, namely the `range()` builtin and an iterable type, like SArray and DArray. During codegen, iterable lists are required to not produce any side-effects (in the following code, `range_scope` forces `iter_list` to be parsed in a constant context, which is checked against `is_constant`). However, this does not prevent the iterator from consuming side effects provided by the body of the loop. For SArrays on the other hand, `iter_list` is instantiated in the body of a `repeat` ir, so it can be evaluated several times. This issue is being addressed and is expected to be available in version 0.4.1. Users are advised to upgrade as soon as the patched release is available. There are no known workarounds for this vulnerability.