CVE-2025-27104: double eval in For List Iter in Vyper

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, rangescope forces iterlist to be parsed in a constant context, which is checked against isconstant).

python def parseForlist(self): with self.context.rangescope(): iterlist = Expr(self.stmt.iter, self.context).irnode ...

def rangescope(self): prevvalue = self.inrangeexpr self.inrangeexpr = True yield self.inrangeexpr = prevvalue

def isconstant(self): return self.constancy is Constancy.Constant or self.inrangeexpr

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, iterlist 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, iterlist 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, iterlist 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]