CVE-2025-47774: Vyper's `slice()` may elide side-effects when output length is 0

Impact the slice() builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (msg.data or <address>.code). the reason is that for these source locations, the check that length >= 1 is skipped: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319

the result is that a 0-length bytestring constructed with slice can be passed to makebytearraycopier, which elides evaluation of its source argument when the max length is 0: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191

the impact is that side effects in the start argument may be elided when the length argument is 0, e.g. slice(msg.data, self.dosideeffect(), 0).

the following example illustrates how the issue would look in user code vyper counter: public(uint256)

@external def test() -> Bytes[10]: b: Bytes[10] = slice(msg.data, self.sideeffect(), 0) return b

def sideeffect() -> uint256: self.counter += 1 return 0

the severity assigned is low, since this is not a very useful pattern and unlikely to be found in user code.

Patches

the fix is tracked in https://github.com/vyperlang/vyper/pull/4645, which disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory.

Workarounds Is there a way for users to fix or remediate the vulnerability without upgrading?

References Are there any links users can visit to find out more?