🛡️ CVE-2025-47285 — vyper

Vyper's concat() builtin may elide side-effects for zero-length arguments

Impact

concat() may skip evaluation of side effects when the length of an argument is zero. this is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero:

https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562

in practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal b""; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. b"" if self.do_some_side_effect() else b"".

the following example demonstrates how the issue would look in user code

```vyper

counter: public(uint256)

@external

def test() -> Bytes[256]:

a: Bytes[256] = concat(b"" if self.sideeffect() else b"", b"aaaa")

return a

def sideeffect() -> bool:

self.counter += 1

return True

```

the severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.

Patches

fix is tracked in https://github.com/vyperlang/vyper/pull/4644

Workarounds

don't have side effects in expressions which construct zero-length bytestrings.

References

_Are there any links users can visit to find out more?_