CVE-2025-47285 — Vyper's `concat()` builtin may elide side-effects for zero-length arguments

Affected: vyperlang vyper

Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `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. 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 fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.

GHSA-qhr6-mgqr-mchm — Vyper's `concat()` builtin may elide side-effects for zero-length arguments

Affected: PyPI: vyper

### 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?_

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CVE-2025-47285 — Vyper's `concat()` builtin may elide side-effects for zero-length arguments

GHSA-qhr6-mgqr-mchm — Vyper's `concat()` builtin may elide side-effects for zero-length arguments