CVE-2025-47285

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.

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.

### 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?_

### 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?_

Vyper es el lenguaje de programación Pythonic para la Máquina Virtual de Ethereum. En versiones hasta la 0.4.2rc1 (incluida), `concat()` puede omitir la evaluación de efectos secundarios cuando la longitud de un argumento es cero. Esto se debe a una ruta rápida en la implementación que omite la evaluación de expresiones de argumentos cuando su longitud es cero. En la práctica, sería muy inusual en el código de usuario construir cadenas de bytes de longitud cero utilizando una expresión con efectos secundarios, ya que estas cadenas se construyen típicamente con el literal vacío `b""`; la única manera de construir una cadena de bytes vacía con efectos secundarios sería con el operador ternario introducido en la v0.3.8, por ejemplo, `b"" if self.do_some_side_effect() else b""`. La corrección está disponible en la solicitud de incorporación de cambios 4644 y se espera que forme parte de la versión 0.4.2. Como workaround, no se deben incluir efectos secundarios en expresiones que construyen cadenas de bytes de longitud cero.