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
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 Truethe 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?
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Python (PyPI) | vyper | <= 0.4.2rc1 | sin corrección |
Referencias
- https://github.com/vyperlang/vyper/security/advisories/GHSA-qhr6-mgqr-mchm
- https://nvd.nist.gov/vuln/detail/CVE-2025-47285
- https://github.com/vyperlang/vyper/pull/4644
- https://github.com/vyperlang/vyper
- https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562
- https://pypi.org/project/vyper
- https://github.com/advisories/GHSA-qhr6-mgqr-mchm
- Vector CVSS
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P- Debilidad (CWE)
CWE-691- Plataforma
- Python (PyPI)
- Paquete
vyper- Publicada
- 16.05.2025
- Actualizado
- 11.10.2026 13:18
Fuentes y licencias
OSV / GitHub Advisory Database - PYSEC-2026-2029 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-qhr6-mgqr-mchm - CC BY 4.0
Historial de vulnerabilidades: vyper
Todas las vulnerabilidades publicadas de este paquete (Python (PyPI)), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
- Entradas en total
- 47
- Primera → última
- 03.2020 10.2026
- Sin corrección
- 5
- Explotada en ataques
- 0
Distribución por gravedad
- Crítica 3
- Alta 14
- Media 20
- Baja 10
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- 2020: 1
- 2021: 5
- 2022: 4
- 2023: 14
- 2024: 14
- 2025: 6
- 2026: 3
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