# CVE-2025-47285

## Summary

- **CVE ID:** CVE-2025-47285
- **Severity:** LOW
- **CVSS Score:** 2.9 (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)
- **CWE:** CWE-691
- **Published:** May 15, 2025
- **Last Modified:** Mar 13, 2026

## Description

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.

## Affected Products

- vyperlang — vyper (<= 0.4.2rc1)

## References

- [CNA](https://github.com/vyperlang/vyper/security/advisories/GHSA-qhr6-mgqr-mchm)
- [CNA](https://github.com/vyperlang/vyper/pull/4644)
- [CNA](https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.44%
- **EPSS Percentile:** 37.4

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._