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 ...

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.

Metrics

CVSS 4.0

2.9/10

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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS Probability

0.37%

28.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-691

References

Timeline

Published: May 15, 2025
Last Modified: Jun 17, 2026
Status: Deferred

Frequently Asked Questions

What is 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.

How severe is CVE-2025-47285?

CVE-2025-47285 has a CVSS score of 2.9/10 (LOW severity). The EPSS model estimates a 0.37% probability of exploitation in the next 30 days.

How do I fix CVE-2025-47285?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.