CVE-2025-47285 Vyper's `concat()` builtin may elide side-effects for zero-length arguments
Vyper compiler may skip side effects in zero-length concat arguments, potentially altering smart contract logic execution.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2025-47285?
CVE-2025-47285 is a low-severity vulnerability in vyperlang vyper, published on 15 May 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-691
Which versions are affected by CVE-2025-47285?
3 open source packages ship an affected version, including vyper.
| Package | Affected | Fixed in |
|---|---|---|
vyper PyPI | All versions to 0.4.2rc1 | None |
vyper PyPI | All versions to 0.4.2rc1 | None |
vyper PyPI | All versions to 0.4.2rc1 | None |
How do I fix CVE-2025-47285?
No fix has been published for CVE-2025-47285 yet. Limit exposure of the affected software until one is available.
Is CVE-2025-47285 being exploited?
There is no known exploitation of CVE-2025-47285 in the wild and no public exploit code.
Sources
Also known as GHSA-qhr6-mgqr-mchm, PYSEC-2026-2029.