# 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.

- **Severity:** Low 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 15 May 2025
- **Updated:** 15 May 2025
- **Also known as:** GHSA-qhr6-mgqr-mchm, PYSEC-2026-2029

## 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 (Insufficient Control Flow Management)

## Which versions are affected by CVE-2025-47285?

3 open source packages ship an affected version, including vyper.


| Package | Ecosystem | 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-47285 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2025-47285 in the wild and no public exploit code.

Affected packages and fixed versions:
- vyper (PyPI): affected all versions up to 0.4.2rc1; no fix listed
- vyper (PyPI): affected all versions up to 0.4.2rc1; no fix listed
- vyper (PyPI): affected all versions up to 0.4.2rc1; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-47285/ (Markdown: https://insights.sbomify.com/CVE-2025-47285/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-47285
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-47285
- OSV: https://osv.dev/vulnerability/CVE-2025-47285

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-47285/
Summaries are generated and may contain mistakes; the official description above is the record.