# CVE-2025-47774: Vyper's `slice()` may elide side-effects when output length is 0

> Vyper compiler may skip side effects in slice operations with zero length, 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-3vcg-j39x-cwfm, PYSEC-2026-2028

## What is CVE-2025-47774?

CVE-2025-47774 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, the `slice()` builtin can elide side effects when the output length is 0, and the source bytestring is a builtin (`msg.data` or `<address>.code`). The reason is that for these source locations, the check that `length >= 1` is skipped. The result is that a 0-length bytestring constructed with slice can be passed to `make_byte_array_copier`, which elides evaluation of its source argument when the max length is 0. The impact is that side effects in the `start` argument may be elided when the `length` argument is 0, e.g. `slice(msg.data, self.do_side_effect(), 0)`. The fix in pull request 4645 disallows any invocation of `slice()` with length 0, including for the ad hoc locations discussed in this advisory. The fix is expected to be part of version 0.4.2.

Weakness: CWE-691 (Insufficient Control Flow Management)

## Which versions are affected by CVE-2025-47774?

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-2025-47774?

No fix has been published for CVE-2025-47774 yet. Limit exposure of the affected software until one is available.

## Is CVE-2025-47774 being exploited?

There is no known exploitation of CVE-2025-47774 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-47774 and fix it if it is affected.

What it is: CVE-2025-47774 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-2025-47774 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-2025-47774/ (Markdown: https://insights.sbomify.com/CVE-2025-47774/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-47774
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-47774
- OSV: https://osv.dev/vulnerability/CVE-2025-47774

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-47774/
Summaries are generated and may contain mistakes; the official description above is the record.