Vyper's `slice()` may elide side-effects when output length is 0
Impact
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:
https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319
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:
https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191
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 following example illustrates how the issue would look in user code
counter: public(uint256)
@external
def test() -> Bytes[10]:
b: Bytes[10] = slice(msg.data, self.side_effect(), 0)
return b
def side_effect() -> uint256:
self.counter += 1
return 0the severity assigned is low, since this is not a very useful pattern and unlikely to be found in user code.
Patches
the fix is tracked in https://github.com/vyperlang/vyper/pull/4645, which disallows any invocation of slice() with length 0, including for the ad hoc locations discussed in this advisory.
Workarounds
Is there a way for users to fix or remediate the vulnerability without upgrading?
References
Are there any links users can visit to find out more?
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | vyper | <= 0.4.2rc1 | no fix |
References
- https://github.com/vyperlang/vyper/security/advisories/GHSA-3vcg-j39x-cwfm
- https://nvd.nist.gov/vuln/detail/CVE-2025-47774
- https://github.com/vyperlang/vyper/pull/4645
- https://github.com/vyperlang/vyper
- https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L315-L319
- https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/codegen/core.py#L189-L191
- https://pypi.org/project/vyper
- https://github.com/advisories/GHSA-3vcg-j39x-cwfm
- CVSS vector
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- Weakness (CWE)
CWE-691- Platform
- Python (PyPI)
- Package
vyper- Disclosed
- 16.05.2025
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-2028 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-3vcg-j39x-cwfm - CC BY 4.0
Vulnerability history: vyper
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 47
- First → latest
- 03.2020 10.2026
- Without a fix
- 5
- Exploited in attacks
- 0
Severity breakdown
- Critical 3
- High 14
- Medium 20
- Low 10
Disclosures per year
- 2020: 1
- 2021: 5
- 2022: 4
- 2023: 14
- 2024: 14
- 2025: 6
- 2026: 3
-
Vyper's `slice()` may elide side-effects when output length is 0 this entry
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