Vyper's concat() builtin may elide side-effects for zero-length arguments

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

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 · score source

EPSS

• CVE-2025-47285: 0.46% estimated exploitation activity in the next 30 days; percentile 40.24 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact 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: https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562 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.dosomesideeffect() else b"". the following example demonstrates how the issue would look in user code the severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code. Patches fix is tracked in https://github.com/vyperlang/vyper/pull/4644 Workarounds don't have side effects…

Published by the source

16 May 2025, 14:10 UTC

Last modified by the source

7 Jul 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:32 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-qhr6-mgqr-mchm

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qhr6-mgqr-mchm

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# Patch Next advisory brief: CVE-2025-47285

Advisory: Vyper's `concat()` builtin may elide side-effects for zero-length arguments
IDs: CVE-2025-47285, GHSA-qhr6-mgqr-mchm
Record: https://patchnext.com/advisories/cve-2025-47285 (revision 3, source checked 26 Oct 2026, 13:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI vyper: affected <= 0.4.2rc1

## Suggested inspection (read-only)
- `python -m pip show vyper`: prints the installed version. Run with the environment that serves production active.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on vyper.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-qhr6-mgqr-mchm (OSV record GHSA-qhr6-mgqr-mchm lists affected ranges: PyPI vyper >=0 <=0.4.2rc1)
- Verified: https://github.com/advisories/GHSA-qhr6-mgqr-mchm (GitHub-reviewed advisory GHSA-qhr6-mgqr-mchm lists affected ranges: PyPI vyper >=0 <=0.4.2rc1)

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