CVE-2024-24563: Vyper negative array index bounds checks

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 1.5%, percentile 75: FIRST’s model estimates a 1.5% chance of exploitation activity in the next 30 days, higher than 75% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes vyper (PyPI) < 0.4.0, fixed in 0.4.0. Check your lockfile, including transitive dependencies.

What it is

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 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.

From the GitHub advisory

Summary 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. Typically, negative integers are filtered out at runtime by the bounds checker, but small enough (i.e. large in magnitude, ex. -2255 + 5) quantities combined with large enough arrays (at least 2255 in length) can pass the bounds checker, resulting in unexpected behavior.

A contract search was performed, and no production contracts were found to be impacted.

Details 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. Here is an example from 0.3.10: https://github.com/vyperlang/vyper/blob/c150fc49ee9375a930d177044559b83cb95f7963/vyper/semantics/types/subscriptable.py#L127-L137

As can be seen, the validation is performed against IntegerT.any().

PoC If the array is sufficiently large, it can be indexed with a negative value:

Weakness type: CWE-129.

Product: vyper (PyPI) · Assigned by: GitHub_M · Published: 2024-02-07

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 1.54% probability of exploitation activity in the next 30 days; percentile 75.

Timeline

• 2024-02-07 — CVE record published by GitHub_M
• 2024-02-07 — GitHub-reviewed advisory GHSA-52xq-j7v9-v4v2 published (critical)
• 2025-06-17 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-52xq-j7v9-v4v2 github.com
2. CVE.org record CVE-2024-24563 cve.org
3. NVD entry CVE-2024-24563 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

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