CVE-2024-24563: Vyper array negative index vulnerability
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: python arr: public(uint256[MAXUINT256])
@external def set(idx: int256, num: uint256): self.arr[idx] = num For signed integers, 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): https://github.com/vyperlang/vyper/blob/a1fd228cb9936c3e4bbca6f3ee3fb4426ef45490/vyper/codegen/core.py#L534-L541
Patches Patched in https://github.com/vyperlang/vyper/pull/3817.
Impact There are two potential vulnerability classes: unpredictable behavior and accessing inaccessible elements.
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.
2. If a contract has an invariant in the form assert index < x where both index and x are signed integers, the developer might suppose that no elements on indexes y | y >= x are accessible. However, by using negative indexes this can be bypassed.
The contract search found no production contracts impacted by these two classes of issues.
Other sources
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.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/vyperto a version that resolves this vulnerability.Fixed in 0.4.0
Event History
Frequently Asked Questions
What is the severity of CVE-2024-24563?
CVE-2024-24563 has a medium severity level due to potential undefined behavior from improper array indexing.
How do I fix CVE-2024-24563?
To fix CVE-2024-24563, upgrade Vyper to version 0.4.0 or later.
Which versions of Vyper are affected by CVE-2024-24563?
CVE-2024-24563 affects Vyper versions up to 0.3.10, including 0.4.0 before the fix.
What types of indexing are problematic in CVE-2024-24563?
CVE-2024-24563 highlights issues with using signed integers as indexes for arrays that expect unsigned integers.
Is there a workaround for CVE-2024-24563 before upgrading?
A recommended workaround for CVE-2024-24563 is to avoid using negative integers as array indexes in your code.