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.\n\nThere 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.
EPSS Score:
1%
Comprehensive Technical Analysis of EUVD-2024-0177
1. Vulnerability Assessment and Severity Evaluation
Vulnerability Description: The vulnerability in Vyper, a Pythonic Smart Contract Language for the Ethereum Virtual Machine, arises from the typechecker allowing signed integers to be used as indexes for arrays, which are defined for unsigned integers only. This discrepancy can lead to unpredictable behavior, access to inaccessible elements, and potential denial of service.
Severity Evaluation:
The vulnerability has a CVSS Base Score of 9.8, which is considered critical. The scoring vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H indicates:
- Attack Vector (AV): Network (N)
- Attack Complexity (AC): Low (L)
- Privileges Required (PR): None (N)
- User Interaction (UI): None (N)
- Scope (S): Unchanged (U)
- Confidentiality (C): High (H)
- Integrity (I): High (H)
- Availability (A): High (H)
This high severity score underscores the critical nature of the vulnerability, which can be exploited remotely with low complexity and without requiring any privileges or user interaction.
2. Potential Attack Vectors and Exploitation Methods
Attack Vectors:
- Unpredictable Behavior: An attacker could exploit the vulnerability to index arrays with negative integers, leading to unpredictable behavior in smart contracts.
- Accessing Inaccessible Elements: By using negative indexes, an attacker could bypass invariants set by developers, accessing elements that should be inaccessible.
- Denial of Service: If the state of the contract can be manipulated to force the index to be negative, it could lead to a denial of service by causing the array access to always revert.
Exploitation Methods:
- Negative Indexing: An attacker could craft transactions that use negative integers as array indexes, exploiting the 2's complement representation to access large indexes.
- State Manipulation: By manipulating the contract state, an attacker could force the index to be negative, leading to denial of service.
3. Affected Systems and Software Versions
Affected Systems:
- All versions of Vyper, including
0.3.10.
Software Versions:
- Vyper versions ≤ 0.3.10 are affected.
4. Recommended Mitigation Strategies
Immediate Mitigation:
- Patching: Upgrade to a patched version of Vyper once available.
- Manual Review: Conduct a thorough review of smart contracts to identify and correct any instances where signed integers are used as array indexes.
- Input Validation: Implement additional input validation to ensure that only unsigned integers are used as array indexes.
Long-Term Mitigation:
- Code Audits: Regularly audit smart contracts for potential vulnerabilities.
- Security Best Practices: Follow best practices for smart contract development, including rigorous testing and validation.
5. Impact on European Cybersecurity Landscape
Impact:
- Financial Loss: Unpredictable behavior and access to inaccessible elements could lead to financial losses for users and organizations relying on Vyper-based smart contracts.
- Reputation Damage: Denial of service attacks could damage the reputation of organizations using affected smart contracts.
- Regulatory Compliance: Non-compliance with regulatory requirements could result in legal penalties and fines.
European Context:
- GDPR Compliance: Ensuring the security of smart contracts is crucial for compliance with GDPR, especially for contracts handling personal data.
- Critical Infrastructure: Vulnerabilities in smart contracts could impact critical infrastructure, particularly in sectors like finance and energy.
6. Technical Details for Security Professionals
Technical Analysis:
- Typechecker Issue: The typechecker in Vyper does not throw an error when signed integers are used as array indexes, leading to potential misuse.
- 2's Complement Representation: Negative values are represented as very large numbers due to the 2's complement representation, bypassing bounds checking.
- Array Declaration: Large array declarations exacerbate the issue, as negative indexes can be interpreted as valid large indexes.
References:
- GitHub Advisories: GHSA-52xq-j7v9-v4v2
- Code References:
- NVD Reference: CVE-2024-24563
- PyPA Advisory: PYSEC-2024-150
Conclusion: The vulnerability in Vyper poses a significant risk to smart contracts, necessitating immediate mitigation strategies and long-term security best practices. Organizations and developers should prioritize patching and thorough code reviews to mitigate potential exploitation. The impact on the European cybersecurity landscape underscores the importance of robust security measures in smart contract development and deployment.