CVE-2024-24563: Vyper Signed Integer Index Vulnerability

CVE-2024-24563 Overview

CVE-2024-24563 is an Improper Validation of Array Index vulnerability affecting Vyper, a Pythonic Smart Contract Language for the Ethereum Virtual Machine. The vulnerability stems from the typechecker's failure to properly validate array indexing operations, allowing signed integers to be used as array indexes despite arrays being defined for unsigned integers only.

The core issue lies in Vyper's handling of signed integers when used to index arrays. Due to the 2's complement representation used for signed integers, negative values are represented as very large positive numbers. When a large array is declared, bounds checking passes for these negative-converted values, allowing access to unintended memory locations.

Critical Impact

Smart contracts compiled with affected Vyper versions may exhibit unpredictable behavior, allow unauthorized access to array elements, or become susceptible to denial of service attacks when negative integers are used for array indexing.

Affected Products

• Vyperlang Vyper (all versions including 0.3.10)

Discovery Timeline

• 2024-02-07 - CVE-2024-24563 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-24563

Vulnerability Analysis

This vulnerability represents an input validation flaw in Vyper's typechecker component. The typechecker, responsible for ensuring type safety in smart contract code, fails to reject signed integers (int) when used as array indexes. Arrays in Vyper are fundamentally designed for unsigned integer indexing, but this validation gap allows developers to inadvertently or maliciously use signed values.

The vulnerability manifests in three distinct attack classes:

Class 1 - Unpredictable Behavior: When an array is indexed with a negative integer without causing a revert, the resulting behavior is not anticipated by developers. This can lead to contracts operating in undefined states.

Class 2 - Access Control Bypass: Contracts implementing invariants such as assert index < x assume elements at indexes greater than or equal to x are inaccessible. Negative indexing bypasses this assumption since negative values, when interpreted as unsigned, represent very large numbers that may still pass bounds checking on large arrays.

Class 3 - Denial of Service: When array indexes depend on contract state, attackers who can manipulate the state to produce negative index values can force persistent reverts, effectively denying service to legitimate users.

Root Cause

The root cause is located in Vyper's semantic type checking module. The typechecker at the subscriptable type handling layer does not validate that array index expressions are of unsigned integer type. The relevant code paths in vyper/semantics/types/subscriptable.py and vyper/codegen/core.py permit signed integer expressions to flow through to array access operations without raising type errors.

Additionally, the code generation module does not include runtime checks specifically for signed index values, relying solely on standard bounds checking which can be bypassed with sufficiently large array declarations.

Attack Vector

The vulnerability is exploitable via network-accessible smart contract interactions. An attacker can craft transactions that invoke contract functions using signed integer parameters for array indexing operations. The attack requires no privileges or user interaction.

The exploitation scenario involves:

1. Identifying a Vyper-compiled contract with array access patterns accepting external input
2. Crafting transaction data with negative integer values for array indexes
3. Observing unpredictable state changes, bypassing access invariants, or triggering denial of service

While the vulnerability has significant potential impact, the advisory notes that exploitation scenarios are highly unlikely in practice, as the most probable outcome is a revert on bounds checking unless arrays are declared extremely large.

Detection Methods for CVE-2024-24563

Indicators of Compromise

• Unexpected state changes in Vyper-compiled smart contracts after transactions with unusual index parameters
• Contract function reverts occurring at array access operations with valid-appearing unsigned index values
• Anomalous gas consumption patterns during array access operations
• Transaction traces showing array accesses at unexpectedly high index positions

Detection Strategies

• Static analysis of Vyper source code to identify array access patterns using signed integer types
• Review of compiled contract bytecode for array bounds checking logic inconsistencies
• Monitoring transaction logs for contracts with known array indexing operations receiving large unsigned values
• Automated smart contract auditing tools configured to flag signed integer array indexing

Monitoring Recommendations

• Implement transaction monitoring for Vyper-compiled contracts to detect anomalous array access patterns
• Deploy smart contract analysis tools that specifically check for type confusion between signed and unsigned integers
• Establish alerts for unexpected reverts in production contracts that may indicate exploitation attempts
• Monitor blockchain explorers for suspicious transaction patterns targeting known vulnerable contracts

How to Mitigate CVE-2024-24563

Immediate Actions Required

• Audit all deployed Vyper-compiled smart contracts for array indexing patterns that accept external input
• Review contract logic for invariants that rely on array bounds assumptions
• Consider implementing additional validation logic at the application layer for array index parameters
• Evaluate upgradeability options for contracts that cannot be directly patched

Patch Information

As of the publication date, no fixed version of Vyper exists that addresses this vulnerability. The development team has acknowledged the issue through their GitHub Security Advisory.

Developers should monitor the Vyper project for future releases that address this type checking gap. Technical details about the vulnerable code paths can be found in the codegen module and subscriptable types module.

Workarounds

• Explicitly cast all array index values to unsigned integers (uint256) before array access operations
• Implement manual bounds checking with assertions that verify index values are non-negative before use
• Avoid accepting external input directly as array indexes without validation
• Consider using mapping types instead of arrays where dynamic key access is required
• Add explicit type annotations to ensure index variables are declared as unsigned types

# Verification check for Vyper contracts
# Identify contracts using signed integer array indexing
grep -rn "int.*\[" --include="*.vy" ./contracts/
# Review flagged files for potential vulnerability exposure