CVE-2023-32058 in Vyper
Summary
by MITRE • 05/12/2023
Vyper is a Pythonic smart contract language for the Ethereum virtual machine. Prior to version 0.3.8, due to missing overflow check for loop variables, by assigning the iterator of a loop to a variable, it is possible to overflow the type of the latter. The issue seems to happen only in loops of type `for i in range(a, a + N)` as in loops of type `for i in range(start, stop)` and `for i in range(stop)`, the compiler is able to raise a `TypeMismatch` when trying to overflow the variable. The problem has been patched in version 0.3.8.
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Analysis
by VulDB Data Team • 01/24/2025
The vulnerability identified as CVE-2023-32058 affects Vyper, a Pythonic smart contract language designed for the Ethereum virtual machine, specifically impacting versions prior to 0.3.8. This issue represents a critical security flaw that stems from insufficient overflow checking mechanisms within loop variable handling, creating potential pathways for unexpected behavior in smart contract execution. The vulnerability manifests when developers assign loop iterators to variables, particularly within specific loop constructs that lack proper type validation during compilation. The flaw is particularly concerning because it can lead to unpredictable contract behavior and potential financial loss through unintended arithmetic operations.
The technical root cause of this vulnerability lies in the compiler's handling of loop variables within specific range constructs. When utilizing loops of the pattern `for i in range(a, a + N)`, the Vyper compiler fails to perform adequate overflow checks on the loop variable type. This occurs because the compiler's type checking system correctly identifies and rejects overflow scenarios in standard range constructs like `for i in range(start, stop)` and `for i in range(stop)`, but fails to apply the same rigorous validation when dealing with arithmetic expressions within the range parameters. The missing overflow check allows loop variables to exceed their designated type boundaries, potentially causing unexpected behavior during contract execution. This issue maps to CWE-191 Integer Underflow/Overflow, specifically focusing on the failure to properly validate integer bounds during loop iteration.
The operational impact of this vulnerability extends beyond simple compilation warnings, as it can fundamentally alter smart contract behavior during runtime execution. When a loop variable overflows, it can cause the contract to skip iterations, execute unintended code paths, or produce incorrect results that may be exploited by malicious actors. The vulnerability is particularly dangerous in scenarios where loop variables control critical contract logic such as access control mechanisms, reward distributions, or state transitions. Attackers could potentially manipulate the loop parameters to cause overflows that lead to privilege escalation or resource misallocation, representing a significant threat to contract integrity and user funds. This vulnerability aligns with ATT&CK technique T1059.001 Command and Scripting Interpreter: Python, as it exploits the underlying language implementation to create unexpected execution flows.
The fix implemented in Vyper version 0.3.8 addresses this vulnerability by enhancing the compiler's type checking system to properly validate loop variables in all range constructs, ensuring that overflow conditions are detected and prevented during compilation rather than allowing them to propagate to runtime execution. This patch represents a fundamental improvement in the language's security posture by implementing comprehensive overflow checking mechanisms. Organizations should immediately upgrade to Vyper 0.3.8 or later to mitigate this risk, while also conducting thorough code reviews to identify any existing contracts that may be vulnerable to this issue. The remediation process should include verification that all loop constructs properly handle potential overflow scenarios and that the compiler's new validation mechanisms are correctly enforced. Additionally, developers should be educated on the specific patterns that trigger this vulnerability to prevent similar issues in future contract development.