CVE-2020-37289 in Vyper
Summary
by MITRE • 10/21/2026
Vyper before 0.3.1 lacks a uint8 type, allowing interface return values declared as uint8, such as ERC20.decimals(), to be stored in uint256 variables without bounds checking. Malicious contracts can return values greater than 255 to manipulate or bypass contract logic that relies on these values.
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Analysis
by VulDB Data Team • 10/21/2026
The absence of a native unsigned eight-bit integer type, specifically uint8, in Vyper versions prior to 0.3.1 introduces a significant semantic gap in the language's type system. This limitation forces developers to utilize larger data types, such as uint256, for values that are conceptually constrained to an eight-bit range. While this may appear to be a minor syntactic inconvenience, it creates a critical vulnerability surface when interacting with external smart contracts or interfaces that do not strictly enforce their declared return type constraints at the ABI level. In many blockchain environments, particularly those adhering to Ethereum standards like ERC20 tokens, functions such as decimals() are expected to return small integer values typically ranging from zero to eighteen. However, because Vyper lacks a mechanism to automatically truncate or validate these returns against an eight-bit boundary during assignment, any value returned by the external contract is accepted into the uint256 variable without modification or bounds checking.
This technical flaw allows malicious contracts to exploit this lack of validation by returning values that exceed the maximum limit for an unsigned eight-bit integer, which is 255. When a Vyper smart contract calls such a function and stores the result in a uint256 variable, it implicitly trusts the external source without verifying if the value fits within the expected logical range. If subsequent logic within the vulnerable contract assumes that this value behaves like an eight-bit integer—for instance by using it as an index for array access, performing bitwise operations expecting small values, or making critical business decisions based on thresholds set below 256—the injection of a large malicious value can lead to severe unintended consequences. The attacker does not need to break the type system itself but rather exploits the logical assumptions made by the contract developer regarding data integrity and range constraints.
The operational impact of this vulnerability is profound, as it enables logic bypasses that can compromise the security and financial stability of affected contracts. For example, if a contract uses the return value from decimals() to calculate token amounts or determine fee structures under the assumption that the divisor will be small, an attacker providing a large decimal count could manipulate these calculations drastically. This might result in incorrect asset transfers, unauthorized minting of tokens, or complete bypass of access control mechanisms that rely on specific numeric thresholds. In more complex scenarios involving array indexing based on such values, this could lead to out-of-bounds memory accesses, potentially causing contract reverts or unexpected state corruption depending on the implementation details and gas constraints.
To mitigate this vulnerability, developers must implement explicit bounds checking whenever interacting with external interfaces that return small integer types in older Vyper versions. This involves adding conditional statements after calling functions like decimals() to verify that the returned value is less than 256 before proceeding with any logic dependent on its magnitude. Alternatively, upgrading to Vyper version 0.3.1 or later resolves this issue by introducing native support for uint8 and other smaller integer types, allowing the compiler to enforce type safety at compile time rather than relying solely on runtime checks. Adopting these practices ensures that contracts remain resilient against malicious inputs from external sources and aligns with secure coding standards such as CWE-20 Improper Input Validation and ATT&CK techniques related to smart contract exploitation where attackers manipulate data flow to bypass security controls.