Vulnerabilities

Vulnerability · CVE-2020-37289

Vyper interfaces returning integer types less than 256 bits can be manipulated if uint256 is used

Published Mar 25, 2020 · catalog updated Oct 24, 2026 · Vyper interfaces returning integer

Check if you run it

No confirmed exploitation is on record for CVE-2020-37289. EPSS is 0.26%. Act only if a listed version of Vyper interfaces returning integer is installed.

Severity

Low—

Known exploitation

Not in CISA/VulnCheck KEV

EPSS

0.26%

Remediation

Advisory link

Affected products

PyPI vyper

# VVE-2020-0001 Earlier today, we received a responsible disclosure of a potential issue from @montyly (security researcher at @trailofbits) for Vyper users who make assumptions about what values certain interface types can return. ### Impact We determined the issue to be mild and unlikely to be exploited, with an easy workaround while the correct resolution is in process. The issue stems from a number of things, which we will detail here. (1) The ABI Specification is under-defined such that function return type is not always reflected in how you use it This means that a function which returns under the hood actually returns a 32 byte integer, making it identical to a function that returns . This allows users to read an interface that returns a value to be stored into a variable without any explicit casting or input validation. (2) Vyper doesn't have types When Vyper was originally created, it only had one numeric type, but we added just enough types to be able to work with the majority of ERC interfaces that exist. Unfortunately, we never added , because it's only majority usage was for as the return type, which isn't reflected in the method ID. Because of (1), it didn't matter that we didn't have these types implemented because you could capture the return value as and use it just fine. (3) returns (which is an optional function) returns a type. While it was never intentioned to be used directly within a smart contract (hence being optional), someone could easily make the decision to rely on it to perform important functionality within their Vyper smart contract. This might lead to a scenario where an unexpectedly large value (> 255) returned by calling this function (which a malicious contract writer might write) would allow an attacker to manipulate or bypass certain logic depending on this value. In summary, because of (1), it isn't necessary to have to cast the return value of a function that returns to , and because of (2) it isn't possible to have the type s