Headline
CVE-2023-42460: _abi_decode input not validated in certain complex expressions
Vyper is a Pythonic Smart Contract Language for the EVM. The _abi_decode() function does not validate input when it is nested in an expression. Uses of _abi_decode() can be constructed which allow for bounds checking to be bypassed resulting in incorrect results. This issue has not yet been fixed, but a fix is expected in release 0.3.10. Users are advised to reference pull request #3626.
Impact
_abi_decode() does not validate input when it is nested in an expression. the following example gets correctly validated (bounds checked):
x: int128 = _abi_decode(slice(msg.data, 4, 32), int128)
however, the following example is not bounds checked
@external def abi_decode(x: uint256) -> uint256: a: uint256 = convert(_abi_decode(slice(msg.data, 4, 32), (uint8)), uint256) + 1 return a # abi_decode(256) returns: 257
the issue can be triggered by constructing an example where the output of _abi_decode is not internally passed to make_setter (an internal codegen routine) or other input validating routine.
Patches
#3626
Workarounds
Is there a way for users to fix or remediate the vulnerability without upgrading?
References
Are there any links users can visit to find out more?
Related news
### Impact `_abi_decode()` does not validate input when it is nested in an expression. the following example gets correctly validated (bounds checked): ```vyper x: int128 = _abi_decode(slice(msg.data, 4, 32), int128) ``` however, the following example is not bounds checked ```vyper @external def abi_decode(x: uint256) -> uint256: a: uint256 = convert(_abi_decode(slice(msg.data, 4, 32), (uint8)), uint256) + 1 return a # abi_decode(256) returns: 257 ``` the issue can be triggered by constructing an example where the output of `_abi_decode` is not internally passed to `make_setter` (an internal codegen routine) or other input validating routine. ### Patches https://github.com/vyperlang/vyper/pull/3626 ### Workarounds _Is there a way for users to fix or remediate the vulnerability without upgrading?_ ### References _Are there any links users can visit to find out more?_