CVE-2024-24560 — Vyper external calls can overflow return data to return input buffer

Affected: vyperlang vyper

Vyper is a Pythonic Smart Contract Language for the Ethereum Virtual Machine. When calls to external contracts are made, we write the input buffer starting at byte 28, and allocate the return buffer to start at byte 0 (overlapping with the input buffer). When checking RETURNDATASIZE for dynamic types, the size is compared only to the minimum allowed size for that type, and not to the returned value's length. As a result, malformed return data can cause the contract to mistake data from the input buffer for returndata. When the called contract returns invalid ABIv2 encoded data, the calling contract can read different invalid data (from the dirty buffer) than the called contract returned.

GHSA-gp3w-2v2m-p686 — Vyper's external calls can overflow return data to return input buffer

Affected: PyPI: vyper

## Summary

When calls to external contracts are made, we write the input buffer starting at byte 28, and allocate the return buffer to start at byte 0 (overlapping with the input buffer). When checking `RETURNDATASIZE` for dynamic types, the size is compared only to the minimum allowed size for that type, and not to the returned value's `length`. As a result, malformed return data can cause the contract to mistake data from the input buffer for returndata.

This advisory is given a severity of "Low" because when the called contract returns invalid ABIv2 encoded data, the calling contract can read different invalid data (from the dirty buffer) than the called contract returned.

## Details

When arguments are packed for an external call, we create a buffer of size `max(args, return_data) + 32`. The input buffer is placed in this buffer (starting at byte 28), and the return buffer is allocated to start at byte 0. The assumption is that we can reuse the memory becase we will not be able to read past `RETURNDATASIZE`.

```python if fn_type.return_type is not None: return_abi_t = calculate_type_for_external_return(fn_type.return_type).abi_type

# we use the same buffer for args and returndata, # so allocate enough space here for the returndata too. buflen = max(args_abi_t.size_bound(), return_abi_t.size_bound()) else: buflen = args_abi_t.size_bound()

buflen += 32 # padding for the method id ```

When data is returned, we unpack the return data by starting a…

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CVE-2024-24560 — Vyper external calls can overflow return data to return input buffer

GHSA-gp3w-2v2m-p686 — Vyper's external calls can overflow return data to return input buffer