OpenZeppelin Contracts Bytes's lastIndexOf function with position argument performs out-of-bound memory access on empty buffers

Impact

The lastIndexOf(bytes,byte,uint256) function of the Bytes.sol library may access uninitialized memory when the following two conditions hold: 1) the provided buffer length is empty (i.e. buffer.length == 0) and position is not 2**256 - 1 (i.e. pos != type(uint256).max).

The pos argument could be used to access arbitrary data outside of the buffer bounds. This could lead to the operation running out of gas, or returning an invalid index (outside of the empty buffer). Processing this invalid result for accessing the buffer would cause a revert under normal conditions.

When triggered, the function reads memory at offset buffer + 0x20 + pos. If memory at that location (outside the buffer) matches the search pattern, the function would return an out of bound index instead of the expected type(uint256).max. This creates unexpected behavior where callers receive a valid-looking index pointing outside buffer bounds.

Subsequent memory accesses that don't check bounds and use the returned index must carefully review the potential impact depending on their setup. Code relying on this function returning type(uint256).max for empty buffers or using the returned index without bounds checking could exhibit undefined behavior.

Patches

Upgrade to 5.4.0