Roughly Enough Items (REI) v.16.0.729 and before contains an Improper Validation of Specified Index, Position, or Offset in Input vulnerability. The specific issue is a failure to validate slot index and decrement stack count in the Roughly Enough Items (REI) mod for Minecraft, which allows in-game item duplication.
The product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array.