The issue is that the DEC_TBL has a size of 128, however the input string could contain non-ascii characters leading to an out of bound access. This does not raise a bounds check error, given that it happens in a #no_bounds_check block. I'm not sure what the best way to fix this is, either increasing the table size or adding bounds checks ig. I would imagine the security impact of this to be relatively small, but given that base64 decoding happens rather frequently in a network context this could be a potential attack vector and should be fixed.