Affected versions of bam set the length of an internal buffer using self.compressed.set_len(block_size - HEADER_SIZE - MIN_EXTRA_SIZE) and then wrote into it. While block_size was constrained to a proper maximum, when it was too small the subtraction could overflow negatively to a large number past the capacity of self.compressed. This can result in memory corruption in the form of writing out of bounds when loading a bgzip file with a small block_size. Commit 061eee38d4 fixed this issue by checking for the underflow when setting the buffer size.