What functions and functionality is relevant?

json_parse_ex() in src/json.c.

Why is this vulnerable?

“Buffer Over-read #2” is a one-byte heap based “CWE-126 Buffer Over-read” in json_parse_ex().

Is this a replay and/or is inspired by anything?

This is not a replay.

What makes it interesting?

This vulnerability enables denial of service by triggering a segmentation fault through a one-byte heap over-read.