singlevar in lparser.c in Lua from (including) 5.4.0 up to (excluding) 5.4.4 lacks a certain luaK_exp2anyregup call, leading to a heap-based buffer over-read that might affect a system that compiles untrusted Lua code.
https://github.com/lua/lua/commit/1f3c6f4534c6411313361697d98d1145a1f030fa