Summary: Write side effects in MCallGetProperty opcode not accounted for
"In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free condition."
For observers: note that the appropriate versions have already been stabilised and we're just pending a GLSA now.
This issue was resolved and addressed in
GLSA 202011-07 at https://security.gentoo.org/glsa/202011-07
by GLSA coordinator Sam James (sam_c).