I surprised that Project Zero waited the last 30 days for disclosure since Github themselves publicly announced at 1. October that the whole searching for magic strings in stdout as RPC channel is broken by design.
When I saw the announcement month ago I assumed that Github fully disabled the reading commands from stdout mechanism but looking at current docs it seems that they disabled only 5 most easily exploitable out of 14 commands which can be called using stdout. This seems like typical "we fixed your POC doesn't work anymore" but the main cause remains. As can be sometimes seen with hardware vendors "we fixed hard-coded backdoor password, now there is new hard-coded backdoor password".
When I saw the announcement month ago I assumed that Github fully disabled the reading commands from stdout mechanism but looking at current docs it seems that they disabled only 5 most easily exploitable out of 14 commands which can be called using stdout. This seems like typical "we fixed your POC doesn't work anymore" but the main cause remains. As can be sometimes seen with hardware vendors "we fixed hard-coded backdoor password, now there is new hard-coded backdoor password".