Sabah belongs to Sultanate of Sulu.@badoc thanks bro, we in ASEAN need to clean up the mess our colonizer had left behind, the policy they implement Divide and Conquer really leave a lasting Scars in the native psychic. In our case our Muslim brethren enclave which was the whole of Mindanao had shrank to the present 3 provinces, you can understand the grievances they hold, they become a minority in their owned land.
And there is a lingering problem regarding Sabah, It is part of Sultanate of Zulu and there is strong linkage with the Tausog in Jolo, It will become a future flash point as nationalism both in Malaysia and the Philippine may led to the dissolution of ASEAN.
So by association, should rightly belong to the Philippines.
Philippines should take back Sabah similar to how China took Hong Kong after lease to the Colonialist ends.
Philippines should engage Chinese help to strengthen its economy and military instead of the US.
And negotiate with Malaysia from a position of strength.