Puranas are the scriptures of Hinduism which are accepted by some people and rejected by some people. There are 18 Mahapuranas which are the most important. The basic theory that puranas are interpolated or not written by Sage Vyasa arises because many verses of Puranas contradict each other. Vishnu Purana says Lord Vishnu is supreme and other Devas are His subordinates and the same Shiva Mahapurana says that Lord Shiva is supreme and other devas are His subordinates (Same thing in Devi Purana etc). Some Puranic stories also contradict other but to defend these puranas the theory is that the contradiction is arised due to Kalpa-Bheda. It could be true also.
Now I want to know what did Adi Shankara thought about Puranas? He mainly wrote a commentary on Upanishads and Bhagavad Gita, did he mention anything about Puranas? Are Puranas authentic according to Shankara?