OP_RETURN malleability? Can anyone alter the data after OP_return?  

My question needs to have more characters, so I insert an example, that doesn't hurt:

"vout" : [
    {
        "value" : 0.00000000,
        "n" : 0,
        "scriptPubKey" : {
            "asm" : "OP_RETURN 636861726c6579206c6f766573206865696469",
            "hex" : "6a13636861726c6579206c6f766573206865696469",
            "type" : "nulldata"
        }
    }
],
The OP_RETURN is in the scriptPubKey, which is signed as part of the transaction, so it can't be altered. So it is not malleable.

The known types of malleability are described in BIP 62.