wordpress error after update

Hello there,

i have recently update my wordpress websites to latest wordpress version, but in all i go same http 500 error, i have try many ways to solve it but i was unable. finally i find that after update wordpress i has changes file permission of 2 or 3 file in every websites. Those are post.php in wp-includes and functions.php inside wp-content/themes/[my-website-theme]/, it change permission 0644 to 0000. Now i resolve these issue but now i face this issue again in my new website(developing), every 2 day it’s change both files permission 0644 to 0000. Help me how i can resolve this issue.