Manual:$wgApiFrameOptions

Details
Disallow framing of API pages directly, by setting the X-Frame-Options header. Since the API returns CSRF tokens, allowing the results to be framed can compromise your user's account security.

Options are:
 * 'DENY': Do not allow framing. This is recommended for most wikis.
 * 'SAMEORIGIN': Allow framing by pages on the same domain.
 * false: Allow all framing.