Manual:$wgCrossSiteAJAXdomains

Details
Allows Ajax requests from certain domains to make cross-site requests to a wiki's API (see  for example usage). This uses the Access-Control-Allow-Origin HTTP header. Note that some older browsers [http://caniuse.com/#feat=cors don't support this]. This only affects requests to the api>Special:MyLanguage/API:Main page|API. Other entry points (index.php) are not affected.

The value must be a list of allowed domain names, which can include shell-style wildcards ( to match any character,   to match any number (including zero) of characters). An empty array means no external access is allowed.

Some examples:

Allow any domain to access the API via Ajax (This is insecure):

Allow two specific domains:

Allow all subdomains of a domain (including "deep" subdomains such as  ):

See 9624 for a usage example.

Until MediaWiki 1.34, there could be logs   referring to the wiki itself, which could be fixed by adding the wiki’s server name in this parameter to avoid these logs. This was fixed in MediaWiki 1.35 in T243908.