Manual:$wgMangleFlashPolicy

Output: $wgMangleFlashPolicy
Whether to mangle any <cross-domain-policy> (Adobe cross-domain policy) tags, to prevent XSS attacks.
Introduced in version:1.23.7 (Gerrit change 174289; git #92f22cd4)
Deprecated in version:1.39.0 (Gerrit change 815827; git #51ddd706)
Removed in version:1.40.0 (Gerrit change 838769; git #bb10b7d5)
Allowed values:(boolean)
Default value:true

DetailsEdit

When this is set to true, any occurrences of <cross-domain-policy> in sanitised output will be altered to <NOT-cross-domain-policy>. Without this, an attacker can potentially send their own Adobe cross-domain policy unless it is prevented by the crossdomain.xml file at the domain root.

You should only set this to false if you have a crossdomain.xml file in the root of your website (e.g. http://example.com/crossdomain.xml).