Feature-Policy: sync-xhr

The HTTP Feature-Policy header sync-xhr directive controls whether the current document is allowed to make synchronous XMLHttpRequest requests.

Feature-Policy: sync-xhr <allowlist>;

<allowlist>

An allowlist is a list of origins that takes one or more of the following values, separated by spaces:

• *: The feature will be allowed in this document, and all nested browsing contexts (iframes) regardless of their origin.
• 'self': The feature will be allowed in this document, and in all nested browsing contexts (iframes) in the same origin.
• 'src': (In an iframe allow attribute only) The feature will be allowed in this iframe, as long as the document loaded into it comes from the same origin as the URL in the iframe's src attribute.
  The 'src' origin is used in the iframe allow attribute only, and is the default allowlist value. 
• 'none': The feature is disabled in top-level and nested browsing contexts.
• <origin(s)>: The feature is allowed for specific origins (for example, https://example.com). Origins should be separated by a space.

The values * (enable for all origins) or 'none' (disable for all origins) may only be used alone, while 'self' and 'src' may be used with one or more origins.

Features are each defined to have a default allowlist, which is one of:

• *: The feature is allowed by default in top-level browsing contexts and all nested browsing contexts (iframes).
• 'self': The feature is allowed by default in top-level browsing contexts and in nested browsing contexts (iframes) in the same origin. The feature is not allowed in cross-origin documents in nested browsing contexts.
• 'none': The feature is disabled in top-level and nested browsing contexts.

By default the policy is set to *, which means synchronous requests are allowed in all frames.

• Feature Policy
• Using Feature Policy