SharePoint :The security validation for this page is invalid

Question

I called a web service in SharePoint designer with HTTP delete:

_api/web/lists/getbytitle('title1')/items(1)/roleassignments/getbyprincipalid('1')

The workflow returned "Forbidden" response status and the ULS log :

Exception occured in scope Microsoft.SharePoint.SPRoleAssignment.DeleteObject. Exception=System.Runtime.InteropServices.COMException (0x8102006D): The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.

Answer 1

You need to send a security token (Request Digest) within your request header. It is stored in a html input with the DOM ID __REQUESTDIGEST on every site. E.g. an ajax call:

$.ajax({
    headers: {
        "Accept": "application/json; odata=verbose",
        "content-type": "application/json;odata=verbose",
        "X-RequestDigest": $("#__REQUESTDIGEST").val()
    },

The lack of the token is mostly the course of the mentioned error

Answer 2

As @Dave mentioned, you need to specify the request digest to perform write operations in a REST call. But:

$("#__REQUESTDIGEST").val()

is JavaScript/jQuery code for obtaining the request digest, suitable for putting on a browser based form, but you can't use JavaScript in SharePoint designer.

In designer, you need to first call the web service _api/contextinfo to get the request digest, and then assign that request digest to the X-RequestDigest header and make the delete call.

Ref: SharePoint Designer Call HTTP Web service with RequestDigest