Varnish and WSL anyone?

Ran into this myself today. Here’s a fix that works.

The default.vcl script is most likely removing cookies needed for social login to work. Skip caching of anything passed to a URL that starts with
/wp-content/plugins/wordpress-social-login/hybridauth/?

Here’s what I have:

sub vcl_recv {
    # lines skipped here

    # Do not cache these paths
    if (req.url ~ "^/wp-cron\.php$" ||
        req.url ~ "^/xmlrpc\.php$" ||
        req.url ~ "^/wp-admin/.*$" ||
        req.url ~ "^/wp-includes/.*$" ||
        # Allow WP Social login to bypass Varnish cache
        req.url ~ "^/wp-content/plugins/wordpress-social-login/hybridauth/\?" ||
        req.url ~ "\?s=") {
            return (pass);
    }

    # Define the default grace period to serve cached content
    set req.grace = 6h;

    # By ignoring any other cookies, it is now ok to get a page
    unset req.http.Cookie;
    return (lookup);
}

Hope it helps.

Thanks for the reply, Boris.
As it turns out, I ditched Varnish and went with Nginx, memcached, php-fpm 5.5 and W3total Cache, for my situation and sites. A lot less issues and lots faster for my sites.
Thanks again though and hope it helps someone else

My varnish config was stripping cookies so i had to make another mod in addition to this one.
in the vcl_hash section I had to include the following
hash_data(req.http.Cookie);