Set default `forwarded_for_headers` to 'HTTP_X_FORWARDED_FOR'

config/config.sample.php

 
 /**
  * Headers that should be trusted as client IP address in combination with
- * `trusted_proxies`
+ * `trusted_proxies`. If the HTTP header looks like 'X-Forwarded-For', then use
+ * 'HTTP_X_FORWARDED_FOR' here.
+ *
+ * If set incorrectly, a client can spoof their IP address as visible to
+ * ownCloud, bypassing access controls and making logs useless!
+ *
+ * Defaults to 'HTTP_X_FORWARED_FOR' if unset
  */
 'forwarded_for_headers' => array('HTTP_X_FORWARDED', 'HTTP_FORWARDED_FOR'),
 

lib/private/appframework/http/request.php

 		$trustedProxies = $this->config->getSystemValue('trusted_proxies', []);
 
 		if(is_array($trustedProxies) && in_array($remoteAddress, $trustedProxies)) {
-			$forwardedForHeaders = $this->config->getSystemValue('forwarded_for_headers', []);
+			$forwardedForHeaders = $this->config->getSystemValue('forwarded_for_headers', [
+				'HTTP_X_FORWARDED_FOR'
+				// only have one default, so we cannot ship an insecure product out of the box
+			]);
 
 			foreach($forwardedForHeaders as $header) {
 				if(isset($this->server[$header])) {