diff --git a/actions/userauthorization.php b/actions/userauthorization.php
index a6dc2a5b07..0d3b71ac98 100644
--- a/actions/userauthorization.php
+++ b/actions/userauthorization.php
@@ -365,6 +365,10 @@ class UserauthorizationAction extends Action {
 			throw new OAuthException("Listener URI '$listener' not found here");
 		}
 		$listenee = $req->get_parameter('omb_listenee');
+		if (!Validate::uri($listenee) &&
+			!common_valid_tag($listenee)) {
+			throw new OAuthException("Listenee URI '$listenee' not a recognizable URI");
+		}
 		if (strlen($listenee) > 255) {
 			throw new OAuthException("Listenee URI '$listenee' too long");
 		}
diff --git a/lib/util.php b/lib/util.php
index 03b1e42a90..771a4880c6 100644
--- a/lib/util.php
+++ b/lib/util.php
@@ -598,3 +598,11 @@ function common_debug($msg, $filename=NULL) {
 function common_valid_http_url($url) {
 	return Validate::uri($url, array('allowed_schemes' => array('http', 'https')));
 }
+
+function common_valid_tag($tag) {
+	if (preg_match('/^tag:(.*?),(\d{4}(-\d{2}(-\d{2})?)?):(.*)$/', $tag, $matches)) {
+		return (Validate::email($matches[1]) || 
+				preg_match('/^([\w-\.]+)$/', $matches[1]));
+	}
+	return false;
+}