diff --git a/classes/backend.php b/classes/backend.php
index 122e28c65..5bd724728 100644
--- a/classes/backend.php
+++ b/classes/backend.php
@@ -88,7 +88,7 @@ class Backend extends Handler {
 	}
 
 	function help() {
-		$topic = clean_filename($_REQUEST["topic"]); // only one for now
+		$topic = basename(clean($_REQUEST["topic"])); // only one for now
 
 		if ($topic == "main") {
 			$info = get_hotkeys_info();
diff --git a/classes/diskcache.php b/classes/diskcache.php
index 68829b8e3..74415189c 100644
--- a/classes/diskcache.php
+++ b/classes/diskcache.php
@@ -191,7 +191,7 @@ class DiskCache {
 	];
 
 	public function __construct($dir) {
-		$this->dir = CACHE_DIR . "/" . clean_filename($dir);
+		$this->dir = CACHE_DIR . "/" . basename(clean($dir));
 	}
 
 	public function getDir() {
@@ -227,9 +227,7 @@ class DiskCache {
 	}
 
 	public function getFullPath($filename) {
-		$filename = clean_filename($filename);
-
-		return $this->dir . "/" . $filename;
+		return $this->dir . "/" . basename(clean($filename));
 	}
 
 	public function put($filename, $data) {
diff --git a/classes/feeds.php b/classes/feeds.php
index 55a514cc0..58ba1b6f8 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -1124,9 +1124,9 @@ class Feeds extends Handler_Protected {
 
 		$pdo = Db::pdo();
 
-		$url = Feeds::fix_url($url);
+		$url = validate_url($url);
 
-		if (!$url || !Feeds::validate_feed_url($url)) return array("code" => 2);
+		if (!$url) return array("code" => 2);
 
 		$contents = @fetch_file_contents($url, false, $auth_login, $auth_pass);
 
@@ -1924,7 +1924,7 @@ class Feeds extends Handler_Protected {
 	}
 
 	static function get_feeds_from_html($url, $content) {
-		$url     = Feeds::fix_url($url);
+		$url     = validate_url($url);
 		$baseUrl = substr($url, 0, strrpos($url, '/') + 1);
 
 		$feedUrls = [];
@@ -1955,56 +1955,6 @@ class Feeds extends Handler_Protected {
 		return preg_match("/<html|DOCTYPE html/i", substr($content, 0, 8192)) !== 0;
 	}
 
-	static function validate_feed_url($url) {
-		$parts = parse_url($url);
-
-		return ($parts['scheme'] == 'http' || $parts['scheme'] == 'feed' || $parts['scheme'] == 'https');
-	}
-
-	/**
-	 * Fixes incomplete URLs by prepending "http://".
-	 * Also replaces feed:// with http://, and
-	 * prepends a trailing slash if the url is a domain name only.
-	 *
-	 * @param string $url Possibly incomplete URL
-	 *
-	 * @return string Fixed URL.
-	 */
-	static function fix_url($url) {
-
-		// support schema-less urls
-		if (strpos($url, '//') === 0) {
-			$url = 'https:' . $url;
-		}
-
-		if (strpos($url, '://') === false) {
-			$url = 'http://' . $url;
-		} else if (substr($url, 0, 5) == 'feed:') {
-			$url = 'http:' . substr($url, 5);
-		}
-
-		//prepend slash if the URL has no slash in it
-		// "http://www.example" -> "http://www.example/"
-		if (strpos($url, '/', strpos($url, ':') + 3) === false) {
-			$url .= '/';
-		}
-
-		//convert IDNA hostname to punycode if possible
-		if (function_exists("idn_to_ascii")) {
-			$parts = parse_url($url);
-			if (mb_detect_encoding($parts['host']) != 'ASCII')
-			{
-				$parts['host'] = idn_to_ascii($parts['host']);
-				$url = build_url($parts);
-			}
-		}
-
-		if ($url != "http:///")
-			return $url;
-		else
-			return '';
-	}
-
 	static function add_feed_category($feed_cat, $parent_cat_id = false, $order_id = 0) {
 
 		if (!$feed_cat) return false;
diff --git a/classes/handler/public.php b/classes/handler/public.php
index e6d94e223..135cdcbc7 100755
--- a/classes/handler/public.php
+++ b/classes/handler/public.php
@@ -1234,7 +1234,7 @@ class Handler_Public extends Handler {
 	public function pluginhandler() {
 		$host = new PluginHost();
 
-		$plugin_name = clean_filename($_REQUEST["plugin"]);
+		$plugin_name = basename(clean($_REQUEST["plugin"]));
 		$method = clean($_REQUEST["pmethod"]);
 
 		$host->load($plugin_name, PluginHost::KIND_USER, 0);
diff --git a/classes/pluginhost.php b/classes/pluginhost.php
index 4fec13000..c6c036783 100755
--- a/classes/pluginhost.php
+++ b/classes/pluginhost.php
@@ -193,7 +193,7 @@ class PluginHost {
 
 		foreach ($plugins as $class) {
 			$class = trim($class);
-			$class_file = strtolower(clean_filename($class));
+			$class_file = strtolower(basename(clean($class)));
 
 			if (!is_dir(__DIR__."/../plugins/$class_file") &&
 					!is_dir(__DIR__."/../plugins.local/$class_file")) continue;
diff --git a/classes/pref/feeds.php b/classes/pref/feeds.php
index 9d29ab478..8cdb1577c 100755
--- a/classes/pref/feeds.php
+++ b/classes/pref/feeds.php
@@ -1701,7 +1701,7 @@ class Pref_Feeds extends Handler_Protected {
 		foreach ($feeds as $feed) {
 			$feed = trim($feed);
 
-			if (Feeds::validate_feed_url($feed)) {
+			if (validate_url($feed)) {
 
 				$this->pdo->beginTransaction();
 
diff --git a/classes/rpc.php b/classes/rpc.php
index 208551075..7f809f29b 100755
--- a/classes/rpc.php
+++ b/classes/rpc.php
@@ -572,7 +572,7 @@ class RPC extends Handler_Protected {
 
 	function log() {
 		$msg = clean($_REQUEST['msg']);
-		$file = clean_filename($_REQUEST['file']);
+		$file = basename(clean($_REQUEST['file']));
 		$line = (int) clean($_REQUEST['line']);
 		$context = clean($_REQUEST['context']);
 
diff --git a/include/functions.php b/include/functions.php
index 16953a0a1..43e9eb8f6 100644
--- a/include/functions.php
+++ b/include/functions.php
@@ -238,8 +238,9 @@
 		$url = ltrim($url, ' ');
 		$url = str_replace(' ', '%20', $url);
 
-		if (strpos($url, "//") === 0)
-			$url = 'http:' . $url;
+		$url = validate_url($url);
+
+		if (!$url) return false;
 
 		$url_host = parse_url($url, PHP_URL_HOST);
 		$fetch_domain_hits[$url_host] += 1;
@@ -623,10 +624,6 @@
 		}
 	}
 
-	function clean_filename($filename) {
-		return basename(preg_replace("/\.\.|[\/\\\]/", "", clean($filename)));
-	}
-
 	function make_password($length = 12) {
 		$password = "";
 		$possible = "0123456789abcdfghjkmnpqrstvwxyzABCDFGHJKMNPQRSTVWXYZ*%+^";
@@ -1517,13 +1514,6 @@
 		return $parts['scheme'] . "://" . $parts['host'] . $parts['path'];
 	}
 
-	function cleanup_url_path($path) {
-		$path = str_replace("/./", "/", $path);
-		$path = str_replace("//", "/", $path);
-
-		return $path;
-	}
-
 	/**
 	 * Converts a (possibly) relative URL to a absolute one.
 	 *
@@ -1533,33 +1523,35 @@
 	 * @return string Absolute URL
 	 */
 	function rewrite_relative_url($url, $rel_url) {
-		if (strpos($rel_url, "://") !== false) {
+
+		$rel_parts = parse_url($rel_url);
+
+		if ($rel_parts['host'] && $rel_parts['scheme']) {
 			return $rel_url;
 		} else if (strpos($rel_url, "//") === 0) {
 			# protocol-relative URL (rare but they exist)
+			return "https:" . $rel_url;
+		} else if (strpos($rel_url, "magnet:") === 0) {
+			# allow magnet links
 			return $rel_url;
-		} else if (preg_match("/^[a-z]+:/i", $rel_url)) {
-			# magnet:, feed:, etc
-			return $rel_url;
-		} else if (strpos($rel_url, "/") === 0) {
-			$parts = parse_url($url);
-			$parts['path'] = $rel_url;
-			$parts['path'] = cleanup_url_path($parts['path']);
-
-			return build_url($parts);
-
 		} else {
 			$parts = parse_url($url);
+
 			if (!isset($parts['path'])) {
 				$parts['path'] = '/';
 			}
+
 			$dir = $parts['path'];
+
 			if (substr($dir, -1) !== '/') {
 				$dir = dirname($parts['path']);
 				$dir !== '/' && $dir .= '/';
 			}
+
 			$parts['path'] = $dir . $rel_url;
-			$parts['path'] = cleanup_url_path($parts['path']);
+
+			$parts['path'] = str_replace("/./", "/", $parts['path']);
+			$parts['path'] = str_replace("//", "/", $parts['path']);
 
 			return build_url($parts);
 		}
@@ -1837,6 +1829,15 @@
 			if ($mimetype == "application/octet-stream")
 				$mimetype = "video/mp4";
 
+			/* only serve video and images */
+			if (!preg_match("/(image|video)\//", $mimetype)) {
+				http_response_code(400);
+				header("Content-type: text/plain");
+
+				print "Stored file has disallowed content type ($mimetype)";
+				return false;
+			}
+
 			header("Content-type: $mimetype");
 
 			$stamp = gmdate("D, d M Y H:i:s", filemtime($filename)) . " GMT";
@@ -1924,3 +1925,34 @@
 
 		return $ttrss_version['version'];
 	}
+
+	function validate_url($url) {
+
+		# fix protocol-relative URLs
+		if (strpos($url, "//") === 0)
+			$url = "https:" . $url;
+
+		if (filter_var($url, FILTER_VALIDATE_URL) === FALSE)
+			return false;
+
+		$tokens = parse_url($url);
+
+		if (!$tokens['host'])
+			return false;
+
+		if (!in_array($tokens['scheme'], ['http', 'https']))
+			return false;
+
+		//convert IDNA hostname to punycode if possible
+		if (function_exists("idn_to_ascii")) {
+			if (mb_detect_encoding($tokens['host']) != 'ASCII') {
+				$parts['host'] = idn_to_ascii($tokens['host']);
+				$url = build_url($tokens);
+			}
+		}
+
+		/* if ($tokens['host'] == 'localhost' || $tokens['host'] == '127.0.0.1')
+			return false; */
+
+		return $url;
+	}
diff --git a/plugins/af_proxy_http/init.php b/plugins/af_proxy_http/init.php
index 80100160d..936942387 100644
--- a/plugins/af_proxy_http/init.php
+++ b/plugins/af_proxy_http/init.php
@@ -45,8 +45,7 @@ class Af_Proxy_Http extends Plugin {
 	}
 
 	public function imgproxy() {
-
-		$url = rewrite_relative_url(get_self_url_prefix(), $_REQUEST["url"]);
+		$url = validate_url(clean($_REQUEST["url"]));
 
 		// called without user context, let's just redirect to original URL
 		if (!$_SESSION["uid"]) {
@@ -59,7 +58,6 @@ class Af_Proxy_Http extends Plugin {
 		if ($this->cache->exists($local_filename)) {
 			header("Location: " . $this->cache->getUrl($local_filename));
 			return;
-			//$this->cache->send($local_filename);
 		} else {
 			$data = fetch_file_contents(["url" => $url, "max_size" => MAX_CACHE_FILE_SIZE]);
 
@@ -97,14 +95,13 @@ class Af_Proxy_Http extends Plugin {
 					imagedestroy($img);
 
 				} else {
-					header("Content-type: text/html");
+					header("Content-type: text/plain");
 
 					http_response_code(400);
 
-					print "<h1>Proxy request failed.</h1>";
-					print "<p>Fetch error $fetch_last_error ($fetch_last_error_code)</p>";
-					print "<p>URL: $url</p>";
-					print "<textarea cols='80' rows='25'>" . htmlspecialchars($fetch_last_error_content) . "</textarea>";
+					print "Proxy request failed.\n".
+						"Fetch error $fetch_last_error ($fetch_last_error_code)\n".
+						"Requested URL: $url";
 				}
 			}
 		}