fix(zone.js): a path traversal attack in test (angular#32392)

`simple-server.js` is vulnerable to a trivial path traversal attack, i.e. an
attacker can supply a path like `../../etc/passwd` to read arbitrary files on
the server. This change fixes the issue by properly resolving the path, and then
only serving files under the current directory (as intended).

This is not really a security issue, given the code is not part of Angular, but
rather just testing infrastructure for Angular itself, and the CI servers are
not expected to contain confidential information, but still worth fixing for
code hygiene.

PR Close angular#32392

packages/zone.js/simple-server.js

@@ -13,22 +13,31 @@ let server;
 
 const localFolder = __dirname;
 
+function writeNotFound(res) {
+  res.writeHead(404, {'Content-Type': 'text/html'});
+  res.end('<h1>404, Not Found!</h1>');
+}
+
 function requestHandler(req, res) {
   if (req.url === '/close') {
     res.end('server closing');
     setTimeout(() => { process.exit(0); }, 1000);
   } else {
-    const file = localFolder + req.url;
+    const file = path.resolve(localFolder, req.url);
+    if (!file.startsWith(localFolder + '/')) {
+      writeNotFound(res);
+      return;
+    }
 
     fs.readFile(file, function(err, contents) {
       if (!err) {
         res.end(contents);
       } else {
-        res.writeHead(404, {'Content-Type': 'text/html'});
-        res.end('<h1>404, Not Found!</h1>');
+        writeNotFound(res);
+        return;
       };
     });
   };
 };
 
-server = http.createServer(requestHandler).listen(8080);
+server = http.createServer(requestHandler).listen(8080);