i personally use many techniques too login directly into joomla admin panel. however the came back value is same az the login page. even if the password are correct.

example:

WebClient Client = new WebClient()

System.Collections.Specialized.NameValueCollection Collection =

    new System.Collections.Specialized.NameValueCollection()

Collection.Add("username", "--my username--")

Collection.Add("passwd", "--my password--")

Collection.Add("option", "com_login")

Colletion.Add("e0484cdc56d8ccc42187d26a813324ba", "1")

Collection.Add("lang", "")

Client.Proxy = null

byte[] res = Client.UploadValues(

    "http://127...1/administrator/index.php", "Publish", Collection)

textBox1.Text = Encoding.UTF8.GetString(res, , res.Length)

the issue is with this particular line:

Colletion.Add("e0484cdc56d8ccc42187d26a813324ba", "1")

that is joomla's CSRF anti-spoofing token. Joomla! tries to safeguard againt CSRF by placing a this token into each Publish form and every GET query string that's in a position to modify something within the Joomla! system. This random string provides protection because besides the jeopardized site have to know the Link to the prospective site along with a valid request format for that target site, additionally, it have to know the random string which changes for every session and every user.

To be able to sent a proper token together with your login request you'd need to:

  1. First of all request a proper log-in form by GET while using "Client object" request
  2. Retrieve the token with regex /title="([a-zA-z0-9])"/
  3. Send the log-in request using the token

Best Of Luck

EDIT: For your "collection" add yet another param:

Collection.Add("task", "login")