Troubleshoot PuTTY X11 proxy: unsupported authorization protocol

PuTTY X11 is a mechanism to gain remote access to applications and servers by forwarding the display and control of these applications to the local host machine. It enables the user’s local machine to interact with the GUI/SSH of remote machines which helps in easy access.

PuTTY X11 proxy unsupported authorization protocol
PuTTY X11 proxy unsupported authorization protocol Fix

However, sometimes trying to access the servers or applications, the user can run into the PuTTY X11 proxy: Unsupported authorization protocol error. This error can be caused by a number of reasons which include:

  • Wrong ownership of the Xauthority file (and Locked Files)
  • Corrupt Xauthority file
  • Invalid configuration of PuTTY
  • Incorrect authorization settings Firewall
  • X11forwarding is not configured in SSH config

This article demonstrates the methods to solve the unsupported authorization error for PuTTY.

Solution 1: Change Xauthority file ownership

The Xauthority file is the main component to use PuTTY because this file stores the cookies and credentials for the Xserver for the purpose of connection authentication. An unsupported authorization protocol error occurs when the Xauthority file is missing or the user is not the owner of the file. Let us follow the steps and solve this error.

  1. Use the given list command to find the Xauthority file and confirm its ownership:
    ls -al .Xauthority
    command to find Xauthority file
    Find the Xauthority file

    The Xauthority file is currently owned by the root user, let’s Access the server using PuTTY to see how it responds to this ownership issue:

    Unsupported authorization protocol error
    Unsupported authorization protocol

    The above output shows that it is necessary for the user to have control over the Xauthority.

  2. Shift the ownership of the Xauthority file to the logged-in user and to do that execute the following command:
    Note: Write the username that you want to use instead of “linux-user” given in the command:

    sudo chown linux-user .Xauthority
    command to change the ownership of Xauthority
    Change Xauthority ownership
  3. Once the unsupported authorization protocol error occurs, the Xauthority file is often locked. When this file is locked, having the correct ownership will also not resolve the issue. Therefore, the user needs to check for the locked files and remove them. run the ls -al command to check if Xauthority-c and Xauthority-l files exist:
    ls -al
    ls -al list show command
    Show list to find Xauthority files

    If these files exist then remove the Xauthority-c and Xauthority-l files by using the following command:

    rm .Xauthority-c .Xauthority-l

    : Restart your machine and PuTTY connection for the changes to take effect.

  4. Reconnect to the PuTTY server and call “xclock” to test the server:
    PuTTY connected
    Connected to PuTTY

    The authorization error has been successfully resolved.

Solution 2: Replace Xauthority corrupt file

In some cases, the Xauthority file can be corrupted and due to that, access to the server is not authorized. To solve this issue you will need to do the following steps

  1. Run the “ls – al” command to check if our Xauthority file exists:
    List command
  2. To remove the Xauthority file, use the following command:
    rm .Xauthority
    Command to remove Xauthority file
    Remove Xauthority file
  3. Once you have deleted the Xauthority file reconnect the server using PuTTy, this will create your new Xauthority file in the directory, and run the “xclock” command:
    Accessing server using PuTTY
    Connection established successfully

    You have successfully logged into PuTTY and accessed the Xclock.

Solution 3: Reconfigure the Invalid configuration of PuTTY

Setting up your PuTTY is also essential and sometimes, users make mistakes while configuring or setting up their PuTTY. To solve this basic error, follow the steps provided below:

  1. Open up PuTTY on your PC, click the Default Settings file, and load it:
    PuTTY configuration settings
    Load default settings
  2. Now, locate the SSH tab and click on the X11 option. Enable the X11Forwarding and set “X display location” to “localhost0”:
    SSH and X11 configuration in PuTTY
    Configuring X11 forwarding in PuTTY
  3. Now you will need to input the IP address or Host Name of the server you are going to access. After setting the Hostname or IP address, set the Port to 22 (or whatever port you have set for SSH) and then save the“Default File”:
    Host name or IP address in PuTTY
  4. Click the Open button and you will see the PuTTY terminal:
    PuTTY login
    PuTTY login

    In the PuTTY terminal, you will have to input the Username and the password of the server you are going to access. Once you pass these two arguments you will gain access to the remote server:

    PuTTY gained access
    PuTTY accessed server

    As you can see you have successfully set up PuTTY and accessed the server.

Solution 4: Allow Firewall configuration for port 22

To use the X11Forwarding and PuTTY, you will need to configure the Firewall and enable Port 22. Here, the UFW utility is used to configure the Firewall:

  1. Execute the following command in your terminal to allow the SSH server to gain access to port 22 through the Firewall:
    sudo ufw allow ssh
    Allow SSH in Firewall
    Give access to SSH server through Firewall
  2. Enable the Firewall using the following command:
    sudo ufw enable
    Enable Firewall
    Enable Firewall
  3. Check the Firewall status with the following command:
    sudo ufw status
    Check Firewall status
    Check Firewall status

    As we can see that both 22/tcp and 22/tcp (v6) are now enabled.Once we have completed these steps, reconnect the PuTTY X11 and see if the error is resolved. To connect to the PuTTY server, input the remote system login name and password to access the server.

    PuTTY accessed remote server
    PuTTY server access

    You have successfully resolved the error and gained access to the remote server.

Solution 5: Configure X11Forwarding in SSH Server config

Another configuration error due to which PuTTY X11 authorization is denied is the invalid configuration of the SSH server. To configure the SSH server you will run the following command in your terminal.

sudo nano /etc/ssh/sshd/config
  1. By running this command you will gain access to the SSH server’s config file:
    Access SSH server
    Access SSH server
  2. In the SSH configuration, find X11Forwarding, then uncomment it to enable it. Set the X11Forwarding parameter to “yes”:
    X11Forwarding enabled
    Enable X11Forwarding
  3. After that find “X11Displayoffset” also enable this setting and set the display to “10”:
    X11DisplayOffset enabled
    Enable X11Displayoffset
  4. After enabling the above settings enable the set “X11UseLocalhost” to “no”:
    X11UseLocalhost to no
    Set X11UseLocalhost to no
  5. Furthermore, enable and set the “PrintMotd” setting to “no”:
    set PrintMotd to no
    Set PrintMotd to no
  6. After completing all the above steps, save the settings by using CTRL+O and then exit the config file by using CTRL+X. After that, try to reconnect to the server using PuTTY:
    PuTTY access
    PuTTY connected to server
  7. You have successfully logged into the server. Try and access the xclock by using the following command in your PuTTY terminal:
    PuTTY gain access
    PuTTY connected and xclock accessed

    You have successfully solved the error and gained access to the remote machine.

The PuTTY X11 proxy: unsupported authorization protocol error is where you can’t gain access to the remote server using PuTTY. The reasons for this error to occur have been mentioned in this post along with their solutions.


Kevin Arrows

Kevin Arrows is a highly experienced and knowledgeable technology specialist with over a decade of industry experience. He holds a Microsoft Certified Technology Specialist (MCTS) certification and has a deep passion for staying up-to-date on the latest tech developments. Kevin has written extensively on a wide range of tech-related topics, showcasing his expertise and knowledge in areas such as software development, cybersecurity, and cloud computing. His contributions to the tech field have been widely recognized and respected by his peers, and he is highly regarded for his ability to explain complex technical concepts in a clear and concise manner.