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:bulb: Authentication errors and resolution when running Snap Firefox over SSH + X11 forwarding from Windows MobaXterm to an Ubuntu server.

Environment: Windows + MobaXterm (X11 proxy) → Ubuntu 22.04 server + Firefox snap 149.0.2

[01] The Problem

After SSH’ing into the Ubuntu server with X11 forwarding enabled in MobaXterm:



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$ firefox &
libpxbackend-1.0.so: cannot open shared object file: No such file or directory
Failed to load module: /home/user/snap/firefox/common/.cache/gio-modules/libgiolibproxy.so
MobaXterm X11 proxy: Unsupported authorisation protocol
Error: cannot open display: localhost:10.0

What the errors say:

Error Meaning
libpxbackend-1.0.so missing Snap sandbox library loading issue
Unsupported authorisation protocol X11 auth protocol mismatch
cannot open display: localhost:10.0 Final display connection failure

[02] Root Cause — DISPLAY vs xauth Mismatch

2-1. Check Environment



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$ echo $DISPLAY
localhost:10.0

$ xauth list
myserver/unix:10  MIT-MAGIC-COOKIE-1  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The core issue:

Item Value Format
DISPLAY localhost:10.0 TCP form
xauth entry myserver/unix:10 Unix socket form

MobaXterm’s X11 proxy connects via TCP (localhost:10.0), but xauth only has a Unix-socket cookie (/unix:10). The two formats don’t match.

2-2. Other X11 Apps Work — Why Only Firefox?



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$ xdpyinfo
name of display:    localhost:10.0
version number:    11.0
vendor string:    The X.Org Foundation
...

xdpyinfo works fine. The X11 server itself isn’t the problem.

[03] Real Cause — Snap Sandbox Blocks Environment Variables

3-1. Inside the Snap Environment



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$ snap run firefox env | grep -i xauth
XAUTHORITY=

XAUTHORITY is empty! The Snap sandbox didn’t inherit the parent environment variables.

More serious:



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$ snap run firefox which xauth
(no output)

There is no xauth binary inside Snap. It can’t create or verify the cookie.

3-2. The Problem Diagram



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Regular X11 apps (xdpyinfo etc.)
  → use system xauth → cookie verified → O connection succeeds

Snap Firefox
  → enters Snap sandbox
  → XAUTHORITY empty
  → no xauth binary
  → cookie verification impossible
  → X "Unsupported authorisation protocol"

[04] Attempted Fixes

4-1. Try 1 — xauth add for TCP Cookie



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$ xauth add localhost:10 MIT-MAGIC-COOKIE-1 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xauth:  unable to write authority file /home/user/.Xauthority

xauth refused to add the TCP-form entry.

4-2. Try 2 — xauth generate 



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$ xauth generate :10
xauth:  (null):0:  unable to resolve "SECURITY" extension to get random data

Security extension query failed too.

4-3. The Real Fix — Set XAUTHORITY Explicitly

Since the root cause is missing XAUTHORITY inside the Snap sandbox, specify it explicitly.



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$ XAUTHORITY=$HOME/.Xauthority firefox --no-remote &

Firefox appears in MobaXterm.

xhost for extra permissions also helps:



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$ xhost +local:
access control enabled, only authorized clients can connect
LOCAL:

[05] Permanent Setup

Add to ~/.bashrc so you don’t have to type it every time.



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# Append to ~/.bashrc
export XAUTHORITY="$HOME/.Xauthority"





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$ source ~/.bashrc
$ firefox --no-remote &
# Now works normally

[06] Bonus — Broken Korean Font

If Firefox launches but Korean text shows as boxes (□):



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$ fc-list | grep -i hangul
(almost no output)

No Korean font installed.

Fix:



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$ sudo apt update
$ sudo apt install fonts-noto-cjk fonts-nanum -y
$ fc-cache -fv

Run Firefox again — Korean renders correctly.

[07] Summary

7-1. Final Commands



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# One-shot
XAUTHORITY=$HOME/.Xauthority firefox --no-remote &

# Permanent (add to ~/.bashrc)
export XAUTHORITY="$HOME/.Xauthority"

7-2. Key Lessons

Lesson Description
Snap doesn’t inherit env vars For security, doesn’t pass XAUTHORITY from parent
Check xauth format mismatch Unix socket (/unix:10) ≠ TCP (localhost:10)
One X11 app working doesn’t mean all do xdpyinfo works but Snap apps may fail due to sandbox
No xauth binary inside Snap Provide XAUTHORITY path explicitly
Install fonts separately fonts-noto-cjk, fonts-nanum for Korean rendering

:bulb: This isn’t a simple X11 auth issue — it’s the incomplete interaction between the Snap sandbox and remote X11 forwarding. The non-Snap Firefox package wouldn’t have this problem.

참고