Error: The WebSocket handshake or connection to the server ended in failure message when accessing the cPanel Terminal

Question

When accessing the Terminal option in the cPanel UI, why do I get the following error?

Error: The WebSocket handshake failed
or
Error: The connection to the server ended in failure at 3:56:05 PM. (ABORTED)

Answer

If you're running CloudLinux, one reason might be that the /bin/cagefs_enter.proxied file no longer has the setuid bit set.  

# stat /bin/cagefs_enter.proxied
  File: ‘/bin/cagefs_enter.proxied’
  Size: 11544      Blocks: 24         IO Block: 4096   regular file
Device: fd00h/64768d    Inode: 6559052     Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-09-14 23:19:10.980052726 +0200
Modify: 2020-09-02 09:53:43.000000000 +0200
Change: 2020-09-14 23:17:45.822484844 +0200
 Birth: -

Permissions should be 4755. CloudLinux is tracking this issue in case CAG-1082. The solution is to run the following:

chmod u+s /bin/cagefs_enter.proxied

This should fix that, and the Terminal should work again.

See also this page for a potential cause of the same error message in the WHM Terminal.