Cannot access Mailman in Plesk: Bug in Mailman version 2.1.15. OSError: [Errno 13] Permission denied

Symptoms

• When accessing Mailmail via Plesk > Domains > example.com > Mail Settings > Mailing Lists > list_name > Manage, the error below is displayed in a browser:

  CONFIG_TEXT: Bug in Mailman version 2.1.15
  We're sorry, we hit a bug!
  Please inform the webmaster for this site of this problem. Printing of
  traceback and other system information has been explicitly inhibited, but
  the webmaster can find this information in the Mailman error logs.

• In the /var/log/mailman/error log file the following can be found:

  CONFIG_TEXT: admin(3365556): [----- Mailman Version: 2.1.15 -----]
  admin(3365556): [----- Traceback ------]
  admin(3365556): Traceback (most recent call last):
  admin(3365556): File "/usr/lib/mailman/scripts/driver", line 112, in run_main
  admin(3365556): main()
  admin(3365556): File "/usr/lib/mailman/Mailman/Cgi/admin.py", line 227, in main
  admin(3365556): mlist.Save()
  admin(3365556): File "/usr/lib/mailman/Mailman/MailList.py", line 578, in Save
  admin(3365556): self.__save(dict)
  admin(3365556): File "/usr/lib/mailman/Mailman/MailList.py", line 555, in __save
  admin(3365556): os.link(fname, fname_last)
  admin(3365556): OSError: [Errno 13] Permission denied

• The server is running with CloudLinux.

Cause

Link traversal protection is enabled:

# sysctl fs.protected_symlinks_create
fs.protected_symlinks_create = 1

# sysctl fs.protected_hardlinks_create
fs.protected_hardlinks_create = 1

Note: According to the Plesk Obsidian Installation requirements such an option should be disabled to avoid potential issues.

Resolution

1. Connect to the server via SSH.

2. Open the file /etc/sysctl.conf in any text editor, for example, in the vi text editor:

   # vi /etc/sysctl.conf

3. Edit the file and change the lines below:
   Before:

   CONFIG_TEXT: fs.protected_symlinks_create = 1
   fs.protected_hardlinks_create = 1

   After:

   CONFIG_TEXT: fs.protected_symlinks_create = 0
   fs.protected_hardlinks_create = 0

4. Save changes and close the file.

5. Apply changes by executing the command below:

   # sysctl -p​