Backup: PHP Exception on Missing SFTP Public Key

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Describe the bug

I was doing some maintenance and I deleted the SSH key used to backup configurations via SFTP. This caused PHP to throw an exception and log the following in Crash Reporter. A more user friendly message may be needed instead.

[15-Feb-2026 01:20:25 America/Chicago] PHP Fatal error:  Uncaught Exception: ** WARNING: connection is not using a post-quantum key exchange algorithm.

** This session may be vulnerable to "store now, decrypt later" attacks.

** The server may need to be upgraded. See https://openssh.com/pq.html

opnsense@host.domain.tld: Permission denied (publickey).

Connection closed

 in /usr/local/opnsense/mvc/app/library/OPNsense/Backup/Sftp.php:174
Stack trace:
#0 /usr/local/opnsense/mvc/app/library/OPNsense/Backup/Sftp.php(202): OPNsense\Backup\Sftp->sftpCmd('ls -lnt config-...')
opnsense/core#1 /usr/local/opnsense/mvc/app/library/OPNsense/Backup/Sftp.php(251): OPNsense\Backup\Sftp->ls('config-*.xml')
opnsense/core#2 /usr/local/opnsense/scripts/system/remote_backup.php(17): OPNsense\Backup\Sftp->backup()
opnsense/core#3 {main}
  thrown in /usr/local/opnsense/mvc/app/library/OPNsense/Backup/Sftp.php on line 174

To Reproduce

Steps to reproduce the behavior:

1. Configure a working sftp backup configuration
2. Delete the SSH key under ~/.ssh/authorized_keys
3. Make a configuration change that triggers a backup

Expected behavior

A message under System Status that says "Configuration backups via SFTP failed."

Environment

OPNsense 26.1.2-amd64
Intel(R) Core(TM) i3-N305