How to fix subsystem request failed on channel 0 in SSH

First, check your sshd_config file for syntax errors and confirm the Subsystem directive is present and uncommented.

On Windows:

C:\ProgramData\ssh\sshd_config

On Linux/macOS:

/etc/ssh/sshd_config

Look for the line:

Subsystem sftp /usr/libexec/openssh/sftp-server

or on Windows:

Subsystem sftp sftp-server.exe

To validate the configuration syntax:

# Linux/macOS
sshd -t

# Windows (PowerShell as admin)
C:\Windows\System32\OpenSSH\sshd.exe -t

Confirm the sftp-server binary is present at the path specified in sshd_config.

On Linux/macOS:

ls -la /usr/libexec/openssh/sftp-server
# or try the alternative location
ls -la /usr/lib/openssh/sftp-server

On Windows (PowerShell as admin):

Test-Path C:\Windows\System32\OpenSSH\sftp-server.exe

If the binary is missing, reinstall or repair OpenSSH on your system.

On Windows, ensure the SSH user account has a valid user profile.

# Check if user profile exists
Test-Path C:\Users\username

# Verify SSH service runs as Local System or correct account
Get-Service sshd | Select-Object StartType, Status

If the user profile is missing, create it:

# Log in as that user once locally or via RDP to initialize the profile

If you still have issues, try connecting with a different local admin account to isolate whether it's user-specific.

After making any changes to sshd_config, restart the SSH daemon for changes to take effect.

On Windows (PowerShell as admin):

Restart-Service -Name sshd -Force

On Linux/macOS:

sudo systemctl restart sshd
# or
sudo service ssh restart

Use verbose SSH flags to see detailed error messages during the subsystem request.

# SFTP verbose test
sftp -vvv username@hostname

# SCP verbose test
scp -vvv file.txt username@hostname:/tmp/

Verbose output often reveals whether the subsystem path is wrong, permissions are denied, or the binary is missing.

Ensure the SSH daemon can execute the subsystem binary.

On Linux/macOS:

# Check file exists and is executable
ls -la /usr/libexec/openssh/sftp-server

# If not executable, fix it
sudo chmod 755 /usr/libexec/openssh/sftp-server

On Windows:
Temporarily disable Windows Defender or antivirus software and retry. Some security software blocks sftp-server.exe. If that fixes it, add an exclusion for:

C:\Windows\System32\OpenSSH\sftp-server.exe

If you're using chroot with SFTP, the traditional sftp-server path won't work inside the chroot jail. Instead, use internal-sftp:

In sshd_config:

Match User sftpuser
  ChrootDirectory /var/sftp
  ForceCommand internal-sftp
  AllowTcpForwarding no
  X11Forwarding no
EndMatch

Then restart sshd. The internal-sftp subsystem doesn't require an external binary.

If the above steps don't resolve the issue, uninstall and reinstall the OpenSSH feature.

On Windows (PowerShell as admin):

# Uninstall
Remove-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0

# Reinstall
Add-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0

# Start the service
Start-Service sshd
Set-Service -Name sshd -StartupType Automatic

On Linux (example for Ubuntu/Debian):

sudo apt remove openssh-server openssh-client
sudo apt install openssh-server openssh-client
sudo systemctl restart sshd