Coder vulnerable to SSH config injection via unsanitized server-supplied values in `coder config-ssh`

Summary

coder config-ssh wrote server-supplied SSH settings (HostnameSuffix, SSHConfigOptions) into the user's ~/.ssh/config without sanitizing embedded newlines or restricting directives so a malicious or compromised Coder server could inject arbitrary SSH configuration.

Note: Practical exploitation requires control of the server-supplied values through a malicious or compromised deployment, a man-in-the-middle position or admin access to the HostnameSuffix and SSHConfigOptions settings.

Impact

A server administrator or an attacker who controlled the server, could inject a directive such as ProxyCommand and achieve arbitrary code execution on any developer workstation that ran coder config-ssh. Injected commands ran with the local user's privileges and applied to all SSH connections, not just Coder workspaces.

Patches

The fix validates HostnameSuffix and SSHConfigOptions against a strict character set that rejects newlines and other control characters.

The fix was backported to all supported release lines:

Workarounds

Inspect coder config-ssh --dry-run output before applying changes.

Resources

• Fix: #26154

Credits

Coder would like to thank Anthropic's Security Team (ANT-2026-22437) for independently disclosing this issue!