Kopia: RCE via SSH ProxyCommand Injection

Summary

Kopia's HTTP server, when started with --without-password , accepts unauthenticated requests to /api/v1/repo/exists. The handler forwards an attacker-supplied storage configuration to blob.NewStorage. For SFTP backends with externalSSH: true, that path constructs a process command line by splitting sshArguments on spaces and passes the result directly to exec.CommandContext("ssh"). An -oProxyCommand= token in sshArguments causes OpenSSH to invoke ` via $SHELL -c` before any TCP connection is attempted, giving the requester arbitrary command execution as the Kopia process user.

Analysis

`internal/server/server_authz_checks.go` lines 61–73:

when the server is started without --server-username or --server-password, getAuthenticator() returns nil and requireUIUser unconditionally authorizes the request. Every endpoint registered through handleUIPossiblyNotConnected becomes accessible without credentials.

`repo/blob/sftp/sftp_storage.go` lines 448–468:

opt.SSHArguments is populated from the JSON request body (storage.config.sshArguments). The string is split only on the literal ASCII space character, there is no shell style tokenizer, no quote handling, and no allowlist. Whatever tokens the caller supplies are appended to the ssh argv.

OpenSSH treats -oProxyCommand= as a directive to execute ` via the user's shell ($SHELL -c `) and pipe the SSH transport over its stdio. The shell invocation happens before SSH attempts a TCP connection, so the command runs even when the target host is unreachable.

Impact

No user interaction is required. No valid credentials are required. The exploit is a single HTTP request.

Credits

This vulnerability was discovered and responsibly disclosed by Daniele Berardinelli.

## Mitigation https://github.com/kopia/kopia/pull/5354 disallows starting of a server without a password which also listens on a non-loopback interface.