security(offbox): validate host/user/repo before the ssh exec (option-injection guard)

Background commit review flagged command/option injection: operator-provided host/user/
repo_path flow into restic's ssh -s sftp command. Reject leading '-' (ssh option
injection, e.g. -oProxyCommand) + metacharacters/traversal; OffboxConfigured fails closed
on an invalid target. Companion test covers the injection cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HxLA1mZurFq9kt8hneFeCs

controller/internal/backup/offbox.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"time"
 
@@ -108,13 +109,49 @@ func generateOffboxPassword() (string, error) {
 	return hex.EncodeToString(b), nil
 }
 
-// OffboxConfigured reports whether the target is set, enabled, and the key + password files exist (so the
-// UI/scheduler can gate a run without leaking why).
+// Off-box target field validation — the security boundary for the values that flow into the `ssh … -s
+// sftp` command restic runs. Host/user are charset-restricted AND must not start with '-' (an ssh
+// OPTION-INJECTION vector: a host like "-oProxyCommand=evil" would make ssh execute an arbitrary command).
+// RepoPath is an absolute, traversal-free, metacharacter-free path. This mirrors the agent's validate.go
+// discipline: validate before any value reaches an exec.
+var (
+	reOffboxHost = regexp.MustCompile(`^[A-Za-z0-9._-]+$`)
+	reOffboxUser = regexp.MustCompile(`^[A-Za-z0-9._-]+$`)
+	reOffboxPath = regexp.MustCompile(`^/[A-Za-z0-9._/-]+$`)
+)
+
+// ValidateOffboxTarget rejects values that could inject into the ssh command line (option injection via a
+// leading '-', shell/space metacharacters, path traversal). Returns nil for a safe target.
+func ValidateOffboxTarget(t *settings.OffboxTarget) error {
+	if t == nil {
+		return fmt.Errorf("no off-box target")
+	}
+	if t.Host == "" || len(t.Host) > 255 || !reOffboxHost.MatchString(t.Host) || strings.HasPrefix(t.Host, "-") || strings.HasPrefix(t.Host, ".") {
+		return fmt.Errorf("invalid NAS host (letters, digits, '.', '-', '_'; must not start with '-' or '.')")
+	}
+	if t.User == "" || len(t.User) > 64 || !reOffboxUser.MatchString(t.User) || strings.HasPrefix(t.User, "-") {
+		return fmt.Errorf("invalid user (letters, digits, '.', '-', '_'; must not start with '-')")
+	}
+	if len(t.RepoPath) > 512 || !reOffboxPath.MatchString(t.RepoPath) || strings.Contains(t.RepoPath, "..") {
+		return fmt.Errorf("invalid repo path (absolute, no spaces/metacharacters, no '..')")
+	}
+	if p := t.Port; p != 0 && (p < 1 || p > 65535) {
+		return fmt.Errorf("invalid port")
+	}
+	return nil
+}
+
+// OffboxConfigured reports whether the target is set, enabled, VALID, and the key + password files exist
+// (so the UI/scheduler can gate a run without leaking why). A target that fails validation is treated as
+// not-configured — fail-closed, so a bad/hostile persisted target can never reach the ssh exec.
 func (m *Manager) OffboxConfigured() bool {
 	t := m.settings.GetOffboxTarget()
 	if t == nil || !t.Enabled || t.Host == "" || t.User == "" || t.RepoPath == "" {
 		return false
 	}
+	if err := ValidateOffboxTarget(t); err != nil {
+		return false
+	}
 	if _, err := os.Stat(m.offboxKeyPath()); err != nil {
 		return false
 	}

controller/internal/backup/offbox_test.go

@@ -265,6 +265,31 @@ func TestOffbox_SecretsAre0600(t *testing.T) {
 
 // --- tiny test helpers ---
 
+// TestOffbox_ValidateRejectsInjection is the security companion: host/user/repo values that could inject
+// an ssh option (leading '-' → e.g. -oProxyCommand) or a shell metacharacter must be REFUSED; a clean
+// target is accepted. A build without this guard would let a hostile target reach the ssh exec → FAIL.
+func TestOffbox_ValidateRejectsInjection(t *testing.T) {
+	ok := &settings.OffboxTarget{Host: "nas.local", Port: 22, User: "felhom", RepoPath: "/srv/repo"}
+	if err := ValidateOffboxTarget(ok); err != nil {
+		t.Fatalf("clean target rejected: %v", err)
+	}
+	bad := []settings.OffboxTarget{
+		{Host: "-oProxyCommand=touch /tmp/pwn", User: "felhom", RepoPath: "/srv/repo"}, // ssh option injection
+		{Host: "nas;rm -rf /", User: "felhom", RepoPath: "/srv/repo"},                  // metacharacters
+		{Host: "nas.local", User: "-oProxyCommand=x", RepoPath: "/srv/repo"},           // user option injection
+		{Host: "nas.local", User: "felhom", RepoPath: "/srv/repo; evil"},              // path metacharacters
+		{Host: "nas.local", User: "felhom", RepoPath: "/srv/../etc"},                  // traversal
+		{Host: "nas local", User: "felhom", RepoPath: "/srv/repo"},                    // space
+		{Host: "nas.local", User: "felhom", RepoPath: "relative/path"},                // non-absolute
+	}
+	for i, b := range bad {
+		bb := b
+		if err := ValidateOffboxTarget(&bb); err == nil {
+			t.Errorf("case %d (%+v) must be rejected", i, bb)
+		}
+	}
+}
+
 func runtimeIsUnix() bool { return os.PathSeparator == '/' }
 
 func contains(ss []string, want string) bool {