diff --git a/cmd/attesto/main.go b/cmd/attesto/main.go
index d31ed85..859382b 100644
--- a/cmd/attesto/main.go
+++ b/cmd/attesto/main.go
@@ -1801,7 +1801,47 @@ func writeConfig(path string, cfg cliConfig) error {
 	if err != nil {
 		return err
 	}
-	return os.WriteFile(path, append(raw, '\n'), 0o600)
+	return writeFileAtomic0600(path, append(raw, '\n'))
+}
+
+func writeFileAtomic0600(path string, body []byte) error {
+	dir := filepath.Dir(path)
+	tmp, err := os.CreateTemp(dir, "."+filepath.Base(path)+".tmp-")
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	removeTmp := true
+	defer func() {
+		if removeTmp {
+			_ = os.Remove(tmpName)
+		}
+	}()
+	if err := tmp.Chmod(0o600); err != nil {
+		_ = tmp.Close()
+		return err
+	}
+	written, err := tmp.Write(body)
+	if err != nil {
+		_ = tmp.Close()
+		return err
+	}
+	if written != len(body) {
+		_ = tmp.Close()
+		return fmt.Errorf("short atomic write: wrote %d of %d bytes", written, len(body))
+	}
+	if err := tmp.Sync(); err != nil {
+		_ = tmp.Close()
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		return err
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		return err
+	}
+	removeTmp = false
+	return nil
 }
 
 func redactValue(value any) any {
diff --git a/cmd/attesto/main_test.go b/cmd/attesto/main_test.go
index 5e2d61f..ce5137d 100644
--- a/cmd/attesto/main_test.go
+++ b/cmd/attesto/main_test.go
@@ -156,6 +156,36 @@ func TestConfigSetRedactsSecrets(t *testing.T) {
 	if !strings.Contains(string(raw), cliTestAPIKey) {
 		t.Fatalf("config did not persist api key")
 	}
+	info, err := os.Stat(config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info.Mode().Perm() != 0o600 {
+		t.Fatalf("config mode = %o, want 600", info.Mode().Perm())
+	}
+	matches, err := filepath.Glob(filepath.Join(dir, ".config.json.tmp-*"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(matches) != 0 {
+		t.Fatalf("atomic config temp files leaked: %v", matches)
+	}
+}
+
+func TestConfigSetFailsWhenConfigPathIsDirectory(t *testing.T) {
+	dir := t.TempDir()
+	env := testEnv(t, map[string]string{
+		"ATTESTO_CONFIG": dir,
+		"ATT_API_KEY":    cliTestAPIKey,
+	})
+	var stdout, stderr bytes.Buffer
+	code := run([]string{"--json", "config", "set", "--api-key-env", "ATT_API_KEY"}, &stdout, &stderr, env)
+	if code == 0 {
+		t.Fatal("config set must fail when config path is a directory")
+	}
+	if strings.Contains(stdout.String(), cliTestAPIKey) || strings.Contains(stderr.String(), cliTestAPIKey) {
+		t.Fatalf("secret leaked on write failure: stdout=%s stderr=%s", stdout.String(), stderr.String())
+	}
 }
 
 func TestLocalVaultSpoolAndStatus(t *testing.T) {