[Security][Low] Harden local credential-at-rest handling (plaintext tokens, TOCTOU perms, unvalidated bearer target)

[data-wamf]

Severity: Low · Category: Credential-at-rest / hardening
Assessed at: main head 673a32d9699e869be910ed1caa3cef3aa87d94ce (read-only security health-check, 2026-07-29)

Summary

slack_cli is well-hardened overall — GitHub Actions are SHA-pinned with least-privilege permissions: contents: read (resolving closed #24/#25/#26), and no secrets are committed (.env.example is an empty template, .env is gitignored). The items below are Low-severity hardening opportunities in local credential handling; filing one consolidated issue rather than several.

Findings (all Low, confirmed)

1. Plaintext token storage. OAuth access token (xoxp-…) and Socket Mode app token (xapp-…) are written unencrypted to ~/.dart_slack/:
   • lib/src/auth/credentials_store.dart:37-40
   • lib/src/auth/app_config_store.dart:47-51
     Standard for CLIs but no OS-keychain integration.
2. File permissions applied after write (TOCTOU) and not on Windows. _setFilePermissions() runs chmod 600 only on macOS/Linux and only after the file is created with the default umask:
   • lib/src/auth/credentials_store.dart:59-63, lib/src/auth/app_config_store.dart:68-72, lib/src/auth/oauth_flow.dart:174-179 (TLS key/cert).
     Brief world/group-readable window; on Windows no restriction is applied.
3. Bearer token forwarded to API-supplied URLs without host validation. lib/src/slack_api/slack_api_client.dart:307-313 attaches Authorization: Bearer to whatever url_private_download URL the API returned; lib/src/socket_mode/socket_mode_client.dart:690 connects to the API-supplied WS URL. Low (relies on authenticated TLS Slack responses).

Minor (informational): command_runner.dart:1026-1028 prints full stack traces to stderr on parse errors (gate behind --verbose); credentials_store.dart:17-19 interpolates HOME without the resilient fallback used by AppConfigStore; .gitignore ignores pubspec.lock (commit it for reproducible builds of an executable package).

Remediation

• Create credential/key files atomically with mode 0600 (temp-file + rename) and apply an equivalent ACL on Windows.
• Optionally integrate an OS keyring (Keychain / libsecret / Windows Credential Manager).
• Validate API-returned URL hosts against a Slack-domain allowlist before attaching the bearer token.