Follow-up hardening on top of the C14 cherry-picks (#57860/#44026/#66742/#60009):
- Slack file downloads (_download_slack_file/_download_slack_file_bytes)
now require an https URL on a Slack CDN host (files.slack.com,
*.slack.com Enterprise Grid, *.slack-files.com legacy shares) before
attaching the bot token. url_private/url_private_download only ever
point at the Slack CDN, so a forged file object from a malicious
workspace app or compromised event stream pointing the Bearer-token
download at an arbitrary PUBLIC host (token exfiltration) is now
refused — a hole #44026's generic private-IP SSRF check alone could
not close.
- The same two download paths now use create_ssrf_safe_async_client
(from #57860) so the preflight-validated hostname is resolved once,
validated, and dialed by IP — closing the DNS-rebinding TOCTOU window
for the token-bearing inbound fetches as well.
- #60009's slack_tokens.json permission warning is generalized into
utils.warn_if_credential_file_broadly_readable() (POSIX-only,
fail-quiet) and wired into the other read path with the same gap:
google_chat's load_user_credentials(). google_chat already writes
0o600 via _write_private_json; the read-time warning covers
hand-provisioned/legacy files. Nothing in-repo writes
slack_tokens.json (user/OAuth-provisioned), so there is no write
path to chmod for Slack.
Security tests both directions: non-CDN/lookalike/http URLs and
connect-time DNS rebinds are blocked before any TCP connect; real
files.slack.com, Enterprise Grid, and slack-files.com URLs still reach
the network layer; 0o600 files stay silent while 0o644/0o640 warn with
a chmod hint. A/B: all 10 new download-guard tests fail with the
hardening reverted and pass with it applied.
