register_credential_file() takes a skill-declared relative path from
required_credential_files frontmatter and bind-mounts it read-only into the
remote sandbox the skill's own code runs in. It validates that the resolved
path stays inside HERMES_HOME — the docstring names the threat directly:
so that a malicious skill cannot declare
required_credential_files: ['../../.ssh/id_rsa'] and exfiltrate
sensitive host files into a container sandbox
Containment is the wrong boundary on its own, because HERMES_HOME is exactly
where the master credential stores live. Traversal is blocked; asking for the
keys by name is not:
skill declares mounted? agent may read it?
.env YES DENIED
auth.json YES DENIED
.anthropic_oauth.json YES DENIED
cache/bws_cache.json YES DENIED
mcp-tokens/srv.json YES DENIED
google_token.json YES allowed
../../.ssh/id_rsa no n/a
Every row marked DENIED is refused by the canonical read guard
(agent.file_safety.get_read_block_error) — the agent cannot read_file them —
yet one line of hub-installed skill frontmatter gets them bind-mounted where
that skill can cat them. .env alone is every provider API key.
Reuse the canonical deny-list as the mount bar: what the agent is forbidden
to read is not mountable either, so the mount surface cannot hand a skill
what the read surface denies it. Fails CLOSED — if the guard can't be
consulted the mount is refused rather than risked.
The module keeps doing its job: a skill still mounts its own service token
(google_token.json, skills/*), and a refused entry is reported back through
register_credential_files' missing list instead of failing the batch.
The three prior PRs here (#3946, #3951, #4316) all hardened traversal; this
closes the half that traversal validation never covered.
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