[gold-silver-copper]

Description

On unix, RIG_HOME/sessions/ and every session directory were created with the default umask, usually 0755. With a traversable RIG_HOME and the default directory store, other local users could read every conversation, spill file and traffic log. auth/ was already 0700.

What users see change: on unix, sessions/ and each new session directory are now mode 0700. An existing wider sessions/ is tightened to 0700 the next time a session starts, by whichever path creates it. RIG_HOME itself, existing session directories inside it, and file modes are left as they are; a 0700 sessions/ already keeps them from other users. Windows and wasm are untouched (everything is cfg(unix)).

Both creation paths change:

• the launcher's Home::new_session (crates/rig-harness-protocol/src/lib.rs): sessions/ is set to 0700 after create_dir_all, and the session directory is created with DirBuilder mode 0700. If sessions/ can't be made 0700 (another owner, or a filesystem that refuses chmod), it keeps its mode and the session starts, as on main; the new session's own directory is still 0700.
• the harness started without the launcher, paths_in (crates/rig-harness/src/host/session.rs): the same two steps. Errors are still ignored there, as before, and show up as a failed log write.

The umask can only narrow the DirBuilder mode, so 0700 holds whatever the umask is. Tightening an existing directory needs set_permissions. No pub item is added or changed. The two crates can't share one helper without a new public item, so each crate has the few lines inline.

Bug	Verdict	Test
Architecture audit 2026-11-07, d-7: session directories created 0755	reproduced on base 63431d7, fixed	rig-harness-protocol tests::a_new_session_directory_is_private_to_the_user (fresh home with sessions/ pre-made 0755: sessions/ and the new session directory end up & 0o077 == 0); rig-harness host::session::tests::a_session_directory_made_without_the_launcher_is_private_to_the_user (paths_in on a fresh home)

There is no issue or outside PR for this. This change differs from the audit's repro diff in one way: the repro had only a test, and this test also covers tightening an existing sessions/.

Changelog

• (harness) On unix, sessions/ and every session directory under RIG_HOME are created mode 0700, and an existing wider sessions/ is tightened when a session starts, so other local users can no longer read conversations and logs.

Migration

None.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

• Both tests fail on the test-only commit (mode 0755) and pass on the head, run on a Linux build machine
• cargo xtask verify --affected --base origin/main passes at the head

Checklist:

• I have updated READMEs and Rust docs affected by this change (new_session's # Errors)
• I have added tests that prove my fix is effective or that my feature works
• I did not edit CHANGELOG.md or MIGRATING.md (they are generated at release)

Notes

Line deltas: production +21 / -2 (2 files), tests +54 / -1 (2 files).

Real use: an independent tester ran real rig (launcher, interactive, and the agent alone) on main and this PR: sessions/ and session directories 755 on main, 700 here; an existing 0755/0777/0750 sessions/ tightened; RIG_HOME, files and old session directories unchanged; resume works.

Second fix commit (from review): new_session ignores a failed chmod of sessions/, as paths_in does, so a group-shared sessions/ owned by another user, or one on vfat, still starts a new session as on main.

Not verified: macOS.

Follow-ups (not in this PR): RIG_HOME itself and session directories from before this change keep their modes. A 0700 sessions/ already protects those directories. Other parts of the audit's proposal 3 (secret leaks in MCP header expansion and the cloak browser opener) are handled separately.