fix: prevent path traversal via session_id in session_store

load_session/save_session built the session file path directly from the
caller-supplied session_id with no validation, so a value like
'../../../tmp/secrets' (reachable from the 'load-session' CLI subcommand)
could read or write JSON files outside the session directory.

Add _validate_session_id() to reject anything outside [A-Za-z0-9_-]
(path separators, '..', absolute/Windows paths) before the id is used to
build a filename. Legitimate ids (uuid4 hex, 'session-<digits>-<n>') are
unaffected.

Adds SessionStorePathTraversalTests covering traversal rejection on both
load and save plus a legitimate-id round-trip.

src/session_store.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import json
+import re
 from dataclasses import asdict, dataclass
 from pathlib import Path
 
@@ -15,16 +16,31 @@ class StoredSession:
 
 DEFAULT_SESSION_DIR = Path('.port_sessions')
 
+# Session ids become filenames inside the session directory, so they must not be
+# able to escape it via path separators or traversal. Restrict to the charset
+# produced by legitimate ids (uuid4 hex, `session-<digits>-<n>`).
+_SESSION_ID_PATTERN = re.compile(r'\A[A-Za-z0-9_-]+\Z')
+
+
+def _validate_session_id(session_id: str) -> str:
+    if not _SESSION_ID_PATTERN.match(session_id):
+        raise ValueError(
+            f'invalid session_id {session_id!r}: only [A-Za-z0-9_-] characters are allowed'
+        )
+    return session_id
+
 
 def save_session(session: StoredSession, directory: Path | None = None) -> Path:
+    session_id = _validate_session_id(session.session_id)
     target_dir = directory or DEFAULT_SESSION_DIR
     target_dir.mkdir(parents=True, exist_ok=True)
-    path = target_dir / f'{session.session_id}.json'
+    path = target_dir / f'{session_id}.json'
     path.write_text(json.dumps(asdict(session), indent=2))
     return path
 
 
 def load_session(session_id: str, directory: Path | None = None) -> StoredSession:
+    session_id = _validate_session_id(session_id)
     target_dir = directory or DEFAULT_SESSION_DIR
     data = json.loads((target_dir / f'{session_id}.json').read_text())
     return StoredSession(

tests/test_security_scope.py

@@ -9,9 +9,58 @@ from src.models import PermissionDenial
 from src.path_scope import WorkspacePathScope, extract_path_candidates
 from src.permissions import ToolPermissionContext
 from src.query_engine import QueryEnginePort
+from src.session_store import StoredSession, load_session, save_session
 from src.tools import execute_tool
 
 
+class SessionStorePathTraversalTests(unittest.TestCase):
+    def test_traversal_session_id_is_rejected_on_load(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            store = Path(tmp) / 'store'
+            store.mkdir()
+            # A secret living outside the session store that traversal would reach.
+            (Path(tmp) / 'secret.json').write_text('{}')
+
+            with self.assertRaises(ValueError):
+                load_session('../secret', directory=store)
+
+    def test_absolute_and_separator_session_ids_are_rejected(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            store = Path(tmp) / 'store'
+            store.mkdir()
+            for bad_id in ('/etc/passwd', 'nested/child', '..', 'a/../../b'):
+                with self.assertRaises(ValueError):
+                    load_session(bad_id, directory=store)
+
+    def test_traversal_session_id_is_rejected_on_save(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            store = Path(tmp) / 'store'
+            store.mkdir()
+            session = StoredSession(
+                session_id='../escape',
+                messages=('hello',),
+                input_tokens=1,
+                output_tokens=2,
+            )
+            with self.assertRaises(ValueError):
+                save_session(session, directory=store)
+
+    def test_legitimate_session_id_round_trips(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            store = Path(tmp) / 'store'
+            session = StoredSession(
+                session_id='session-1775386832313-0',
+                messages=('hi', 'there'),
+                input_tokens=3,
+                output_tokens=4,
+            )
+            path = save_session(session, directory=store)
+
+            self.assertEqual(store, path.parent)
+            loaded = load_session('session-1775386832313-0', directory=store)
+            self.assertEqual(session, loaded)
+
+
 class WorkspacePathScopeTests(unittest.TestCase):
     def test_direct_parent_escape_is_denied(self) -> None:
         with tempfile.TemporaryDirectory() as tmp: