From 4d91554fe6fcab7ef4925da1b36602125101ed45 Mon Sep 17 00:00:00 2001
From: Imran Siddique <imran.siddique@opaque.co>
Date: Sat, 6 Jun 2026 17:08:34 -0700
Subject: [PATCH] fix(config): reject path traversal in policy_bundle_path and
 catalog_path (CONF-004)

Closes #182.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 src/cmcp_gateway/config.py | 24 ++++++++++++++++++++++--
 tests/unit/test_config.py  | 31 +++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/src/cmcp_gateway/config.py b/src/cmcp_gateway/config.py
index 4b5a3bcb..1a98c8c2 100644
--- a/src/cmcp_gateway/config.py
+++ b/src/cmcp_gateway/config.py
@@ -5,6 +5,7 @@
 import os
 from dataclasses import dataclass, field
 from enum import StrEnum
+from pathlib import PurePosixPath, PureWindowsPath
 from typing import Any
 
 import yaml
@@ -55,6 +56,20 @@ class Config:
 _KNOWN_ATTEST_KEYS = {"provider", "enforcement_mode", "validity_seconds", "staleness_policy"}
 
 
+def _check_no_traversal(field_name: str, path_str: str) -> None:
+    """Reject paths that contain '..' components to prevent directory traversal (CONF-004)."""
+    for part in PurePosixPath(path_str).parts:
+        if part == "..":
+            raise ConfigError(
+                f"'{field_name}' must not contain '..' path components: {path_str!r}"
+            )
+    for part in PureWindowsPath(path_str).parts:
+        if part == "..":
+            raise ConfigError(
+                f"'{field_name}' must not contain '..' path components: {path_str!r}"
+            )
+
+
 def load_config(path: str) -> Config:
     """Load and validate cmcp-config.yaml. Raises ConfigError on invalid input."""
     raw: dict[str, Any]
@@ -114,6 +129,11 @@ def load_config(path: str) -> Config:
     dev_mode = os.environ.get("CMCP_DEV_MODE", "0") == "1"
     bearer_token = os.environ.get("CMCP_BEARER_TOKEN") or None
 
+    policy_bundle_path = raw.get("policy_bundle_path", "policy/")
+    catalog_path = raw.get("catalog_path", "catalog.json")
+    _check_no_traversal("policy_bundle_path", policy_bundle_path)
+    _check_no_traversal("catalog_path", catalog_path)
+
     return Config(
         attestation=AttestationConfig(
             provider=provider,
@@ -121,8 +141,8 @@ def load_config(path: str) -> Config:
             validity_seconds=validity_seconds,
             staleness_policy=staleness_policy,
         ),
-        policy_bundle_path=raw.get("policy_bundle_path", "policy/"),
-        catalog_path=raw.get("catalog_path", "catalog.json"),
+        policy_bundle_path=policy_bundle_path,
+        catalog_path=catalog_path,
         listen_addr=raw.get("listen_addr", "0.0.0.0:8443"),
         max_response_size_bytes=max_bytes,
         dev_mode=dev_mode,
diff --git a/tests/unit/test_config.py b/tests/unit/test_config.py
index aa1e96f1..3df717a3 100644
--- a/tests/unit/test_config.py
+++ b/tests/unit/test_config.py
@@ -98,3 +98,34 @@ def test_non_mapping_config(config_file):
 def test_missing_file():
     with pytest.raises(ConfigError, match="Cannot read"):
         load_config("/nonexistent/path/config.yaml")
+
+
+# ── CONF-004: path traversal rejection ───────────────────────────────────────
+
+def test_policy_bundle_path_traversal_rejected(config_file):
+    """CONF-004: '..' components in policy_bundle_path must be rejected."""
+    path = config_file("policy_bundle_path: ../../etc/passwd\n")
+    with pytest.raises(ConfigError, match=r"\.\."):
+        load_config(path)
+
+
+def test_catalog_path_traversal_rejected(config_file):
+    """CONF-004: '..' components in catalog_path must be rejected."""
+    path = config_file("catalog_path: ../../../etc/shadow\n")
+    with pytest.raises(ConfigError, match=r"\.\."):
+        load_config(path)
+
+
+def test_embedded_traversal_in_policy_path_rejected(config_file):
+    """CONF-004: embedded '..' (e.g. /safe/../etc) must also be rejected."""
+    path = config_file("policy_bundle_path: /safe/../etc/passwd\n")
+    with pytest.raises(ConfigError, match=r"\.\."):
+        load_config(path)
+
+
+def test_legitimate_absolute_path_accepted(config_file):
+    """CONF-004: absolute paths without '..' remain valid."""
+    path = config_file("policy_bundle_path: /opt/cmcp/policy\ncatalog_path: /opt/cmcp/catalog.json\n")
+    cfg = load_config(path)
+    assert cfg.policy_bundle_path == "/opt/cmcp/policy"
+    assert cfg.catalog_path == "/opt/cmcp/catalog.json"