diff --git a/src/core-skills/bmad-brainstorming/scripts/brain.py b/src/core-skills/bmad-brainstorming/scripts/brain.py
index 1028cf6edb..b559535f1d 100644
--- a/src/core-skills/bmad-brainstorming/scripts/brain.py
+++ b/src/core-skills/bmad-brainstorming/scripts/brain.py
@@ -130,7 +130,11 @@ def resolve_detail(row: dict, csv_dir: Path) -> str | None:
     (or the file is missing — a missing file is reported to stderr, not fatal)."""
     if not row.get("detail"):
         return None
-    path = (csv_dir / row["detail"]).resolve()
+    base = csv_dir.resolve()
+    path = (base / row["detail"]).resolve()
+    if not path.is_relative_to(base):
+        print(f"# detail path escapes the catalog directory for {row['technique_name']}: {row['detail']}", file=sys.stderr)
+        return None
     if not path.is_file():
         print(f"# detail file not found for {row['technique_name']}: {row['detail']}", file=sys.stderr)
         return None
diff --git a/src/core-skills/bmad-brainstorming/scripts/tests/test_brain.py b/src/core-skills/bmad-brainstorming/scripts/tests/test_brain.py
index 329a6a5f3c..549def26b6 100644
--- a/src/core-skills/bmad-brainstorming/scripts/tests/test_brain.py
+++ b/src/core-skills/bmad-brainstorming/scripts/tests/test_brain.py
@@ -74,6 +74,33 @@ def test_resolve_detail_missing_file_warns_not_fatal(lib, capsys):
     assert "not found" in capsys.readouterr().err
 
 
+def test_resolve_detail_refuses_path_traversal(lib, tmp_path, capsys):
+    """Paths that escape the catalog directory must be refused, not read."""
+    rows = brain.load(lib)
+    secret = tmp_path.parent / ".env"
+    secret.write_text("SECRET=hunter2", encoding="utf-8")
+    # relative traversal
+    rows[1]["detail"] = "../.env"
+    assert brain.resolve_detail(rows[1], lib.parent) is None
+    assert "escapes" in capsys.readouterr().err
+    # absolute path (pathlib '/' lets the right operand win)
+    rows[1]["detail"] = str(secret.resolve())
+    assert brain.resolve_detail(rows[1], lib.parent) is None
+    assert "escapes" in capsys.readouterr().err
+    # symlink inside catalog pointing outside
+    link = lib.parent / "techniques" / "escape.md"
+    link.symlink_to(secret)
+    rows[1]["detail"] = "techniques/escape.md"
+    assert brain.resolve_detail(rows[1], lib.parent) is None
+    assert "escapes" in capsys.readouterr().err
+
+
+def test_resolve_detail_still_reads_valid_detail(lib):
+    """A legitimate detail file inside the catalog directory still resolves."""
+    row = next(r for r in brain.load(lib) if r["detail"])
+    assert "multi-step instructions" in brain.resolve_detail(row, lib.parent)
+
+
 def test_show_inlines_detail(lib, capsys):
     assert brain.main(["--file", str(lib), "show", "Quantum Superposition"]) == 0
     out = capsys.readouterr().out