diff --git a/scripts/release_train.py b/scripts/release_train.py
index c538487..b6c0ead 100644
--- a/scripts/release_train.py
+++ b/scripts/release_train.py
@@ -50,6 +50,17 @@ def check(repos: list[str], extra: list[str]) -> None:
             raise SystemExit(f"{opt!r} is not an option a train passes to cut_release.py")
 
 
+def logs_dir(name: str) -> Path:
+    """The log folder: a plain relative path inside the current directory, nothing outside it."""
+    base = Path.cwd().resolve()
+    if not re.fullmatch(r"[A-Za-z0-9._/-]+", name) or Path(name).is_absolute():
+        raise SystemExit(f"{name!r}: --logs is a relative folder name inside the current directory")
+    path = (base / name).resolve()
+    if path != base and base not in path.parents:
+        raise SystemExit(f"{name!r}: --logs must stay inside {base}")
+    return path
+
+
 def run_app(repo: str, extra: list[str], logs: Path) -> tuple[str, int, str]:
     log = logs / f"{repo.split('/')[-1]}.log"
     with log.open("w", encoding="utf-8") as out:
@@ -80,7 +91,7 @@ def main(argv: list[str] | None = None) -> int:
         p.error("name the apps, or pass --all")
     extra += ["--publish"] * a.publish + ["--dry-run"] * a.dry_run
     check(repos, extra)
-    logs = Path(a.logs)
+    logs = logs_dir(a.logs)
     logs.mkdir(parents=True, exist_ok=True)
 
     start = time.time()
diff --git a/tests/test_release_train.py b/tests/test_release_train.py
index fbd9e61..daeb184 100644
--- a/tests/test_release_train.py
+++ b/tests/test_release_train.py
@@ -1,5 +1,6 @@
 """scripts/release_train.py: which apps a train releases, and what it passes on."""
 import importlib.util
+import os
 import tempfile
 import unittest
 from pathlib import Path
@@ -19,8 +20,13 @@ class TrainTests(unittest.TestCase):
             calls.append((repo, extra))
             return repo, rc, "done"
 
-        with mock.patch.object(release_train, "run_app", side_effect=fake):
-            code = release_train.main(["--logs", tempfile.mkdtemp()] + argv)
+        cwd = os.getcwd()
+        os.chdir(tempfile.mkdtemp())
+        try:
+            with mock.patch.object(release_train, "run_app", side_effect=fake):
+                code = release_train.main(["--logs", "logs"] + argv)
+        finally:
+            os.chdir(cwd)
         return code, calls
 
     def test_all_but_the_skipped_apps_with_the_flags_passed_on(self):
@@ -39,6 +45,17 @@ class TrainTests(unittest.TestCase):
             with self.assertRaises(SystemExit, msg=f"{repos} {extra}"):
                 release_train.check(repos, extra)
 
+    def test_the_log_folder_stays_inside_the_current_directory(self):
+        cwd = os.getcwd()
+        os.chdir(tempfile.mkdtemp())
+        try:
+            self.assertEqual(release_train.logs_dir("logs/today"), Path.cwd().resolve() / "logs" / "today")
+            for bad in ["/tmp/x", "../outside", "logs/../../x", "C:/Windows", "a b"]:
+                with self.assertRaises(SystemExit, msg=bad):
+                    release_train.logs_dir(bad)
+        finally:
+            os.chdir(cwd)
+
     def test_one_stopped_app_fails_the_train(self):
         code, _ = self.run_train(["storytold/pdfcraft"], rc=1)
         self.assertEqual(code, 1)