diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 3ae152f..cdd097c 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -2,3 +2,8 @@
 **Vulnerability:** Unbounded memory consumption DoS risk due to JSON file parsing without size limits in file-backed repositories and stores.
 **Learning:** `json.loads(file.read_text())` reads the entire file content into memory. This can lead to out-of-memory errors or denial-of-service if an attacker or misconfiguration provides an excessively large file. The application relies heavily on file-backed adapters (e.g., `JsonFilePracticeRepository`, `JsonFileProgressSnapshotStore`, `CheckpointStore`).
 **Prevention:** Implement a strict file size limit check using `path.stat().st_size` (e.g., standardized at 10MB or `10 * 1024 * 1024` bytes) before reading the file content into memory in all file-based adapters.
+
+## 2025-02-23 - [TOCTOU and DoS via File Stat Checks]
+**Vulnerability:** Using `path.stat().st_size` to check file size limits before reading the entire file (TOCTOU), and using `.exists()` instead of `.is_file()`, which allows device files (like `/dev/zero`) to report a size of 0 and bypass the check, leading to unbounded memory consumption DoS.
+**Learning:** Checking size before reading is susceptible to Time-of-Check to Time-of-Use race conditions if the file is modified in between. Furthermore, device files report incorrect sizes, circumventing the limit entirely if `.exists()` is used instead of `.is_file()`.
+**Prevention:** Always verify a path is a regular file using `path.is_file()` first. To prevent TOCTOU when enforcing size limits, implement a secure bounded read (e.g., `content = f.read(limit + 1)`) and raise a ValueError if the returned length exceeds the limit, rather than relying on pre-read stat checks.
diff --git a/src/python_learning_orchestrated/adapters/checkpoint_store.py b/src/python_learning_orchestrated/adapters/checkpoint_store.py
index 0d9c5d5..cc444d1 100644
--- a/src/python_learning_orchestrated/adapters/checkpoint_store.py
+++ b/src/python_learning_orchestrated/adapters/checkpoint_store.py
@@ -231,11 +231,15 @@ def _to_int(value: object, default: int) -> int:
 
 
 def _read_json(path: Path) -> dict[str, object]:
-    if not path.exists():
+    if not path.is_file():
         return {}
-    if path.stat().st_size > 10 * 1024 * 1024:
-        raise ValueError(f"Checkpoint file {path} exceeds 10MB size limit")
-    parsed = json.loads(path.read_text(encoding="utf-8"))
+
+    with open(path, encoding="utf-8") as f:
+        content = f.read(10 * 1024 * 1024 + 1)
+        if len(content) > 10 * 1024 * 1024:
+            raise ValueError(f"Checkpoint file {path} exceeds 10MB size limit")
+
+    parsed = json.loads(content)
     return parsed if isinstance(parsed, dict) else {}
 
 
diff --git a/src/python_learning_orchestrated/adapters/json_file_practice_repository.py b/src/python_learning_orchestrated/adapters/json_file_practice_repository.py
index 15da61b..12c554f 100644
--- a/src/python_learning_orchestrated/adapters/json_file_practice_repository.py
+++ b/src/python_learning_orchestrated/adapters/json_file_practice_repository.py
@@ -103,15 +103,18 @@ def record_attempts(self, attempts: list[Attempt]) -> None:
         self._save_storage(storage)
 
     def _load_storage(self) -> dict[str, object]:
-        if not self._file_path.exists():
+        if not self._file_path.is_file():
             return {"items": [], "attempts": []}
-        if self._file_path.stat().st_size > 10 * 1024 * 1024:
-            raise ValueError(
-                f"Practice repository file {self._file_path} exceeds 10MB size limit"
-            )
 
         try:
-            content = self._file_path.read_text(encoding="utf-8")
+            with open(self._file_path, encoding="utf-8") as f:
+                content = f.read(10 * 1024 * 1024 + 1)
+                if len(content) > 10 * 1024 * 1024:
+                    raise ValueError(
+                        f"Practice repository file {self._file_path} "
+                        "exceeds 10MB size limit"
+                    )
+
             if not content.strip():
                 return {"items": [], "attempts": []}
             parsed = json.loads(content)
diff --git a/src/python_learning_orchestrated/adapters/json_file_progress_repository.py b/src/python_learning_orchestrated/adapters/json_file_progress_repository.py
index 7c956af..5299463 100644
--- a/src/python_learning_orchestrated/adapters/json_file_progress_repository.py
+++ b/src/python_learning_orchestrated/adapters/json_file_progress_repository.py
@@ -40,15 +40,18 @@ def reset_progress(self, user_id: str) -> None:
 
     def _load_storage(self) -> dict[str, LessonProgress]:
         """Load all persisted progress payloads."""
-        if not self._file_path.exists():
+        if not self._file_path.is_file():
             return {}
-        if self._file_path.stat().st_size > 10 * 1024 * 1024:
-            raise ValueError(
-                f"Progress repository file {self._file_path} exceeds 10MB size limit"
-            )
 
         try:
-            content = self._file_path.read_text(encoding="utf-8")
+            with open(self._file_path, encoding="utf-8") as f:
+                content = f.read(10 * 1024 * 1024 + 1)
+                if len(content) > 10 * 1024 * 1024:
+                    raise ValueError(
+                        f"Progress repository file {self._file_path} "
+                        "exceeds 10MB size limit"
+                    )
+
             if not content.strip():
                 return {}
 
diff --git a/src/python_learning_orchestrated/adapters/json_file_progress_snapshot_store.py b/src/python_learning_orchestrated/adapters/json_file_progress_snapshot_store.py
index ac34e1c..e556f41 100644
--- a/src/python_learning_orchestrated/adapters/json_file_progress_snapshot_store.py
+++ b/src/python_learning_orchestrated/adapters/json_file_progress_snapshot_store.py
@@ -34,14 +34,18 @@ def save(self, snapshot: ProgressSnapshot) -> None:
         self._save_payload(progress_snapshot_to_payload(snapshot))
 
     def _load_payload(self) -> dict[str, object]:
-        if not self._file_path.exists():
+        if not self._file_path.is_file():
             return {}
-        if self._file_path.stat().st_size > 10 * 1024 * 1024:
-            raise ValueError(
-                f"Progress snapshot file {self._file_path} exceeds 10MB size limit"
-            )
+
         try:
-            parsed = json.loads(self._file_path.read_text(encoding="utf-8"))
+            with open(self._file_path, encoding="utf-8") as f:
+                content = f.read(10 * 1024 * 1024 + 1)
+                if len(content) > 10 * 1024 * 1024:
+                    raise ValueError(
+                        f"Progress snapshot file {self._file_path} "
+                        "exceeds 10MB size limit"
+                    )
+            parsed = json.loads(content)
         except (OSError, json.JSONDecodeError):
             return {}
         return parsed if isinstance(parsed, dict) else {}