diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 3ae152f..0406c04 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.
+
+## 2025-03-23 - [TOCTOU and Device File Size Limit Bypass]
+**Vulnerability:** File size checks using `path.stat().st_size` can be bypassed if the path points to a device file (like `/dev/zero`, which reports a size of 0). Additionally, there is a Time-of-Check to Time-of-Use (TOCTOU) vulnerability where a file could be modified between the `stat()` check and the `read_text()` call.
+**Learning:** Checking file size before reading is insufficient on its own. Device files bypass `st_size` checks, and files can grow between the size check and the read operation, leading to unbounded memory consumption or application hangs.
+**Prevention:** Always verify a path is a regular file first using `path.is_file()`. Then, instead of reading the whole file with `read_text()`, open the file and use a bounded read (e.g., `f.read(limit + 1)`) and check if the returned content length exceeds the limit.
diff --git a/src/python_learning_orchestrated/adapters/checkpoint_store.py b/src/python_learning_orchestrated/adapters/checkpoint_store.py
index 0d9c5d5..4f3b9c4 100644
--- a/src/python_learning_orchestrated/adapters/checkpoint_store.py
+++ b/src/python_learning_orchestrated/adapters/checkpoint_store.py
@@ -233,9 +233,15 @@ def _to_int(value: object, default: int) -> int:
 def _read_json(path: Path) -> dict[str, object]:
     if not path.exists():
         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"))
+    if not path.is_file():
+        raise ValueError(f"Checkpoint file {path} is not a regular file")
+
+    with path.open("r", 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..c4412de 100644
--- a/src/python_learning_orchestrated/adapters/json_file_practice_repository.py
+++ b/src/python_learning_orchestrated/adapters/json_file_practice_repository.py
@@ -105,13 +105,19 @@ def record_attempts(self, attempts: list[Attempt]) -> None:
     def _load_storage(self) -> dict[str, object]:
         if not self._file_path.exists():
             return {"items": [], "attempts": []}
-        if self._file_path.stat().st_size > 10 * 1024 * 1024:
+        if not self._file_path.is_file():
             raise ValueError(
-                f"Practice repository file {self._file_path} exceeds 10MB size limit"
+                f"Practice repository file {self._file_path} is not a regular file"
             )
 
         try:
-            content = self._file_path.read_text(encoding="utf-8")
+            with self._file_path.open("r", 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..8de05bd 100644
--- a/src/python_learning_orchestrated/adapters/json_file_progress_repository.py
+++ b/src/python_learning_orchestrated/adapters/json_file_progress_repository.py
@@ -42,13 +42,20 @@ def _load_storage(self) -> dict[str, LessonProgress]:
         """Load all persisted progress payloads."""
         if not self._file_path.exists():
             return {}
-        if self._file_path.stat().st_size > 10 * 1024 * 1024:
+        if not self._file_path.is_file():
             raise ValueError(
-                f"Progress repository file {self._file_path} exceeds 10MB size limit"
+                f"Progress repository file {self._file_path} is not a regular file"
             )
 
         try:
-            content = self._file_path.read_text(encoding="utf-8")
+            with self._file_path.open("r", 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..040d1b1 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
@@ -36,12 +36,20 @@ def save(self, snapshot: ProgressSnapshot) -> None:
     def _load_payload(self) -> dict[str, object]:
         if not self._file_path.exists():
             return {}
-        if self._file_path.stat().st_size > 10 * 1024 * 1024:
+        if not self._file_path.is_file():
             raise ValueError(
-                f"Progress snapshot file {self._file_path} exceeds 10MB size limit"
+                f"Progress snapshot file {self._file_path} is not a regular file"
             )
+
         try:
-            parsed = json.loads(self._file_path.read_text(encoding="utf-8"))
+            with self._file_path.open("r", 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 {}