diff --git a/.env.example b/.env.example
index ed38205b..d24dee8e 100644
--- a/.env.example
+++ b/.env.example
@@ -104,8 +104,11 @@ VIRUSTOTAL_API_KEY=
 # Application Configuration
 # ------------------------------------------------------------------------------
 
-# Storage paths (usually set by Docker/Railway automatically)
+# Storage paths and upload limits (usually set by Docker/Railway automatically)
 EXTENSION_STORAGE_PATH=/app/extensions_storage
+UPLOAD_MAX_BYTES=104857600
+ZIP_EXTRACT_MAX_FILES=10000
+ZIP_EXTRACT_MAX_UNCOMPRESSED_BYTES=524288000
 # Only used when DB_BACKEND=sqlite (local file). Ignored when using Supabase.
 DATABASE_PATH=/app/data/extension-shield.db
 
diff --git a/deploy/env.production.template b/deploy/env.production.template
index 8eecb2f1..1a8b09bc 100644
--- a/deploy/env.production.template
+++ b/deploy/env.production.template
@@ -84,8 +84,11 @@ VIRUSTOTAL_API_KEY=
 # Application Configuration
 # ------------------------------------------------------------------------------
 
-# Storage paths (usually set by Docker/Railway automatically)
+# Storage paths and upload limits (usually set by Docker/Railway automatically)
 EXTENSION_STORAGE_PATH=/app/extensions_storage
+UPLOAD_MAX_BYTES=104857600
+ZIP_EXTRACT_MAX_FILES=10000
+ZIP_EXTRACT_MAX_UNCOMPRESSED_BYTES=524288000
 DATABASE_PATH=/app/data/extension-shield.db
 
 # ------------------------------------------------------------------------------
diff --git a/deploy/railway.toml b/deploy/railway.toml
index 9f25f150..0dd52f7e 100644
--- a/deploy/railway.toml
+++ b/deploy/railway.toml
@@ -46,6 +46,7 @@ PYTHONDONTWRITEBYTECODE = "1"
 PYTHONUNBUFFERED = "1"
 LLM_PROVIDER = "openai"
 EXTENSION_STORAGE_PATH = "/app/extensions_storage"
+UPLOAD_MAX_BYTES = "104857600"
 
 [static]
 # Not used since we serve static files via FastAPI
diff --git a/railway.toml b/railway.toml
index 4e57ab17..9a770a1c 100644
--- a/railway.toml
+++ b/railway.toml
@@ -46,6 +46,7 @@ PYTHONDONTWRITEBYTECODE = "1"
 PYTHONUNBUFFERED = "1"
 LLM_PROVIDER = "openai"
 EXTENSION_STORAGE_PATH = "/app/extensions_storage"
+UPLOAD_MAX_BYTES = "104857600"
 EXTSHIELD_MODE = "cloud"
 
 [static]
diff --git a/src/extension_shield/api/main.py b/src/extension_shield/api/main.py
index cbcd50c6..5cd3b13a 100644
--- a/src/extension_shield/api/main.py
+++ b/src/extension_shield/api/main.py
@@ -169,6 +169,36 @@ def _noop(func):
     return _noop
 
 
+def _format_byte_limit(max_bytes: int) -> str:
+    """Return a human-readable upload limit for API error messages."""
+    if max_bytes % (1024 * 1024) == 0:
+        mb = max_bytes // (1024 * 1024)
+        return f"{mb}MB"
+    return f"{max_bytes} bytes"
+
+
+async def _read_upload_with_limit(file: UploadFile, max_bytes: int) -> bytes:
+    """Read an UploadFile without accepting payloads larger than max_bytes."""
+    chunk_size = 1024 * 1024
+    chunks = []
+    total = 0
+
+    while True:
+        remaining = max_bytes - total + 1
+        chunk = await file.read(min(chunk_size, remaining))
+        if not chunk:
+            break
+        total += len(chunk)
+        if total > max_bytes:
+            raise HTTPException(
+                status_code=413,
+                detail=f"File too large. Maximum size is {_format_byte_limit(max_bytes)}.",
+            )
+        chunks.append(chunk)
+
+    return b"".join(chunks)
+
+
 @app.exception_handler(RateLimitExceeded)
 async def rate_limit_handler(request: Request, exc: RateLimitExceeded):
     return JSONResponse(status_code=429, content={"detail": "Rate limit exceeded"})
@@ -2450,14 +2480,9 @@ async def upload_and_scan(
             detail="Invalid file type. Only .crx and .zip files are supported"
         )
 
-    # Validate file size (max 100MB)
-    max_size = 100 * 1024 * 1024  # 100MB
-    file_content = await file.read()
-    if len(file_content) > max_size:
-        raise HTTPException(
-            status_code=400,
-            detail=f"File too large. Maximum size is {max_size / (1024*1024):.0f}MB"
-        )
+    # Validate file size before saving/analysis. Read with a bounded loop rather than
+    # slurping an arbitrary body into memory first.
+    file_content = await _read_upload_with_limit(file, settings.upload_max_bytes)
 
     # Validate MIME type (additional security check)
     import mimetypes
diff --git a/src/extension_shield/core/config.py b/src/extension_shield/core/config.py
index 4e94c7f3..c3c357ca 100644
--- a/src/extension_shield/core/config.py
+++ b/src/extension_shield/core/config.py
@@ -22,6 +22,8 @@
 StorageBackend = Literal["local", "supabase"]
 DbBackend = Literal["sqlite", "postgres", "supabase"]
 
+DEFAULT_UPLOAD_MAX_BYTES = 104857600  # 100 MiB
+
 
 def _normalize_env(value: Optional[str]) -> Optional[str]:
     if value is None:
@@ -122,7 +124,8 @@ class Settings:
     admin_api_key: Optional[str]
     telemetry_admin_key: Optional[str]
 
-    # Zip extract limits (zip-bomb protection)
+    # Upload and zip extract limits (DoS/zip-bomb protection)
+    upload_max_bytes: int
     zip_extract_max_files: int
     zip_extract_max_uncompressed_bytes: int
 
@@ -180,6 +183,8 @@ def get_settings() -> Settings:
     - ADMIN_API_KEY, TELEMETRY_ADMIN_KEY (optional, for admin endpoints)
     - STORAGE_BACKEND: local|supabase (currently only local FS is used)
     - DB_BACKEND: sqlite|supabase (Postgres via Supabase; sqlite is dev fallback)
+    - UPLOAD_MAX_BYTES: max accepted CRX/ZIP upload size before request rejection
+    - ZIP_EXTRACT_MAX_FILES, ZIP_EXTRACT_MAX_UNCOMPRESSED_BYTES: zip-bomb protection
     """
 
     env = _parse_env_name(
@@ -207,7 +212,16 @@ def get_settings() -> Settings:
     admin_api_key = os.environ.get("ADMIN_API_KEY")
     telemetry_admin_key = os.environ.get("TELEMETRY_ADMIN_KEY")
 
-    # Zip-bomb protection: max file count and max total uncompressed size
+    # Upload and zip-bomb protection: reject large uploads before saving/analysis,
+    # then bound extracted archive complexity separately.
+    _upload_max_bytes = os.environ.get("UPLOAD_MAX_BYTES", str(DEFAULT_UPLOAD_MAX_BYTES))
+    try:
+        upload_max_bytes = int(_upload_max_bytes)
+    except ValueError:
+        upload_max_bytes = DEFAULT_UPLOAD_MAX_BYTES
+    if upload_max_bytes <= 0:
+        upload_max_bytes = DEFAULT_UPLOAD_MAX_BYTES
+
     _zip_max_files = os.environ.get("ZIP_EXTRACT_MAX_FILES", "10000")
     _zip_max_bytes = os.environ.get("ZIP_EXTRACT_MAX_UNCOMPRESSED_BYTES", "524288000")  # 500 MiB
     try:
@@ -256,6 +270,7 @@ def get_settings() -> Settings:
         supabase_jwt_aud=supabase_jwt_aud,
         admin_api_key=admin_api_key,
         telemetry_admin_key=telemetry_admin_key,
+        upload_max_bytes=upload_max_bytes,
         zip_extract_max_files=zip_extract_max_files,
         zip_extract_max_uncompressed_bytes=zip_extract_max_uncompressed_bytes,
     )
diff --git a/tests/api/test_upload_limits.py b/tests/api/test_upload_limits.py
new file mode 100644
index 00000000..b90efa23
--- /dev/null
+++ b/tests/api/test_upload_limits.py
@@ -0,0 +1,60 @@
+import asyncio
+from unittest.mock import MagicMock, patch
+
+from fastapi.testclient import TestClient
+
+from extension_shield.api.main import _read_upload_with_limit, app
+from extension_shield.core.config import get_settings
+
+
+class _AsyncChunkFile:
+    def __init__(self, payload: bytes):
+        self._payload = payload
+        self._offset = 0
+
+    async def read(self, size: int = -1) -> bytes:
+        if self._offset >= len(self._payload):
+            return b""
+        if size is None or size < 0:
+            size = len(self._payload) - self._offset
+        chunk = self._payload[self._offset:self._offset + size]
+        self._offset += len(chunk)
+        return chunk
+
+
+def _settings(upload_max_bytes: int) -> MagicMock:
+    settings = MagicMock()
+    settings.is_prod.return_value = False
+    settings.upload_max_bytes = upload_max_bytes
+    return settings
+
+
+def test_upload_rejects_payload_above_configured_limit(tmp_path):
+    """Uploaded CRX/ZIP payloads larger than the configured limit are rejected before saving."""
+    client = TestClient(app)
+
+    with patch("extension_shield.api.main.get_settings", return_value=_settings(4)), \
+         patch("extension_shield.api.main.RESULTS_DIR", tmp_path), \
+         patch("extension_shield.api.main.run_analysis_workflow") as run_analysis:
+        response = client.post(
+            "/api/scan/upload",
+            files={"file": ("extension.zip", b"PK\x03\x04X", "application/zip")},
+        )
+
+    assert response.status_code == 413
+    assert response.json()["detail"] == "File too large. Maximum size is 4 bytes."
+    assert list(tmp_path.iterdir()) == []
+    run_analysis.assert_not_called()
+
+
+def test_bounded_reader_accepts_payload_at_configured_limit():
+    """Payloads exactly at the configured byte limit are accepted by the bounded reader."""
+    result = asyncio.run(_read_upload_with_limit(_AsyncChunkFile(b"1234"), 4))
+
+    assert result == b"1234"
+
+
+def test_invalid_upload_limit_env_falls_back_to_default(monkeypatch):
+    monkeypatch.setenv("UPLOAD_MAX_BYTES", "not-an-int")
+
+    assert get_settings().upload_max_bytes == 104_857_600