diff --git a/backend/secuscan/auth.py b/backend/secuscan/auth.py
new file mode 100644
index 00000000..28c39ab5
--- /dev/null
+++ b/backend/secuscan/auth.py
@@ -0,0 +1,71 @@
+"""
+API key authentication for SecuScan backend.
+
+A random key is generated at startup and written to <data_dir>/.api_key.
+Clients must supply it via:
+  - Authorization: Bearer <key>
+  - X-Api-Key: <key>
+"""
+
+import secrets
+from pathlib import Path
+
+from fastapi import Depends, HTTPException, Security, status
+from fastapi.security import APIKeyHeader, HTTPAuthorizationCredentials, HTTPBearer
+
+_bearer_scheme = HTTPBearer(auto_error=False)
+_api_key_header = APIKeyHeader(name="X-Api-Key", auto_error=False)
+
+_api_key: str | None = None
+
+
+def init_api_key(data_dir: str) -> str:
+    """
+    Load the persisted API key, or generate and persist a new one.
+
+    Called once during application startup; the returned key is also stored in
+    the module-level ``_api_key`` variable so the FastAPI dependency can reach it.
+    """
+    global _api_key
+    key_file = Path(data_dir) / ".api_key"
+    if key_file.exists():
+        _api_key = key_file.read_text().strip()
+    else:
+        _api_key = secrets.token_hex(32)
+        key_file.parent.mkdir(parents=True, exist_ok=True)
+        key_file.write_text(_api_key)
+        key_file.chmod(0o600)
+    return _api_key
+
+
+async def require_api_key(
+    bearer: HTTPAuthorizationCredentials | None = Depends(_bearer_scheme),
+    x_api_key: str | None = Security(_api_key_header),
+) -> str:
+    """
+    FastAPI dependency — rejects requests that do not carry the correct API key.
+
+    Accepts the key in either:
+    - ``Authorization: Bearer <key>``
+    - ``X-Api-Key: <key>``
+    """
+    if _api_key is None:
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail="Authentication service not initialised",
+        )
+
+    candidate: str | None = None
+    if bearer is not None:
+        candidate = bearer.credentials
+    elif x_api_key is not None:
+        candidate = x_api_key
+
+    if candidate is None or not secrets.compare_digest(candidate, _api_key):
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid or missing API key",
+            headers={"WWW-Authenticate": "Bearer"},
+        )
+
+    return candidate
diff --git a/backend/secuscan/main.py b/backend/secuscan/main.py
index 08eb02c2..59c384c8 100644
--- a/backend/secuscan/main.py
+++ b/backend/secuscan/main.py
@@ -13,6 +13,7 @@
 from fastapi.staticfiles import StaticFiles
 
 from .config import settings
+from .auth import init_api_key
 from .cache import init_cache, cache as global_cache
 from .database import init_db, db as global_db
 from .plugins import init_plugins
@@ -42,6 +43,10 @@ async def lifespan(app: FastAPI):
     # Ensure directories exist
     settings.ensure_directories()
     logger.info("✓ Directories initialized")
+
+    # Initialize API key authentication
+    api_key = init_api_key(settings.data_dir)
+    logger.info("✓ API key authentication ready (key file: %s/.api_key)", settings.data_dir)
     
     # Initialize database
     await init_db(settings.database_path)
diff --git a/backend/secuscan/routes.py b/backend/secuscan/routes.py
index f1d53063..5efb8328 100644
--- a/backend/secuscan/routes.py
+++ b/backend/secuscan/routes.py
@@ -2,6 +2,7 @@
 API routes for SecuScan backend
 """
 
+from fastapi import APIRouter, Depends, HTTPException, BackgroundTasks, Response
 from fastapi import APIRouter, HTTPException, BackgroundTasks, Response, Request
 from fastapi.responses import JSONResponse
 from typing import Any, Optional, List, Dict, Callable
@@ -32,12 +33,12 @@ def parse_json_fields(rows: List[Dict], fields: List[str]) -> List[Dict]:
 
 def is_filesystem_target(target: str) -> bool:
     """Best-effort detection for path-based targets that should bypass host validation."""
-    if target.startswith(("/", "./", "../", "~")):
+    # Absolute or relative filesystem roots only — not CIDR notation (e.g. 8.8.8.8/32)
+    if target.startswith(("/", "./", "../", "~/")):
         return True
+    # Windows drive paths (C:\ or C:/)
     if re.match(r"^[A-Za-z]:[\\/]", target):
         return True
-    if "/" in target and not target.startswith(("http://", "https://")):
-        return True
     return False
 
 
@@ -76,10 +77,11 @@ def build_report_filename(task: Dict[str, Any], extension: str) -> str:
 from .reporting import reporting
 from .vault import VaultCrypto
 from .workflows import scheduler
+from .auth import require_api_key
 
 from sse_starlette.sse import EventSourceResponse
 
-router = APIRouter(prefix="/api/v1")
+router = APIRouter(prefix="/api/v1", dependencies=[Depends(require_api_key)])
 
 
 async def get_or_set_cached(key: str, builder):
diff --git a/testing/backend/conftest.py b/testing/backend/conftest.py
index 4805de89..0a74d4e6 100644
--- a/testing/backend/conftest.py
+++ b/testing/backend/conftest.py
@@ -14,6 +14,7 @@
 from backend.secuscan.database import init_db
 from backend.secuscan.main import app
 from backend.secuscan.plugins import init_plugins
+from backend.secuscan import auth as auth_module
 from backend.secuscan.ratelimit import concurrent_limiter, rate_limiter
 
 
@@ -50,7 +51,9 @@ async def setup():
 
     asyncio.run(setup())
 
-    with TestClient(app) as client:
+    api_key = auth_module.init_api_key(settings.data_dir)
+
+    with TestClient(app, headers={"X-Api-Key": api_key}) as client:
         yield client
 
     async def teardown():
diff --git a/testing/backend/integration/test_task_cleanup.py b/testing/backend/integration/test_task_cleanup.py
index 6fbb88af..9f4526e3 100644
--- a/testing/backend/integration/test_task_cleanup.py
+++ b/testing/backend/integration/test_task_cleanup.py
@@ -41,6 +41,8 @@ async def app_client(db_path):
         from backend.secuscan.main import app
         from backend.secuscan import database as db_module
         from backend.secuscan import cache as cache_module
+        from backend.secuscan import auth as auth_module
+        import tempfile
 
         # Initialise a real in-memory cache (it's just a dict, no external deps)
         await cache_module.init_cache()
@@ -48,13 +50,19 @@ async def app_client(db_path):
         # Initialise a fresh DB pointing at our temp file
         test_db = await db_module.init_db(db_path)
 
-        async with AsyncClient(
-            transport=ASGITransport(app=app), base_url="http://test"
-        ) as client:
-            client._mock_executor = mock_executor
-            client._db = test_db
-            client._db_path = db_path
-            yield client
+        # Initialise API key in a temporary directory so the dependency resolves
+        with tempfile.TemporaryDirectory() as tmp_auth_dir:
+            api_key = auth_module.init_api_key(tmp_auth_dir)
+
+            async with AsyncClient(
+                transport=ASGITransport(app=app),
+                base_url="http://test",
+                headers={"X-Api-Key": api_key},
+            ) as client:
+                client._mock_executor = mock_executor
+                client._db = test_db
+                client._db_path = db_path
+                yield client
 
         # Teardown
         await test_db.disconnect()
diff --git a/testing/backend/test_task_pagination.py b/testing/backend/test_task_pagination.py
index 3174fa3e..97f61c4a 100644
--- a/testing/backend/test_task_pagination.py
+++ b/testing/backend/test_task_pagination.py
@@ -3,75 +3,57 @@
 """
 
 import pytest
-from fastapi.testclient import TestClient
-from backend.secuscan.main import app
-from backend.secuscan.database import init_db
-
-# IMPORTANT: Initialize database before any tests run
-@pytest.fixture(scope="session", autouse=True)
-def setup_database():
-    """Initialize database for testing"""
-    import asyncio
-    asyncio.run(init_db())
-
-client = TestClient(app)
 
 
 class TestTasksPagination:
     """Test pagination metadata for /api/v1/tasks endpoint"""
 
-    def test_pagination_has_next_previous_fields(self):
+    def test_pagination_has_next_previous_fields(self, test_client):
         """Test that next and previous fields exist in response"""
-        response = client.get("/api/v1/tasks")
+        response = test_client.get("/api/v1/tasks")
 
-        # Check if we got a response
         if response.status_code == 200:
             data = response.json()
             assert "pagination" in data
             pagination = data["pagination"]
 
-            # These fields should always exist
             assert "next" in pagination
             assert "previous" in pagination
             assert "page" in pagination
             assert "per_page" in pagination
             assert "total_items" in pagination
             assert "total_pages" in pagination
-            print("✅ All pagination fields present!")
         else:
             pytest.fail(f"API returned {response.status_code}")
 
-    def test_default_pagination_values(self):
+    def test_default_pagination_values(self, test_client):
         """Test default page=1, per_page=25"""
-        response = client.get("/api/v1/tasks")
+        response = test_client.get("/api/v1/tasks")
         assert response.status_code == 200
 
         pagination = response.json()["pagination"]
         assert pagination["page"] == 1
         assert pagination["per_page"] == 25
-        print(f"✅ Default values: page={pagination['page']}, per_page={pagination['per_page']}")
 
-    def test_custom_per_page(self):
+    def test_custom_per_page(self, test_client):
         """Test that per_page parameter is respected"""
-        response = client.get("/api/v1/tasks?page=1&per_page=10")
+        response = test_client.get("/api/v1/tasks?page=1&per_page=10")
         assert response.status_code == 200
 
         pagination = response.json()["pagination"]
         assert pagination["per_page"] == 10
-        print(f"✅ Custom per_page=10 works")
 
-    def test_first_page_previous_is_null(self):
+    def test_first_page_previous_is_null(self, test_client):
         """Test that previous is None on first page"""
-        response = client.get("/api/v1/tasks?page=1&per_page=10")
+        response = test_client.get("/api/v1/tasks?page=1&per_page=10")
         assert response.status_code == 200
 
         pagination = response.json()["pagination"]
         assert pagination["previous"] is None
-        print("✅ First page has previous=None")
 
-    def test_next_url_preserves_filters(self):
+    def test_next_url_preserves_filters(self, test_client):
         """Test that next URL keeps filter parameters"""
-        response = client.get(
+        response = test_client.get(
             "/api/v1/tasks?page=1&per_page=5&status=completed&plugin_id=nmap"
         )
         assert response.status_code == 200
@@ -79,10 +61,7 @@ def test_next_url_preserves_filters(self):
         data = response.json()
         next_url = data["pagination"]["next"]
 
-        if next_url:  # If there are more pages
+        if next_url:
             assert "per_page=5" in next_url
             assert "status=completed" in next_url
-            assert "plugin_id=nmap" in next_url
-            print(f"✅ Next URL preserves filters: {next_url}")
-        else:
-            print("ℹ️ No next page (database might be empty)")
\ No newline at end of file
+            assert "plugin_id=nmap" in next_url
\ No newline at end of file
diff --git a/testing/backend/unit/test_api_auth.py b/testing/backend/unit/test_api_auth.py
new file mode 100644
index 00000000..d53c93f6
--- /dev/null
+++ b/testing/backend/unit/test_api_auth.py
@@ -0,0 +1,106 @@
+"""
+Unit tests for API key authentication (issue #199).
+"""
+
+import asyncio
+import tempfile
+from pathlib import Path
+
+import pytest
+from fastapi.testclient import TestClient
+
+from backend.secuscan import auth as auth_module
+from backend.secuscan.main import app
+from backend.secuscan.config import settings
+from backend.secuscan.database import init_db
+from backend.secuscan.plugins import init_plugins
+
+
+@pytest.fixture()
+def client_with_key(setup_test_environment):
+    """TestClient with a valid API key pre-seeded."""
+    asyncio.run(init_db(settings.database_path))
+    asyncio.run(init_plugins(settings.plugins_dir))
+    api_key = auth_module.init_api_key(settings.data_dir)
+    with TestClient(app) as c:
+        yield c, api_key
+
+
+class TestApiKeyInit:
+    def test_key_file_created(self, tmp_path):
+        key = auth_module.init_api_key(str(tmp_path))
+        assert (tmp_path / ".api_key").exists()
+        assert len(key) == 64  # 32 bytes → 64 hex chars
+
+    def test_existing_key_reloaded(self, tmp_path):
+        k1 = auth_module.init_api_key(str(tmp_path))
+        k2 = auth_module.init_api_key(str(tmp_path))
+        assert k1 == k2
+
+    def test_key_file_permissions(self, tmp_path):
+        auth_module.init_api_key(str(tmp_path))
+        mode = (tmp_path / ".api_key").stat().st_mode & 0o777
+        assert mode == 0o600
+
+
+class TestAuthDependency:
+    def test_no_credentials_returns_401(self, client_with_key):
+        client, _ = client_with_key
+        resp = client.get("/api/v1/plugins", headers={})
+        assert resp.status_code == 401
+
+    def test_wrong_key_returns_401(self, client_with_key):
+        client, _ = client_with_key
+        resp = client.get("/api/v1/plugins", headers={"X-Api-Key": "wrong-key"})
+        assert resp.status_code == 401
+
+    def test_valid_x_api_key_header(self, client_with_key):
+        client, api_key = client_with_key
+        resp = client.get("/api/v1/plugins", headers={"X-Api-Key": api_key})
+        assert resp.status_code == 200
+
+    def test_valid_bearer_token(self, client_with_key):
+        client, api_key = client_with_key
+        resp = client.get("/api/v1/plugins", headers={"Authorization": f"Bearer {api_key}"})
+        assert resp.status_code == 200
+
+    def test_bearer_wrong_key_returns_401(self, client_with_key):
+        client, _ = client_with_key
+        resp = client.get("/api/v1/plugins", headers={"Authorization": "Bearer bad"})
+        assert resp.status_code == 401
+
+    def test_health_endpoint_not_protected(self, client_with_key):
+        client, _ = client_with_key
+        resp = client.get("/api/v1/health", headers={})
+        # health check is defined on `app` directly, not inside the authenticated router
+        assert resp.status_code == 200
+
+    def test_root_endpoint_not_protected(self, client_with_key):
+        client, _ = client_with_key
+        resp = client.get("/", headers={})
+        assert resp.status_code == 200
+
+
+class TestIsFilesystemTarget:
+    """Regression tests for is_filesystem_target — CIDR must not be treated as a path."""
+
+    from backend.secuscan.routes import is_filesystem_target
+
+    @pytest.mark.parametrize("target,expected", [
+        ("/etc/passwd", True),
+        ("./relative/path", True),
+        ("../parent/path", True),
+        ("~/home/dir", True),
+        ("C:\\Windows\\System32", True),
+        ("C:/Windows/System32", True),
+        # These are NOT filesystem targets
+        ("8.8.8.8/32", False),
+        ("192.168.1.0/24", False),
+        ("example.com", False),
+        ("http://example.com/path", False),
+        ("https://example.com/path", False),
+        ("10.0.0.1", False),
+    ])
+    def test_filesystem_target_detection(self, target, expected):
+        from backend.secuscan.routes import is_filesystem_target
+        assert is_filesystem_target(target) == expected