diff --git a/services/api-server/src/api_server/main.py b/services/api-server/src/api_server/main.py
index 91907a6..487934f 100644
--- a/services/api-server/src/api_server/main.py
+++ b/services/api-server/src/api_server/main.py
@@ -135,12 +135,14 @@ from ui_semantics import form_semantics
 
 app = FastAPI(title="SFERA API", version="0.1.0")
 _HTML5_ASSETS_DIR = Path(__file__).resolve().parent / "static" / "html5"
+_HTML5_SECURITY_HEADERS = {"X-Content-Type-Options": "nosniff"}
 
 
 class Html5StaticFiles(StaticFiles):
     def file_response(self, *args, **kwargs):
         response = super().file_response(*args, **kwargs)
         response.headers.setdefault("Cache-Control", "public, max-age=86400")
+        response.headers.setdefault("X-Content-Type-Options", "nosniff")
         return response
 
 
@@ -8438,6 +8440,7 @@ def _html5_sse_headers() -> dict[str, str]:
         "Cache-Control": "no-cache, no-transform",
         "Connection": "keep-alive",
         "X-Accel-Buffering": "no",
+        **_HTML5_SECURITY_HEADERS,
     }
 
 
@@ -8445,7 +8448,7 @@ def _html5_response(fragment: str) -> Response:
     return Response(
         fragment,
         media_type="text/html; charset=utf-8",
-        headers={"Cache-Control": "no-cache, no-transform"},
+        headers={"Cache-Control": "no-cache, no-transform", **_HTML5_SECURITY_HEADERS},
     )
 
 
diff --git a/services/api-server/tests/test_api.py b/services/api-server/tests/test_api.py
index 98aabd0..b1977a0 100644
--- a/services/api-server/tests/test_api.py
+++ b/services/api-server/tests/test_api.py
@@ -47,6 +47,7 @@ def assert_html5_contract(text: str, *markers: str, full_page: bool = False) ->
 def assert_html5_response_contract(response, *markers: str, full_page: bool = False) -> None:
     assert "text/html" in response.headers["content-type"]
     assert response.headers["cache-control"] == "no-cache, no-transform"
+    assert response.headers["x-content-type-options"] == "nosniff"
     assert_html5_contract(response.text, *markers, full_page=full_page)
 
 
@@ -176,6 +177,7 @@ def test_html5_server_rendered_project_editor(tmp_path: Path):
         assert events.headers["cache-control"] == "no-cache, no-transform"
         assert events.headers["x-accel-buffering"] == "no"
         assert events.headers["connection"] == "keep-alive"
+        assert events.headers["x-content-type-options"] == "nosniff"
         first_chunk = "".join(events.iter_text())
     assert ": project " in first_chunk
     assert "retry: 5000" in first_chunk
@@ -267,16 +269,19 @@ def test_html5_server_rendered_project_editor(tmp_path: Path):
     assert htmx_asset.status_code == 200
     assert "javascript" in htmx_asset.headers["content-type"]
     assert htmx_asset.headers["cache-control"] == "public, max-age=86400"
+    assert htmx_asset.headers["x-content-type-options"] == "nosniff"
     assert "htmx" in htmx_asset.text
     sse_asset = client.get("/html5/assets/htmx-ext-sse.js")
     assert sse_asset.status_code == 200
     assert "javascript" in sse_asset.headers["content-type"]
     assert sse_asset.headers["cache-control"] == "public, max-age=86400"
+    assert sse_asset.headers["x-content-type-options"] == "nosniff"
     assert "sse" in sse_asset.text
     css_asset = client.get("/html5/assets/html5.css")
     assert css_asset.status_code == 200
     assert "text/css" in css_asset.headers["content-type"]
     assert css_asset.headers["cache-control"] == "public, max-age=86400"
+    assert css_asset.headers["x-content-type-options"] == "nosniff"
     assert ".workspace" in css_asset.text
 
 
@@ -720,6 +725,7 @@ def test_html5_project_setup_renders_server_fragments():
         assert events.headers["cache-control"] == "no-cache, no-transform"
         assert events.headers["x-accel-buffering"] == "no"
         assert events.headers["connection"] == "keep-alive"
+        assert events.headers["x-content-type-options"] == "nosniff"
         first_chunk = "".join(events.iter_text())
     assert ": setup " in first_chunk
     assert "retry: 5000" in first_chunk
@@ -762,6 +768,7 @@ def test_html5_operations_renders_job_monitor_fragments():
         assert events.headers["cache-control"] == "no-cache, no-transform"
         assert events.headers["x-accel-buffering"] == "no"
         assert events.headers["connection"] == "keep-alive"
+        assert events.headers["x-content-type-options"] == "nosniff"
         first_chunk = "".join(events.iter_text())
     assert ": operations heartbeat" in first_chunk
     assert "retry: 5000" in first_chunk