diff --git a/flaskapi_guard/adapters.py b/flaskapi_guard/adapters.py
index d159ed7..483f17c 100644
--- a/flaskapi_guard/adapters.py
+++ b/flaskapi_guard/adapters.py
@@ -97,7 +97,9 @@ def read_body_prefix(self, max_bytes: int) -> bytes:
 
 class FlaskResponseFactory:
     def create_response(self, content: str, status_code: int) -> FlaskGuardResponse:
-        return FlaskGuardResponse(Response(content, status=status_code))
+        return FlaskGuardResponse(
+            Response(content, status=status_code, mimetype="text/plain")
+        )
 
     def create_redirect_response(
         self, url: str, status_code: int
diff --git a/tests/test_adapters.py b/tests/test_adapters.py
index 23d1b4e..9c2bf86 100644
--- a/tests/test_adapters.py
+++ b/tests/test_adapters.py
@@ -110,6 +110,7 @@ def test_flask_response_factory_create() -> None:
     factory = FlaskResponseFactory()
     guard_resp = factory.create_response("error", 403)
     assert guard_resp.status_code == 403
+    assert guard_resp.headers["Content-Type"] == "text/plain; charset=utf-8"
 
 
 def test_flask_response_factory_redirect() -> None:
diff --git a/tests/test_block_response_content_type.py b/tests/test_block_response_content_type.py
new file mode 100644
index 0000000..d7b70dc
--- /dev/null
+++ b/tests/test_block_response_content_type.py
@@ -0,0 +1,72 @@
+import os
+from typing import cast
+
+from flask import Flask
+from guard_core.models import SecurityConfig
+from guard_core.protocols.geo_ip_protocol import GeoIPHandler
+from guard_core.protocols.response_protocol import GuardResponse
+from guard_core.sync.handlers.ipinfo_handler import IPInfoManager
+
+from flaskapi_guard.extension import FlaskAPIGuard
+
+IPINFO_TOKEN = str(os.getenv("IPINFO_TOKEN"))
+
+BLOCKED_IP = "203.0.113.10"
+
+
+def _config(**overrides: object) -> SecurityConfig:
+    return SecurityConfig(
+        geo_ip_handler=cast(GeoIPHandler, IPInfoManager(IPINFO_TOKEN)),
+        blacklist=(BLOCKED_IP,),
+        enable_rate_limiting=False,
+        enable_redis=False,
+        enable_penetration_detection=False,
+        trusted_proxies=("127.0.0.1",),
+        **overrides,  # type: ignore[arg-type]
+    )
+
+
+def _blocked_response_headers(config: SecurityConfig) -> tuple[int, str, str]:
+    app = Flask(__name__)
+    FlaskAPIGuard(app, config=config)
+
+    @app.route("/ping")
+    def ping() -> dict[str, bool]:
+        return {"ok": True}
+
+    with app.test_client() as client:
+        response = client.get("/ping", headers={"X-Forwarded-For": BLOCKED_IP})
+    return (
+        response.status_code,
+        response.headers.get("Content-Type", ""),
+        response.headers.get("X-Content-Type-Options", ""),
+    )
+
+
+def test_block_response_is_plain_text() -> None:
+    status_code, content_type, nosniff = _blocked_response_headers(_config())
+    assert (status_code, content_type, nosniff) == (
+        403,
+        "text/plain; charset=utf-8",
+        "nosniff",
+    )
+
+
+def test_custom_error_message_is_plain_text() -> None:
+    config = _config(custom_error_responses={403: "Access denied"})
+
+    status_code, content_type, _nosniff = _blocked_response_headers(config)
+
+    assert (status_code, content_type) == (403, "text/plain; charset=utf-8")
+
+
+def test_response_modifier_can_still_set_its_own_content_type() -> None:
+    def problem_json(response: GuardResponse) -> GuardResponse:
+        response.headers["Content-Type"] = "application/problem+json"
+        return response
+
+    config = _config(custom_response_modifier=problem_json)
+
+    status_code, content_type, _nosniff = _blocked_response_headers(config)
+
+    assert (status_code, content_type) == (403, "application/problem+json")