diff --git a/app/security.py b/app/security.py
index 6634fd9..7911f03 100644
--- a/app/security.py
+++ b/app/security.py
@@ -10,7 +10,7 @@ from starlette.types import ASGIApp
 
 from .settings import settings
 
-CSP_DIRECTIVES = (
+CSP_DIRECTIVES_BASE = (
     "default-src 'self'; "
     "script-src 'self' https://cdn.jsdelivr.net; "
     "style-src 'self' 'unsafe-inline'; "
@@ -20,11 +20,16 @@ CSP_DIRECTIVES = (
     "base-uri 'self'; "
     "form-action 'self'; "
     "frame-ancestors 'none'; "
-    "object-src 'none'; "
-    "upgrade-insecure-requests"
+    "object-src 'none'"
 )
 
 
+def _csp_directives() -> str:
+    if settings.trust_proxy:
+        return f"{CSP_DIRECTIVES_BASE}; upgrade-insecure-requests"
+    return CSP_DIRECTIVES_BASE
+
+
 class SecurityHeadersMiddleware(BaseHTTPMiddleware):
     def __init__(self, app: ASGIApp):
         super().__init__(app)
@@ -34,7 +39,7 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
         response.headers.setdefault("X-Content-Type-Options", "nosniff")
         response.headers.setdefault("X-Frame-Options", "DENY")
         response.headers.setdefault("Referrer-Policy", "same-origin")
-        response.headers.setdefault("Content-Security-Policy", CSP_DIRECTIVES)
+        response.headers.setdefault("Content-Security-Policy", _csp_directives())
         response.headers.setdefault(
             "Permissions-Policy",
             "camera=(), microphone=(), geolocation=(), payment=()",
diff --git a/tests/test_security_headers.py b/tests/test_security_headers.py
new file mode 100644
index 0000000..3802d67
--- /dev/null
+++ b/tests/test_security_headers.py
@@ -0,0 +1,23 @@
+"""Security header behavior."""
+
+from __future__ import annotations
+
+from app import security
+
+
+def test_csp_does_not_upgrade_http_when_proxy_not_trusted(app_client, monkeypatch):
+    monkeypatch.setattr(security.settings, "trust_proxy", False)
+
+    response = app_client.get("/healthz")
+
+    csp = response.headers["content-security-policy"]
+    assert "upgrade-insecure-requests" not in csp
+
+
+def test_csp_upgrades_requests_when_proxy_is_trusted(app_client, monkeypatch):
+    monkeypatch.setattr(security.settings, "trust_proxy", True)
+
+    response = app_client.get("/healthz")
+
+    csp = response.headers["content-security-policy"]
+    assert "upgrade-insecure-requests" in csp