diff --git a/backend/app/__init__.py b/backend/app/__init__.py
index 3298a3c..70efbda 100644
--- a/backend/app/__init__.py
+++ b/backend/app/__init__.py
@@ -3,13 +3,23 @@
 import logging
 from pathlib import Path
 
-from quart import Quart
+from quart import Quart, request
 
 from .api import all_blueprints
 from .config import get_config
 from .frontend import frontend_bp
 from .services.credential_crypto import CredentialCrypto
 
+# Browser-extension origins. The FabledCurator extension fetches from
+# moz-extension://<uuid>/ on Firefox and chrome-extension://<uuid>/ on
+# Chromium-based browsers. Operator-flagged 2026-05-26: extension's
+# 'Test connection' returned `NetworkError` because the X-Extension-Key
+# header on /api/credentials triggers a CORS preflight that our routes
+# don't handle. Whitelisting only these two schemes (not opening CORS
+# up generally) lets the extension talk to a plain-HTTP self-hosted FC
+# without weakening the no-CORS posture for normal browser usage.
+_EXTENSION_ORIGIN_SCHEMES = ("moz-extension://", "chrome-extension://")
+
 _CREDENTIAL_KEY_PATH = Path("/images/secrets/credential_key.b64")
 
 
@@ -35,6 +45,33 @@ def create_app() -> Quart:
     # Registered last so /api/* routes win over the SPA catch-all.
     app.register_blueprint(frontend_bp)
 
+    @app.before_request
+    async def _extension_cors_preflight():
+        # Short-circuit OPTIONS preflight from the browser extension with a
+        # 204 + CORS headers (the after_request hook below adds them).
+        # Without this, OPTIONS lands on routes that only declared POST/GET
+        # methods and 405s before the after_request gets a chance.
+        if request.method != "OPTIONS":
+            return None
+        origin = request.headers.get("Origin", "")
+        if any(origin.startswith(s) for s in _EXTENSION_ORIGIN_SCHEMES):
+            return "", 204
+        return None
+
+    @app.after_request
+    async def _extension_cors_headers(response):
+        origin = request.headers.get("Origin", "")
+        if any(origin.startswith(s) for s in _EXTENSION_ORIGIN_SCHEMES):
+            response.headers["Access-Control-Allow-Origin"] = origin
+            response.headers["Access-Control-Allow-Methods"] = (
+                "GET, POST, PATCH, DELETE, OPTIONS"
+            )
+            response.headers["Access-Control-Allow-Headers"] = (
+                "Content-Type, X-Extension-Key"
+            )
+            response.headers["Access-Control-Max-Age"] = "86400"
+        return response
+
     @app.after_serving
     async def _dispose_db_engine() -> None:
         from .extensions import dispose_engine
diff --git a/tests/test_extension_cors.py b/tests/test_extension_cors.py
new file mode 100644
index 0000000..da5c336
--- /dev/null
+++ b/tests/test_extension_cors.py
@@ -0,0 +1,86 @@
+"""CORS preflight + response headers for moz-extension:// + chrome-extension://.
+
+Operator-flagged 2026-05-26: extension's first 'Test connection' tap
+returned `NetworkError` because /api/credentials had no OPTIONS handler
+and no Access-Control-Allow-Origin response header. Browser preflight
+failed → fetch blocked.
+
+These tests pin the contract: any request from a moz-extension:// or
+chrome-extension:// origin gets the right ACL headers. Plain browser
+requests (no Origin header, or a regular https:// Origin) get nothing
+— we don't want to open CORS up generally.
+"""
+
+import pytest
+
+from backend.app import create_app
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.fixture
+async def client():
+    app = create_app()
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_extension_preflight_returns_204_with_acl_headers(client):
+    resp = await client.options(
+        "/api/credentials",
+        headers={
+            "Origin": "moz-extension://abcd1234-uuid-fake",
+            "Access-Control-Request-Method": "GET",
+            "Access-Control-Request-Headers": "X-Extension-Key",
+        },
+    )
+    assert resp.status_code == 204
+    assert resp.headers["Access-Control-Allow-Origin"] == "moz-extension://abcd1234-uuid-fake"
+    assert "OPTIONS" in resp.headers["Access-Control-Allow-Methods"]
+    assert "X-Extension-Key" in resp.headers["Access-Control-Allow-Headers"]
+
+
+@pytest.mark.asyncio
+async def test_extension_get_carries_acl_headers(client):
+    # The actual response (post-preflight) also needs ACL headers — the
+    # browser checks them again before exposing the response body.
+    resp = await client.get(
+        "/api/credentials",
+        headers={"Origin": "moz-extension://abcd1234-uuid-fake"},
+    )
+    # 200 with empty list (no creds seeded) — what matters here is the
+    # CORS header is present.
+    assert resp.headers["Access-Control-Allow-Origin"] == "moz-extension://abcd1234-uuid-fake"
+
+
+@pytest.mark.asyncio
+async def test_chrome_extension_origin_also_allowed(client):
+    resp = await client.options(
+        "/api/credentials",
+        headers={
+            "Origin": "chrome-extension://abcd1234-uuid-fake",
+            "Access-Control-Request-Method": "POST",
+        },
+    )
+    assert resp.status_code == 204
+    assert resp.headers["Access-Control-Allow-Origin"] == "chrome-extension://abcd1234-uuid-fake"
+
+
+@pytest.mark.asyncio
+async def test_normal_browser_request_gets_no_cors_headers(client):
+    # A regular browser tab (https://example.com / file:// / no Origin
+    # at all) should NOT get any Access-Control-Allow-* — the extension
+    # whitelist is intentionally narrow.
+    resp = await client.get(
+        "/api/credentials",
+        headers={"Origin": "https://evil.example.com"},
+    )
+    assert "Access-Control-Allow-Origin" not in resp.headers
+
+
+@pytest.mark.asyncio
+async def test_no_origin_header_unaffected(client):
+    # Same-origin requests (no Origin header) — unaffected.
+    resp = await client.get("/api/credentials")
+    assert "Access-Control-Allow-Origin" not in resp.headers