Answer only to the host names of Bindery

Without a login, a web page that the user visits could reach Bindery
under a name of its own that points at the address of Bindery, and read
and change everything. The "none" mode now answers only to addresses
and to names of a private network. ALLOWED_HOSTS sets the names for
each mode.

README.md

@@ -67,6 +67,7 @@ Environment variables set the configuration. **Restart the container after a cha
 | `PROXY_AUTH_TRUSTED_IPS` | The addresses and networks that can send the user header. See "Logins". | Loopback and private addresses |
 | `PROXY_AUTH_LOGOUT_URL` | Where the Log out button goes in proxy mode | None |
 | `BINDERY_API_KEY` | A key for scripts. See "Scripts". | None |
+| `ALLOWED_HOSTS` | The host names that Bindery answers to. See "Host names". | None |
 | `SESSION_COOKIE_SECURE` | `true` if users reach Bindery only over HTTPS | `false` |
 | `TRUST_PROXY` | `true` if a reverse proxy sends the client address. See "Lockouts and sessions". | `false` |
 | `URL_BASE` | A path prefix, such as `/bindery`. See "Reverse proxy". | None |
@@ -209,6 +210,28 @@ open session until that session ends. To stop a CWA user at once, remove the use
 `CWA_ALLOWED_USERS` and restart Bindery. To end all sessions, delete `/config/.flask_secret` and
 restart Bindery.
 
+### Host names
+
+Without a login, a web page that you visit can try to reach Bindery from your browser, under a host
+name of its own that points at the address of Bindery. To stop this, the `none` login mode answers
+only to these host names:
+
+- An address, such as `192.168.1.5`.
+- `localhost`, and a name without a dot, such as `nas` or the name of a container.
+- A name of a private network. These end in `.local`, `.lan`, `.home`, `.home.arpa`, `.internal`,
+  `.localhost` or `.localdomain`.
+
+For each other name, Bindery answers with an error that shows the name. If you use such a name, for
+example behind a reverse proxy, list it in `ALLOWED_HOSTS`:
+
+- `ALLOWED_HOSTS=bindery.example.com` allows that name. Bindery then answers only to the names in
+  the list, in each login mode. Write the names without a port.
+- A leading dot, as in `.example.com`, also allows the subdomains.
+- `ALLOWED_HOSTS=*` allows each name.
+
+The other login modes answer to each name, unless you set a list. The health check at `/api/health`
+works under each name.
+
 ### Scripts
 
 Scripts can use the JSON API of the web interface.

bindery/auth.py

@@ -15,6 +15,7 @@ from collections.abc import Mapping
 from dataclasses import dataclass
 from functools import lru_cache
 from typing import Any
+from urllib.parse import urlsplit
 
 from werkzeug.security import check_password_hash
 
@@ -27,6 +28,8 @@ AUTH_NONE, AUTH_LOCAL, AUTH_CWA, AUTH_PROXY = "none", "local", "cwa", "proxy"
 MAX_FAILURES = 5  # for one user name from one address
 MAX_ADDRESS_FAILURES = 20  # for all user names from one address
 LOCKOUT_SECONDS = 15 * 60
+# Name endings that only a private network can resolve.
+LOCAL_SUFFIXES = (".local", ".lan", ".home", ".home.arpa", ".internal", ".localhost", ".localdomain")
 
 
 class AuthError(Exception):
@@ -72,6 +75,31 @@ def _trusted_proxy(address: str | None) -> bool:
     return any(ip.version == n.version and ip in n for n in _networks(tuple(config.PROXY_AUTH_TRUSTED_IPS)))
 
 
+def host_allowed(host: str) -> bool:
+    """Whether Bindery answers a request with this Host header.
+
+    Without a login, each web page that the user visits could reach Bindery under
+    a name of its own that points at the address of Bindery (DNS rebinding), and
+    read and change everything. Such a name is a public name, so the "none" mode
+    answers only to addresses and to names of a private network.
+    """
+    try:
+        name = (urlsplit(f"//{host}").hostname or "").rstrip(".")
+    except ValueError:
+        return False
+    if config.ALLOWED_HOSTS:
+        return any(
+            h == "*" or name == h.lstrip(".") or (h.startswith(".") and name.endswith(h)) for h in config.ALLOWED_HOSTS
+        )
+    if config.AUTH_METHOD != AUTH_NONE:
+        return True
+    try:
+        ipaddress.ip_address(name)
+        return True
+    except ValueError:
+        return bool(name) and ("." not in name or name.endswith(LOCAL_SUFFIXES))
+
+
 def local_password_version() -> str:
     """Invalidate local sessions when the configured password hash changes."""
     return hashlib.sha256(config.LOCAL_PASSWORD_HASH.encode()).hexdigest()

bindery/config.py

@@ -81,6 +81,9 @@ PROXY_AUTH_TRUSTED_IPS = _list(
     "PROXY_AUTH_TRUSTED_IPS", "127.0.0.0/8, ::1, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, fc00::/7, fe80::/10"
 )
 API_KEY = os.getenv("BINDERY_API_KEY", "").strip()
+# The host names that Bindery answers to. A leading dot also allows the subdomains, and * allows every name.
+# Without a list, the "none" login mode answers only to addresses and local names. The other modes answer to all.
+ALLOWED_HOSTS = [h.lower() for h in _list("ALLOWED_HOSTS")]
 SESSION_COOKIE_SECURE = _bool("SESSION_COOKIE_SECURE")
 # Take the client address from the last entry of X-Forwarded-For, which a reverse proxy adds.
 TRUST_PROXY = _bool("TRUST_PROXY")

bindery/web.py

@@ -139,6 +139,13 @@ def _identity() -> auth.Identity | None:
     return auth.current_identity(request.headers, session, request.remote_addr)
 
 
+@app.before_request
+def check_host() -> Any:
+    if request.path == "/api/health" or auth.host_allowed(request.host):
+        return None
+    return jsonify({"error": f"Bindery does not answer to the host name {request.host}. Add it to ALLOWED_HOSTS."}), 400
+
+
 @app.before_request
 def check_login() -> Any:
     if not request.path.startswith("/api/") or request.path == "/api/health":

docker-compose.yml

@@ -21,6 +21,8 @@ services:
       # CWA_DB_PATH: /cwa-config/app.db
       # CALIBRE_DB_PATH: /calibre-library/metadata.db
       # URL_BASE: /bindery
+      # The host names that Bindery answers to. AUTH_METHOD=none needs this for a public name such as this one.
+      # ALLOWED_HOSTS: bindery.example.com
       # true only if clients reach Bindery through a reverse proxy alone. The login lockout then uses X-Forwarded-For.
       # TRUST_PROXY: "true"
       # For AUTH_METHOD=proxy: the address of the proxy. The default trusts each private address.

tests/test_api.py

@@ -10,6 +10,7 @@ import pytest
 from fake_cwa import FakeCwa
 from itsdangerous import TimestampSigner
 from test_build import PNG
+from werkzeug.security import generate_password_hash
 
 from bindery import auth, config, orchestrator, web
 from bindery.store import Store
@@ -242,6 +243,51 @@ def test_incomplete_login_settings_refuse_every_request(client, monkeypatch, set
     assert client.get("/").status_code == 200
 
 
+@pytest.mark.parametrize(
+    ("mode", "allowed", "host", "accepted"),
+    [
+        # Without a login, only an address or a name of a private network.
+        ("none", [], "localhost", True),
+        ("none", [], "127.0.0.1:8085", True),
+        ("none", [], "192.168.1.5:8085", True),
+        ("none", [], "[fd00::5]:8085", True),
+        ("none", [], "bindery:8085", True),
+        ("none", [], "nas.local", True),
+        ("none", [], "NAS.Home.Arpa.", True),
+        ("none", [], "bindery.example.com", False),
+        ("none", [], "rebind.attacker.example:8085", False),
+        ("none", [], "notlocal", True),
+        ("none", [], "evil.com.local.attacker.example", False),
+        # With a login, each name.
+        ("local", [], "bindery.example.com", True),
+        ("proxy", [], "bindery.example.com", True),
+        # A list counts in each mode.
+        ("none", ["bindery.example.com"], "bindery.example.com:443", True),
+        ("none", ["bindery.example.com"], "localhost", False),
+        ("none", ["bindery.example.com"], "x.bindery.example.com", False),
+        ("local", [".example.com"], "bindery.example.com", True),
+        ("local", [".example.com"], "example.com", True),
+        ("local", [".example.com"], "badexample.com", False),
+        ("local", ["example.com"], "attacker.example", False),
+        ("none", ["*"], "anything.example", True),
+    ],
+)
+def test_bindery_answers_only_to_its_host_names(client, monkeypatch, mode, allowed, host, accepted):
+    monkeypatch.setattr(config, "AUTH_METHOD", mode)
+    monkeypatch.setattr(config, "LOCAL_USERNAME", "reader")
+    monkeypatch.setattr(config, "LOCAL_PASSWORD_HASH", generate_password_hash("pw", method="pbkdf2:sha256:1000"))
+    monkeypatch.setattr(config, "ALLOWED_HOSTS", allowed)
+    for path in ("/", "/api/auth/check", "/static/style.css"):
+        response = client.get(path, headers={"Host": host})
+        assert response.status_code == (200 if accepted else 400), path
+        if not accepted:
+            assert response.json == {
+                "error": f"Bindery does not answer to the host name {host}. Add it to ALLOWED_HOSTS."
+            }
+    # The health check of the container works under each name.
+    assert client.get("/api/health", headers={"Host": host}).status_code == 200
+
+
 def test_api_key(client, cwa, monkeypatch):
     monkeypatch.setattr(config, "API_KEY", "k" * 32)
     assert client.get("/api/stories", headers={"X-Api-Key": "wrong"}).status_code == 401

tests/test_config.py

@@ -75,7 +75,9 @@ def test_values_from_the_environment(monkeypatch, reload_config):
     monkeypatch.setenv("CALIBRE_DB_PATH", "/library/metadata.db")
     monkeypatch.setenv("BINDERY_USERNAME", " reader ")
     monkeypatch.setenv("BINDERY_PASSWORD_HASH", " hash ")
+    monkeypatch.setenv("ALLOWED_HOSTS", "Bindery.Example.com, .home.example")
     reload_config()
+    assert config.ALLOWED_HOSTS == ["bindery.example.com", ".home.example"]
     assert config.URL_BASE == "/bindery"
     assert config.AUTH_METHOD == "cwa"
     assert config.UPDATE_INTERVAL_HOURS == 1.5
@@ -98,6 +100,7 @@ def test_defaults(monkeypatch, reload_config):
         "LOG_LEVEL",
         "PROXY_AUTH_USER_HEADER",
         "PROXY_AUTH_TRUSTED_IPS",
+        "ALLOWED_HOSTS",
         "SESSION_COOKIE_SECURE",
         "TRUST_PROXY",
         "CWA_DB_PATH",
@@ -123,6 +126,7 @@ def test_defaults(monkeypatch, reload_config):
         "fc00::/7",
         "fe80::/10",
     ]
+    assert config.ALLOWED_HOSTS == []
     assert config.SESSION_COOKIE_SECURE is False
     assert config.TRUST_PROXY is False
     assert config.CWA_DB_PATH is None