fix subdomain_matching=False behavior

CHANGES.rst

@@ -23,6 +23,9 @@ Unreleased
 -   Support key rotation with the ``SECRET_KEY_FALLBACKS`` config, a list of old
     secret keys that can still be used for unsigning. Extensions will need to
     add support. :issue:`5621`
+-   Fix how setting ``host_matching=True`` or ``subdomain_matching=False``
+    interacts with ``SERVER_NAME``. Setting ``SERVER_NAME`` no longer restricts
+    requests to only that domain. :issue:`5553`
 
 
 Version 3.0.3

src/flask/app.py

@@ -436,21 +436,24 @@ def create_url_adapter(self, request: Request | None) -> MapAdapter | None:
             matching. Use :attr:`subdomain_matching` instead.
         """
         if request is not None:
-            # If subdomain matching is disabled (the default), use the
-            # default subdomain in all cases. This should be the default
-            # in Werkzeug but it currently does not have that feature.
-            if not self.subdomain_matching:
-                subdomain = self.url_map.default_subdomain or None
-            else:
-                subdomain = None
+            subdomain = None
+            server_name = self.config["SERVER_NAME"]
+
+            if self.url_map.host_matching:
+                # Don't pass SERVER_NAME, otherwise it's used and the actual
+                # host is ignored, which breaks host matching.
+                server_name = None
+            elif not self.subdomain_matching:
+                # Werkzeug doesn't implement subdomain matching yet. Until then,
+                # disable it by forcing the current subdomain to the default, or
+                # the empty string.
+                subdomain = self.url_map.default_subdomain or ""
 
             return self.url_map.bind_to_environ(
-                request.environ,
-                server_name=self.config["SERVER_NAME"],
-                subdomain=subdomain,
+                request.environ, server_name=server_name, subdomain=subdomain
             )
-        # We need at the very least the server name to be set for this
-        # to work.
+
+        # Need at least SERVER_NAME to match/build outside a request.
         if self.config["SERVER_NAME"] is not None:
             return self.url_map.bind(
                 self.config["SERVER_NAME"],

tests/test_basic.py

@@ -4,6 +4,7 @@
 import uuid
 import warnings
 import weakref
+from contextlib import nullcontext
 from datetime import datetime
 from datetime import timezone
 from platform import python_implementation
@@ -1483,6 +1484,51 @@ def test_request_locals():
     assert not flask.g
 
 
+@pytest.mark.parametrize(
+    ("subdomain_matching", "host_matching", "expect_base", "expect_abc", "expect_xyz"),
+    [
+        (False, False, "default", "default", "default"),
+        (True, False, "default", "abc", "<invalid>"),
+        (False, True, "default", "abc", "default"),
+    ],
+)
+def test_server_name_matching(
+    subdomain_matching: bool,
+    host_matching: bool,
+    expect_base: str,
+    expect_abc: str,
+    expect_xyz: str | int,
+) -> None:
+    app = flask.Flask(
+        __name__,
+        subdomain_matching=subdomain_matching,
+        host_matching=host_matching,
+        static_host="example.test" if host_matching else None,
+    )
+    app.config["SERVER_NAME"] = "example.test"
+
+    @app.route("/", defaults={"name": "default"}, host="<name>")
+    @app.route("/", subdomain="<name>", host="<name>.example.test")
+    def index(name: str) -> str:
+        return name
+
+    client = app.test_client()
+
+    r = client.get(base_url="http://example.test")
+    assert r.text == expect_base
+
+    r = client.get(base_url="http://abc.example.test")
+    assert r.text == expect_abc
+
+    with pytest.warns() if subdomain_matching else nullcontext():
+        r = client.get(base_url="http://xyz.other.test")
+
+    if isinstance(expect_xyz, str):
+        assert r.text == expect_xyz
+    else:
+        assert r.status_code == expect_xyz
+
+
 def test_server_name_subdomain():
     app = flask.Flask(__name__, subdomain_matching=True)
     client = app.test_client()

tests/test_blueprints.py

@@ -951,7 +951,10 @@ def index():
 
 
 def test_nesting_subdomains(app, client) -> None:
-    subdomain = "api"
+    app.subdomain_matching = True
+    app.config["SERVER_NAME"] = "example.test"
+    client.allow_subdomain_redirects = True
+
     parent = flask.Blueprint("parent", __name__)
     child = flask.Blueprint("child", __name__)
 
@@ -960,42 +963,31 @@ def index():
         return "child"
 
     parent.register_blueprint(child)
-    app.register_blueprint(parent, subdomain=subdomain)
-
-    client.allow_subdomain_redirects = True
-
-    domain_name = "domain.tld"
-    app.config["SERVER_NAME"] = domain_name
-    response = client.get("/child/", base_url="http://api." + domain_name)
+    app.register_blueprint(parent, subdomain="api")
 
+    response = client.get("/child/", base_url="http://api.example.test")
     assert response.status_code == 200
 
 
 def test_child_and_parent_subdomain(app, client) -> None:
-    child_subdomain = "api"
-    parent_subdomain = "parent"
+    app.subdomain_matching = True
+    app.config["SERVER_NAME"] = "example.test"
+    client.allow_subdomain_redirects = True
+
     parent = flask.Blueprint("parent", __name__)
-    child = flask.Blueprint("child", __name__, subdomain=child_subdomain)
+    child = flask.Blueprint("child", __name__, subdomain="api")
 
     @child.route("/")
     def index():
         return "child"
 
     parent.register_blueprint(child)
-    app.register_blueprint(parent, subdomain=parent_subdomain)
-
-    client.allow_subdomain_redirects = True
-
-    domain_name = "domain.tld"
-    app.config["SERVER_NAME"] = domain_name
-    response = client.get(
-        "/", base_url=f"http://{child_subdomain}.{parent_subdomain}.{domain_name}"
-    )
+    app.register_blueprint(parent, subdomain="parent")
 
+    response = client.get("/", base_url="http://api.parent.example.test")
     assert response.status_code == 200
 
-    response = client.get("/", base_url=f"http://{parent_subdomain}.{domain_name}")
-
+    response = client.get("/", base_url="http://parent.example.test")
     assert response.status_code == 404