wrap IPv6 SERVER_NAME in [] (#2997)

Author: davidism

CHANGES.rst

@@ -9,6 +9,9 @@ Unreleased
     ``list``, ``tuple``, or ``set`` when passing multiple values. It had been
     changed to accept any ``Collection``, but this matched types that should be
     treated as single values, such as ``bytes``. :issue:`2994`
+-   When the ``Host`` header is not set and ``Request.host`` falls back to the
+    WSGI ``SERVER_NAME`` value, if that value is an IPv6 address it is wrapped
+    in ``[]`` to match the ``Host`` header. :issue:`2993`
 
 
 Version 3.1.2

src/werkzeug/sansio/utils.py

@@ -71,6 +71,9 @@ def get_host(
     :return: Host, with port if necessary.
     :raise ~werkzeug.exceptions.SecurityError: If the host is not
         trusted.
+
+    .. versionchanged:: 3.1.3
+        If ``SERVER_NAME`` is IPv6, it is wrapped in ``[]``.
     """
     host = ""
 
@@ -79,6 +82,11 @@ def get_host(
     elif server is not None:
         host = server[0]
 
+        # If SERVER_NAME is IPv6, wrap it in [] to match Host header.
+        # Check for : because domain or IPv4 can't have that.
+        if ":" in host and host[0] != "[":
+            host = f"[{host}]"
+
         if server[1] is not None:
             host = f"{host}:{server[1]}"
 

tests/sansio/test_utils.py

@@ -14,12 +14,16 @@
         ("https", "spam", None, "spam"),
         ("https", "spam:443", None, "spam"),
         ("http", "spam:8080", None, "spam:8080"),
+        ("http", "127.0.0.1:8080", None, "127.0.0.1:8080"),
+        ("http", "[::1]:8080", None, "[::1]:8080"),
         ("ws", "spam", None, "spam"),
         ("ws", "spam:80", None, "spam"),
         ("wss", "spam", None, "spam"),
         ("wss", "spam:443", None, "spam"),
         ("http", None, ("spam", 80), "spam"),
         ("http", None, ("spam", 8080), "spam:8080"),
+        ("http", None, ("127.0.0.1", 8080), "127.0.0.1:8080"),
+        ("http", None, ("::1", 8080), "[::1]:8080"),
         ("http", None, ("unix/socket", None), "unix/socket"),
         ("http", "spam", ("eggs", 80), "spam"),
     ],