diff --git a/CHANGES.rst b/CHANGES.rst
index c0afdb481..de9c3a205 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -6,6 +6,20 @@ Version 3.2.0
 Unreleased
 
 
+Version 3.1.3
+-------------
+
+Released 2024-11-08
+
+-   Initial data passed to ``MultiDict`` and similar interfaces only accepts
+    ``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
 -------------
 
diff --git a/src/werkzeug/datastructures/headers.py b/src/werkzeug/datastructures/headers.py
index e8cbdd8e8..1088e3bc9 100644
--- a/src/werkzeug/datastructures/headers.py
+++ b/src/werkzeug/datastructures/headers.py
@@ -62,7 +62,7 @@ def __init__(
         defaults: (
             Headers
             | MultiDict[str, t.Any]
-            | cabc.Mapping[str, t.Any | cabc.Collection[t.Any]]
+            | cabc.Mapping[str, t.Any | list[t.Any] | tuple[t.Any, ...] | set[t.Any]]
             | cabc.Iterable[tuple[str, t.Any]]
             | None
         ) = None,
@@ -227,7 +227,7 @@ def extend(
         arg: (
             Headers
             | MultiDict[str, t.Any]
-            | cabc.Mapping[str, t.Any | cabc.Collection[t.Any]]
+            | cabc.Mapping[str, t.Any | list[t.Any] | tuple[t.Any, ...] | set[t.Any]]
             | cabc.Iterable[tuple[str, t.Any]]
             | None
         ) = None,
@@ -491,12 +491,14 @@ def update(
         arg: (
             Headers
             | MultiDict[str, t.Any]
-            | cabc.Mapping[str, t.Any | cabc.Collection[t.Any]]
+            | cabc.Mapping[
+                str, t.Any | list[t.Any] | tuple[t.Any, ...] | cabc.Set[t.Any]
+            ]
             | cabc.Iterable[tuple[str, t.Any]]
             | None
         ) = None,
         /,
-        **kwargs: t.Any | cabc.Collection[t.Any],
+        **kwargs: t.Any | list[t.Any] | tuple[t.Any, ...] | cabc.Set[t.Any],
     ) -> None:
         """Replace headers in this object with items from another
         headers object and keyword arguments.
@@ -516,9 +518,7 @@ def update(
                     self.setlist(key, arg.getlist(key))
             elif isinstance(arg, cabc.Mapping):
                 for key, value in arg.items():
-                    if isinstance(value, cabc.Collection) and not isinstance(
-                        value, str
-                    ):
+                    if isinstance(value, (list, tuple, set)):
                         self.setlist(key, value)
                     else:
                         self.set(key, value)
@@ -527,13 +527,16 @@ def update(
                     self.set(key, value)
 
         for key, value in kwargs.items():
-            if isinstance(value, cabc.Collection) and not isinstance(value, str):
+            if isinstance(value, (list, tuple, set)):
                 self.setlist(key, value)
             else:
                 self.set(key, value)
 
     def __or__(
-        self, other: cabc.Mapping[str, t.Any | cabc.Collection[t.Any]]
+        self,
+        other: cabc.Mapping[
+            str, t.Any | list[t.Any] | tuple[t.Any, ...] | cabc.Set[t.Any]
+        ],
     ) -> te.Self:
         if not isinstance(other, cabc.Mapping):
             return NotImplemented
@@ -545,13 +548,11 @@ def __or__(
     def __ior__(
         self,
         other: (
-            cabc.Mapping[str, t.Any | cabc.Collection[t.Any]]
+            cabc.Mapping[str, t.Any | list[t.Any] | tuple[t.Any, ...] | cabc.Set[t.Any]]
             | cabc.Iterable[tuple[str, t.Any]]
         ),
     ) -> te.Self:
-        if not isinstance(other, (cabc.Mapping, cabc.Iterable)) or isinstance(
-            other, str
-        ):
+        if not isinstance(other, (cabc.Mapping, cabc.Iterable)):
             return NotImplemented
 
         self.update(other)
diff --git a/src/werkzeug/datastructures/structures.py b/src/werkzeug/datastructures/structures.py
index fcfa160da..dbb7e8048 100644
--- a/src/werkzeug/datastructures/structures.py
+++ b/src/werkzeug/datastructures/structures.py
@@ -22,7 +22,7 @@
 def iter_multi_items(
     mapping: (
         MultiDict[K, V]
-        | cabc.Mapping[K, V | cabc.Collection[V]]
+        | cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
         | cabc.Iterable[tuple[K, V]]
     ),
 ) -> cabc.Iterator[tuple[K, V]]:
@@ -33,11 +33,11 @@ def iter_multi_items(
         yield from mapping.items(multi=True)
     elif isinstance(mapping, cabc.Mapping):
         for key, value in mapping.items():
-            if isinstance(value, cabc.Collection) and not isinstance(value, str):
+            if isinstance(value, (list, tuple, set)):
                 for v in value:
                     yield key, v
             else:
-                yield key, value  # type: ignore[misc]
+                yield key, value
     else:
         yield from mapping
 
@@ -182,7 +182,7 @@ def __init__(
         self,
         mapping: (
             MultiDict[K, V]
-            | cabc.Mapping[K, V | cabc.Collection[V]]
+            | cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
             | cabc.Iterable[tuple[K, V]]
             | None
         ) = None,
@@ -194,7 +194,7 @@ def __init__(
         elif isinstance(mapping, cabc.Mapping):
             tmp = {}
             for key, value in mapping.items():
-                if isinstance(value, cabc.Collection) and not isinstance(value, str):
+                if isinstance(value, (list, tuple, set)):
                     value = list(value)
 
                     if not value:
@@ -419,7 +419,7 @@ def update(  # type: ignore[override]
         self,
         mapping: (
             MultiDict[K, V]
-            | cabc.Mapping[K, V | cabc.Collection[V]]
+            | cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
             | cabc.Iterable[tuple[K, V]]
         ),
     ) -> None:
@@ -444,7 +444,7 @@ def update(  # type: ignore[override]
             self.add(key, value)
 
     def __or__(  # type: ignore[override]
-        self, other: cabc.Mapping[K, V | cabc.Collection[V]]
+        self, other: cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
     ) -> MultiDict[K, V]:
         if not isinstance(other, cabc.Mapping):
             return NotImplemented
@@ -455,11 +455,12 @@ def __or__(  # type: ignore[override]
 
     def __ior__(  # type: ignore[override]
         self,
-        other: cabc.Mapping[K, V | cabc.Collection[V]] | cabc.Iterable[tuple[K, V]],
+        other: (
+            cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
+            | cabc.Iterable[tuple[K, V]]
+        ),
     ) -> te.Self:
-        if not isinstance(other, (cabc.Mapping, cabc.Iterable)) or isinstance(
-            other, str
-        ):
+        if not isinstance(other, (cabc.Mapping, cabc.Iterable)):
             return NotImplemented
 
         self.update(other)
@@ -600,7 +601,7 @@ def __init__(
         self,
         mapping: (
             MultiDict[K, V]
-            | cabc.Mapping[K, V | cabc.Collection[V]]
+            | cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
             | cabc.Iterable[tuple[K, V]]
             | None
         ) = None,
@@ -744,7 +745,7 @@ def update(  # type: ignore[override]
         self,
         mapping: (
             MultiDict[K, V]
-            | cabc.Mapping[K, V | cabc.Collection[V]]
+            | cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
             | cabc.Iterable[tuple[K, V]]
         ),
     ) -> None:
@@ -1009,7 +1010,7 @@ def __init__(
         self,
         mapping: (
             MultiDict[K, V]
-            | cabc.Mapping[K, V | cabc.Collection[V]]
+            | cabc.Mapping[K, V | list[V] | tuple[V, ...] | set[V]]
             | cabc.Iterable[tuple[K, V]]
             | None
         ) = None,
diff --git a/src/werkzeug/sansio/utils.py b/src/werkzeug/sansio/utils.py
index 14fa0ac88..ff7ceda34 100644
--- a/src/werkzeug/sansio/utils.py
+++ b/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]}"
 
diff --git a/tests/sansio/test_utils.py b/tests/sansio/test_utils.py
index d43de66c2..a63e7c660 100644
--- a/tests/sansio/test_utils.py
+++ b/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"),
     ],
diff --git a/tests/test_datastructures.py b/tests/test_datastructures.py
index dcbc79697..0cd497438 100644
--- a/tests/test_datastructures.py
+++ b/tests/test_datastructures.py
@@ -1,6 +1,9 @@
+from __future__ import annotations
+
 import io
 import pickle
 import tempfile
+import typing as t
 from contextlib import contextmanager
 from copy import copy
 from copy import deepcopy
@@ -43,7 +46,7 @@ def items(self, multi=1):
 
 
 class _MutableMultiDictTests:
-    storage_class: type["ds.MultiDict"]
+    storage_class: type[ds.MultiDict]
 
     def test_pickle(self):
         cls = self.storage_class
@@ -1280,3 +1283,15 @@ def test_range_to_header(ranges):
 def test_range_validates_ranges(ranges):
     with pytest.raises(ValueError):
         ds.Range("bytes", ranges)
+
+
+@pytest.mark.parametrize(
+    ("value", "expect"),
+    [
+        ({"a": "ab"}, [("a", "ab")]),
+        ({"a": ["a", "b"]}, [("a", "a"), ("a", "b")]),
+        ({"a": b"ab"}, [("a", b"ab")]),
+    ],
+)
+def test_iter_multi_data(value: t.Any, expect: list[tuple[t.Any, t.Any]]) -> None:
+    assert list(ds.iter_multi_items(value)) == expect