diff --git a/gunicorn/asgi/parser.py b/gunicorn/asgi/parser.py
index 3f4c8bea..9e3f70f2 100644
--- a/gunicorn/asgi/parser.py
+++ b/gunicorn/asgi/parser.py
@@ -29,6 +29,18 @@ class InvalidProxyHeader(ParseError):
 PP_V2_SIGNATURE = b"\x0D\x0A\x0D\x0A\x00\x0D\x0A\x51\x55\x49\x54\x0A"
 
 
+# RFC 9110 section 6.5.1: fields forbidden in trailers because they alter
+# routing, framing, or authentication.
+RFC9110_6_5_1_FORBIDDEN_TRAILER = frozenset((
+    b"host",
+    b"content-length",
+    b"transfer-encoding",
+    b"trailer",
+    b"authorization",
+    b"te",
+))
+
+
 class PPCommand(IntEnum):
     """PROXY protocol v2 commands."""
     LOCAL = 0x0
@@ -756,6 +768,14 @@ class PythonProtocol:
                         self._on_message_complete()
                     return True
 
+                # RFC 9110 section 6.5.1: reject fields that must not appear
+                # in trailers.
+                colon = line.find(b':')
+                if colon > 0:
+                    name = line[:colon].strip(b' \t').lower()
+                    if name in RFC9110_6_5_1_FORBIDDEN_TRAILER:
+                        raise InvalidHeaderName(name.decode('latin-1'))
+
         return False
 
     def _is_valid_method(self, method):
diff --git a/gunicorn/http/message.py b/gunicorn/http/message.py
index 7e835f9f..b27f40a5 100644
--- a/gunicorn/http/message.py
+++ b/gunicorn/http/message.py
@@ -132,6 +132,18 @@ METHOD_BADCHAR_RE = re.compile("[a-z#]")
 VERSION_RE = re.compile(r"HTTP/(\d)\.(\d)")
 RFC9110_5_5_INVALID_AND_DANGEROUS = re.compile(r"[\0\r\n]")
 
+# RFC 9110 section 6.5.1: fields forbidden in trailers because they alter
+# routing, framing, or authentication. Using the uppercased names stored
+# by parse_headers.
+RFC9110_6_5_1_FORBIDDEN_TRAILER = frozenset((
+    "HOST",
+    "CONTENT-LENGTH",
+    "TRANSFER-ENCODING",
+    "TRAILER",
+    "AUTHORIZATION",
+    "TE",
+))
+
 
 def _ip_in_allow_list(ip_str, allow_list, networks):
     """Check if IP address is in the allow list.
@@ -235,6 +247,10 @@ class Message:
             # b"\xDF".decode("latin-1").upper().encode("ascii") == b"SS"
             name = name.upper()
 
+            # RFC 9110 section 6.5.1
+            if from_trailer and name in RFC9110_6_5_1_FORBIDDEN_TRAILER:
+                raise InvalidHeaderName(name)
+
             value = [value.strip(" \t")]
 
             # Consume value continuation lines..
diff --git a/tests/requests/invalid/rfc9110_trailer_forbidden_cl_01.http b/tests/requests/invalid/rfc9110_trailer_forbidden_cl_01.http
new file mode 100644
index 00000000..7562e1a8
--- /dev/null
+++ b/tests/requests/invalid/rfc9110_trailer_forbidden_cl_01.http
@@ -0,0 +1,9 @@
+POST /p HTTP/1.1\r\n
+Host: example.com\r\n
+Transfer-Encoding: chunked\r\n
+\r\n
+5\r\n
+hello\r\n
+0\r\n
+Content-Length: 99\r\n
+\r\n
diff --git a/tests/requests/invalid/rfc9110_trailer_forbidden_cl_01.py b/tests/requests/invalid/rfc9110_trailer_forbidden_cl_01.py
new file mode 100644
index 00000000..b0f8ccae
--- /dev/null
+++ b/tests/requests/invalid/rfc9110_trailer_forbidden_cl_01.py
@@ -0,0 +1,9 @@
+#
+# This file is part of gunicorn released under the MIT license.
+# See the NOTICE for more information.
+
+# RFC 9110 section 6.5.1: Content-Length in trailers is a classic
+# smuggling vector; origin must reject.
+from gunicorn.http.errors import InvalidHeaderName
+request = InvalidHeaderName
+python_only = True
diff --git a/tests/requests/invalid/rfc9110_trailer_forbidden_host_01.http b/tests/requests/invalid/rfc9110_trailer_forbidden_host_01.http
new file mode 100644
index 00000000..348082e7
--- /dev/null
+++ b/tests/requests/invalid/rfc9110_trailer_forbidden_host_01.http
@@ -0,0 +1,9 @@
+POST /p HTTP/1.1\r\n
+Host: example.com\r\n
+Transfer-Encoding: chunked\r\n
+\r\n
+5\r\n
+hello\r\n
+0\r\n
+Host: evil.example.com\r\n
+\r\n
diff --git a/tests/requests/invalid/rfc9110_trailer_forbidden_host_01.py b/tests/requests/invalid/rfc9110_trailer_forbidden_host_01.py
new file mode 100644
index 00000000..fe897329
--- /dev/null
+++ b/tests/requests/invalid/rfc9110_trailer_forbidden_host_01.py
@@ -0,0 +1,11 @@
+#
+# This file is part of gunicorn released under the MIT license.
+# See the NOTICE for more information.
+
+# RFC 9110 section 6.5.1: certain header fields must not be sent in
+# trailers because they alter routing or message framing (e.g. Host,
+# Content-Length, Transfer-Encoding). Accepting them enables smuggling.
+from gunicorn.http.errors import InvalidHeaderName
+request = InvalidHeaderName
+# The C parser (gunicorn_h1c) does not yet enforce this rule.
+python_only = True
diff --git a/tests/requests/invalid/rfc9110_trailer_forbidden_te_01.http b/tests/requests/invalid/rfc9110_trailer_forbidden_te_01.http
new file mode 100644
index 00000000..e5c98d16
--- /dev/null
+++ b/tests/requests/invalid/rfc9110_trailer_forbidden_te_01.http
@@ -0,0 +1,9 @@
+POST /p HTTP/1.1\r\n
+Host: example.com\r\n
+Transfer-Encoding: chunked\r\n
+\r\n
+5\r\n
+hello\r\n
+0\r\n
+Transfer-Encoding: chunked\r\n
+\r\n
diff --git a/tests/requests/invalid/rfc9110_trailer_forbidden_te_01.py b/tests/requests/invalid/rfc9110_trailer_forbidden_te_01.py
new file mode 100644
index 00000000..caa2f5ac
--- /dev/null
+++ b/tests/requests/invalid/rfc9110_trailer_forbidden_te_01.py
@@ -0,0 +1,9 @@
+#
+# This file is part of gunicorn released under the MIT license.
+# See the NOTICE for more information.
+
+# RFC 9110 section 6.5.1: Transfer-Encoding in trailers alters framing
+# and must not be accepted.
+from gunicorn.http.errors import InvalidHeaderName
+request = InvalidHeaderName
+python_only = True