diff --git a/src/mcp/server/transport_security.py b/src/mcp/server/transport_security.py
index 91b5fa7edb..0012933e0a 100644
--- a/src/mcp/server/transport_security.py
+++ b/src/mcp/server/transport_security.py
@@ -53,17 +53,24 @@ def _validate_host(self, host: str | None) -> bool:
             logger.warning("Missing Host header in request")
             return False
 
+        # Host names are case-insensitive (RFC 9110 Section 4.2.3), and WHATWG-URL
+        # clients (fetch, undici, browsers) lowercase the URL host before sending,
+        # so compare case-insensitively regardless of how allowed_hosts was written.
+        normalized_host = host.lower()
+
         # Check exact match first
-        if host in self.settings.allowed_hosts:
-            return True
+        for allowed in self.settings.allowed_hosts:
+            if allowed.lower() == normalized_host:
+                return True
 
         # Check wildcard port patterns
         for allowed in self.settings.allowed_hosts:
+            allowed = allowed.lower()
             if allowed.endswith(":*"):
                 # Extract base host from pattern
                 base_host = allowed[:-2]
                 # Check if the actual host starts with base host and has a port
-                if host.startswith(base_host + ":"):
+                if normalized_host.startswith(base_host + ":"):
                     return True
 
         logger.warning(f"Invalid Host header: {host}")
@@ -75,17 +82,23 @@ def _validate_origin(self, origin: str | None) -> bool:
         if not origin:
             return True
 
+        # An origin is scheme://host[:port]; scheme and host are case-insensitive
+        # (RFC 6454 Section 4), and clients send them lowercased. Compare case-insensitively.
+        normalized_origin = origin.lower()
+
         # Check exact match first
-        if origin in self.settings.allowed_origins:
-            return True
+        for allowed in self.settings.allowed_origins:
+            if allowed.lower() == normalized_origin:
+                return True
 
         # Check wildcard port patterns
         for allowed in self.settings.allowed_origins:
+            allowed = allowed.lower()
             if allowed.endswith(":*"):
                 # Extract base origin from pattern
                 base_origin = allowed[:-2]
                 # Check if the actual origin starts with base origin and has a port
-                if origin.startswith(base_origin + ":"):
+                if normalized_origin.startswith(base_origin + ":"):
                     return True
 
         logger.warning(f"Invalid Origin header: {origin}")
diff --git a/tests/server/test_transport_security.py b/tests/server/test_transport_security.py
index 67fe4ef1a1..c6eb359707 100644
--- a/tests/server/test_transport_security.py
+++ b/tests/server/test_transport_security.py
@@ -45,6 +45,11 @@ def _request(host: str | None, origin: str | None, content_type: str | None = "a
         pytest.param("good.example", "http://evil.example:9000", 403, id="origin-wildcard-base-mismatch"),
         pytest.param("good.example", "http://good.example", None, id="origin-exact"),
         pytest.param("good.example", "http://wild.example:9000", None, id="origin-wildcard-match"),
+        # Host / Origin are case-insensitive (RFC 9110 / RFC 6454); clients lowercase them.
+        pytest.param("GOOD.EXAMPLE", None, None, id="host-exact-uppercase-request"),
+        pytest.param("WILD.EXAMPLE:9000", None, None, id="host-wildcard-uppercase-request"),
+        pytest.param("good.example", "HTTP://GOOD.EXAMPLE", None, id="origin-exact-uppercase-request"),
+        pytest.param("good.example", "http://WILD.EXAMPLE:9000", None, id="origin-wildcard-uppercase-request"),
     ],
 )
 async def test_validate_request_checks_host_then_origin(
@@ -56,6 +61,25 @@ async def test_validate_request_checks_host_then_origin(
     assert (None if response is None else response.status_code) == expected
 
 
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ("host_header", "expected"),
+    [
+        # The Windows default path into the bug: allowed_hosts derived from
+        # %COMPUTERNAME% (always uppercase), client sends the lowercased host.
+        pytest.param("myhost:8000", None, id="uppercase-config-lowercase-request"),
+        pytest.param("MYHOST:8000", None, id="uppercase-config-uppercase-request"),
+        pytest.param("other:8000", 421, id="uppercase-config-still-rejects-non-match"),
+    ],
+)
+async def test_validate_host_case_insensitive_with_uppercase_allowlist(host_header: str, expected: int | None) -> None:
+    """An uppercase allowed_hosts entry still matches a lowercased Host header (RFC 9110 Section 4.2.3)."""
+    settings = TransportSecuritySettings(enable_dns_rebinding_protection=True, allowed_hosts=["MYHOST:*"])
+    middleware = TransportSecurityMiddleware(settings)
+    response = await middleware.validate_request(_request(host_header, None))
+    assert (None if response is None else response.status_code) == expected
+
+
 @pytest.mark.anyio
 async def test_validate_request_skips_host_and_origin_when_protection_is_disabled() -> None:
     """With DNS-rebinding protection off, any Host/Origin is accepted."""