fix: validate headers in early_hints callback to match process_headers

The early_hints callback constructs 103 Early Hints responses without
any header validation, while process_headers validates against TOKEN_RE
and HEADER_VALUE_RE for normal responses. This inconsistency means a
WSGI app passing unsanitized data to wsgi.early_hints could enable
HTTP response splitting via CRLF injection.

Apply the same TOKEN_RE/HEADER_VALUE_RE checks from process_headers to
the early_hints callback for defense-in-depth consistency.

Closes #3585

gunicorn/http/wsgi.py

@@ -143,6 +143,10 @@ def _make_early_hints_callback(req, sock, resp):
         Args:
             headers: List of (name, value) header tuples, typically Link headers
                      Example: [('Link', '</style.css>; rel=preload; as=style')]
+
+        Raises:
+            InvalidHeaderName: If a header name is not a valid HTTP token.
+            InvalidHeader: If a header value contains invalid characters.
         """
         # Don't send after response has started - would break framing
         if resp.headers_sent:
@@ -159,6 +163,16 @@ def _make_early_hints_callback(req, sock, resp):
                 name = name.decode('latin-1')
             if isinstance(value, bytes):
                 value = value.decode('latin-1')
+
+            # Validate header name and value using the same checks as
+            # Response.process_headers — defense-in-depth against
+            # HTTP response splitting via CRLF injection.
+            if not TOKEN_RE.fullmatch(name):
+                raise InvalidHeaderName('%r' % name)
+            if not HEADER_VALUE_RE.fullmatch(value):
+                raise InvalidHeader('%r' % value)
+
+            value = value.strip(" \t")
             response += f"{name}: {value}\r\n".encode('latin-1')
         response += b"\r\n"
 

tests/test_early_hints.py

@@ -19,6 +19,7 @@ except ImportError:
     H2_AVAILABLE = False
 
 from gunicorn.http import wsgi
+from gunicorn.http.errors import InvalidHeader, InvalidHeaderName
 
 
 class MockConfig:
@@ -225,6 +226,75 @@ class TestWSGIEarlyHints:
         # Should still send 103 response with no headers
         assert sent_data == b"HTTP/1.1 103 Early Hints\r\n\r\n"
 
+    def test_early_hints_rejects_crlf_in_header_value(self):
+        """Test that CRLF in header values is rejected (response splitting)."""
+        cfg = MockConfig()
+        req = MockRequest(version=(1, 1))
+        sock = MockSocket()
+
+        resp, environ = wsgi.create(req, sock, ('127.0.0.1', 12345),
+                                    ('127.0.0.1', 8000), cfg)
+
+        # Attempt CRLF injection in header value
+        with pytest.raises(InvalidHeader):
+            environ['wsgi.early_hints']([
+                ('Link', '</evil>; rel=preload\r\nX-Injected: true'),
+            ])
+
+        # Nothing should have been sent
+        assert sock.get_sent_data() == b""
+
+    def test_early_hints_rejects_crlf_in_header_name(self):
+        """Test that CRLF in header names is rejected."""
+        cfg = MockConfig()
+        req = MockRequest(version=(1, 1))
+        sock = MockSocket()
+
+        resp, environ = wsgi.create(req, sock, ('127.0.0.1', 12345),
+                                    ('127.0.0.1', 8000), cfg)
+
+        with pytest.raises(InvalidHeaderName):
+            environ['wsgi.early_hints']([
+                ('Link\r\nX-Injected', '</evil>'),
+            ])
+
+        assert sock.get_sent_data() == b""
+
+    def test_early_hints_rejects_invalid_header_name(self):
+        """Test that invalid token characters in header name are rejected."""
+        cfg = MockConfig()
+        req = MockRequest(version=(1, 1))
+        sock = MockSocket()
+
+        resp, environ = wsgi.create(req, sock, ('127.0.0.1', 12345),
+                                    ('127.0.0.1', 8000), cfg)
+
+        # Space is not allowed in header names
+        with pytest.raises(InvalidHeaderName):
+            environ['wsgi.early_hints']([
+                ('Invalid Header', '</style.css>'),
+            ])
+
+    def test_early_hints_valid_headers_pass_validation(self):
+        """Test that valid headers still work after adding validation."""
+        cfg = MockConfig()
+        req = MockRequest(version=(1, 1))
+        sock = MockSocket()
+
+        resp, environ = wsgi.create(req, sock, ('127.0.0.1', 12345),
+                                    ('127.0.0.1', 8000), cfg)
+
+        # These should all pass validation
+        environ['wsgi.early_hints']([
+            ('Link', '</style.css>; rel=preload; as=style'),
+            ('Link', '</app.js>; rel=preload; as=script'),
+            ('X-Custom-Header', 'some-value'),
+        ])
+
+        sent_data = sock.get_sent_data()
+        assert b"HTTP/1.1 103 Early Hints\r\n" in sent_data
+        assert b"Link: </style.css>; rel=preload; as=style\r\n" in sent_data
+
 
 @pytest.mark.skipif(not H2_AVAILABLE, reason="h2 library not available")
 class TestHTTP2EarlyHints: