curl_httpclient,http1connection: Prohibit CR and LF in headers

libcurl does not check for CR and LF in headers, making this the
application's responsibility. However, Tornado's other HTTP interfaces
check for linefeeds so we should do the same here so that switching
between the simple and curl http clients does not introduce header
injection vulnerabilties.

http1connection previously checked only for LF in headers (alone or in a
CRLF pair). It now prohibits bare CR as well, following the requirement
in RFC 9112.

tornado/curl_httpclient.py

@@ -19,6 +19,7 @@
 import functools
 import logging
 import pycurl
+import re
 import threading
 import time
 from io import BytesIO
@@ -44,6 +45,8 @@
 
 curl_log = logging.getLogger("tornado.curl_httpclient")
 
+CR_OR_LF_RE = re.compile(b"\r|\n")
+
 
 class CurlAsyncHTTPClient(AsyncHTTPClient):
     def initialize(  # type: ignore
@@ -347,14 +350,15 @@ def _curl_setup_request(
         if "Pragma" not in request.headers:
             request.headers["Pragma"] = ""
 
-        curl.setopt(
-            pycurl.HTTPHEADER,
-            [
-                b"%s: %s"
-                % (native_str(k).encode("ASCII"), native_str(v).encode("ISO8859-1"))
-                for k, v in request.headers.get_all()
-            ],
-        )
+        encoded_headers = [
+            b"%s: %s"
+            % (native_str(k).encode("ASCII"), native_str(v).encode("ISO8859-1"))
+            for k, v in request.headers.get_all()
+        ]
+        for line in encoded_headers:
+            if CR_OR_LF_RE.search(line):
+                raise ValueError("Illegal characters in header (CR or LF): %r" % line)
+        curl.setopt(pycurl.HTTPHEADER, encoded_headers)
 
         curl.setopt(
             pycurl.HEADERFUNCTION,

tornado/http1connection.py

@@ -38,6 +38,8 @@
 
 from typing import cast, Optional, Type, Awaitable, Callable, Union, Tuple
 
+CR_OR_LF_RE = re.compile(b"\r|\n")
+
 
 class _QuietException(Exception):
     def __init__(self) -> None:
@@ -453,8 +455,8 @@ def write_headers(
         )
         lines.extend(line.encode("latin1") for line in header_lines)
         for line in lines:
-            if b"\n" in line:
-                raise ValueError("Newline in header: " + repr(line))
+            if CR_OR_LF_RE.search(line):
+                raise ValueError("Illegal characters (CR or LF) in header: %r" % line)
         future = None
         if self.stream.closed():
             future = self._write_future = Future()

tornado/test/httpclient_test.py

@@ -725,6 +725,22 @@ def test_error_after_cancel(self):
                 if el.logged_stack:
                     break
 
+    def test_header_crlf(self):
+        # Ensure that the client doesn't allow CRLF injection in headers. RFC 9112 section 2.2
+        # prohibits a bare CR specifically and "a recipient MAY recognize a single LF as a line
+        # terminator" so we check each character separately as well as the (redundant) CRLF pair.
+        for header, name in [
+            ("foo\rbar:", "cr"),
+            ("foo\nbar:", "lf"),
+            ("foo\r\nbar:", "crlf"),
+        ]:
+            with self.subTest(name=name, position="value"):
+                with self.assertRaises(ValueError):
+                    self.fetch("/hello", headers={"foo": header})
+            with self.subTest(name=name, position="key"):
+                with self.assertRaises(ValueError):
+                    self.fetch("/hello", headers={header: "foo"})
+
 
 class RequestProxyTest(unittest.TestCase):
     def test_request_set(self):