web: Validate characters in all cookie attributes.

Our previous control character check was missing a check for
U+007F, and also semicolons, which are only allowed in quoted
parts of values. This commit checks all attributes and
updates the set of disallowed characters.

Author: bdarnell

tornado/test/web_test.py

@@ -1,3 +1,5 @@
+import http
+
 from tornado.concurrent import Future
 from tornado import gen
 from tornado.escape import (
@@ -292,11 +294,67 @@ def get(self):
                 self.set_cookie("unicode_args", "blah", domain="foo.com", path="/foo")
 
         class SetCookieSpecialCharHandler(RequestHandler):
+            # "Special" characters are allowed in cookie values, but trigger special quoting.
             def get(self):
                 self.set_cookie("equals", "a=b")
                 self.set_cookie("semicolon", "a;b")
                 self.set_cookie("quote", 'a"b')
 
+        class SetCookieForbiddenCharHandler(RequestHandler):
+            def get(self):
+                # Control characters and semicolons raise errors in cookie names and attributes
+                # (but not values, which are tested in SetCookieSpecialCharHandler)
+                for char in list(map(chr, range(0x20))) + [chr(0x7F), ";"]:
+                    try:
+                        self.set_cookie("foo" + char, "bar")
+                        self.write(
+                            "Didn't get expected exception for char %r in name\n" % char
+                        )
+                    except http.cookies.CookieError as e:
+                        if "Invalid cookie attribute name" not in str(e):
+                            self.write(
+                                "unexpected exception for char %r in name: %s\n"
+                                % (char, e)
+                            )
+
+                    try:
+                        self.set_cookie("foo", "bar", domain="example" + char + ".com")
+                        self.write(
+                            "Didn't get expected exception for char %r in domain\n"
+                            % char
+                        )
+                    except http.cookies.CookieError as e:
+                        if "Invalid cookie attribute domain" not in str(e):
+                            self.write(
+                                "unexpected exception for char %r in domain: %s\n"
+                                % (char, e)
+                            )
+
+                    try:
+                        self.set_cookie("foo", "bar", path="/" + char)
+                        self.write(
+                            "Didn't get expected exception for char %r in path\n" % char
+                        )
+                    except http.cookies.CookieError as e:
+                        if "Invalid cookie attribute path" not in str(e):
+                            self.write(
+                                "unexpected exception for char %r in path: %s\n"
+                                % (char, e)
+                            )
+
+                    try:
+                        self.set_cookie("foo", "bar", samesite="a" + char)
+                        self.write(
+                            "Didn't get expected exception for char %r in samesite\n"
+                            % char
+                        )
+                    except http.cookies.CookieError as e:
+                        if "Invalid cookie attribute samesite" not in str(e):
+                            self.write(
+                                "unexpected exception for char %r in samesite: %s\n"
+                                % (char, e)
+                            )
+
         class SetCookieOverwriteHandler(RequestHandler):
             def get(self):
                 self.set_cookie("a", "b", domain="example.com")
@@ -330,6 +388,7 @@ def get(self):
             ("/get", GetCookieHandler),
             ("/set_domain", SetCookieDomainHandler),
             ("/special_char", SetCookieSpecialCharHandler),
+            ("/forbidden_char", SetCookieForbiddenCharHandler),
             ("/set_overwrite", SetCookieOverwriteHandler),
             ("/set_max_age", SetCookieMaxAgeHandler),
             ("/set_expires_days", SetCookieExpiresDaysHandler),
@@ -387,6 +446,12 @@ def test_cookie_special_char(self):
             response = self.fetch("/get", headers={"Cookie": header})
             self.assertEqual(response.body, utf8(expected))
 
+    def test_set_cookie_forbidden_char(self):
+        response = self.fetch("/forbidden_char")
+        self.assertEqual(response.code, 200)
+        self.maxDiff = 10000
+        self.assertMultiLineEqual(to_unicode(response.body), "")
+
     def test_set_cookie_overwrite(self):
         response = self.fetch("/set_overwrite")
         headers = response.headers.get_list("Set-Cookie")

tornado/web.py

@@ -693,9 +693,30 @@ def set_cookie(
         # The cookie library only accepts type str, in both python 2 and 3
         name = escape.native_str(name)
         value = escape.native_str(value)
-        if re.search(r"[\x00-\x20]", name + value):
-            # Don't let us accidentally inject bad stuff
+        if re.search(r"[\x00-\x20]", value):
+            # Legacy check for control characters in cookie values. This check is no longer needed
+            # since the cookie library escapes these characters correctly now. It will be removed
+            # in the next feature release.
             raise ValueError(f"Invalid cookie {name!r}: {value!r}")
+        for attr_name, attr_value in [
+            ("name", name),
+            ("domain", domain),
+            ("path", path),
+            ("samesite", samesite),
+        ]:
+            # Cookie attributes may not contain control characters or semicolons (except when
+            # escaped in the value). A check for control characters was added to the http.cookies
+            # library in a Feb 2026 security release; as of March it still does not check for
+            # semicolons.
+            #
+            # When a semicolon check is added to the standard library (and the release has had time
+            # for adoption), this check may be removed, but be mindful of the fact that this may
+            # change the timing of the exception (to the generation of the Set-Cookie header in
+            # flush()). We m
+            if attr_value is not None and re.search(r"[\x00-\x20\x3b\x7f]", attr_value):
+                raise http.cookies.CookieError(
+                    f"Invalid cookie attribute {attr_name}={attr_value!r} for cookie {name!r}"
+                )
         if not hasattr(self, "_new_cookie"):
             self._new_cookie = (
                 http.cookies.SimpleCookie()