[3.11] gh-79096: Protect cookie file created by {LWP,Mozilla}CookieJar.save() (GH-93463) (GH-93636)

Note: This change is not effective on Microsoft Windows.

Cookies can store sensitive information and should therefore be protected
against unauthorized third parties. This is also described in issue #79096.

The filesystem permissions are currently set to 644, everyone can read the
file. This commit changes the permissions to 600, only the creater of the file
can read and modify it. This improves security, because it reduces the attack
surface. Now the attacker needs control of the user that created the cookie or
a ways to circumvent the filesystems permissions.

This change is backwards incompatible. Systems that rely on world-readable
cookies will breake. However, one could argue that those are misconfigured in
the first place.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Pascal Wittmann <mail@pascal-wittmann.de>
Co-authored-by: Christian Heimes <christian@python.org>

Lib/http/cookiejar.py

@@ -1890,7 +1890,7 @@ def save(self, filename=None, ignore_discard=False, ignore_expires=False):
             if self.filename is not None: filename = self.filename
             else: raise ValueError(MISSING_FILENAME_TEXT)
 
-        with open(filename, "w") as f:
+        with os.fdopen(os.open(filename, os.O_CREAT | os.O_WRONLY, 0o600), 'w') as f:
             # There really isn't an LWP Cookies 2.0 format, but this indicates
             # that there is extra information in here (domain_dot and
             # port_spec) while still being compatible with libwww-perl, I hope.
@@ -2086,7 +2086,7 @@ def save(self, filename=None, ignore_discard=False, ignore_expires=False):
             if self.filename is not None: filename = self.filename
             else: raise ValueError(MISSING_FILENAME_TEXT)
 
-        with open(filename, "w") as f:
+        with os.fdopen(os.open(filename, os.O_CREAT | os.O_WRONLY, 0o600), 'w') as f:
             f.write(NETSCAPE_HEADER_TEXT)
             now = time.time()
             for cookie in self:

Lib/test/test_http_cookiejar.py

@@ -1,6 +1,8 @@
 """Tests for http/cookiejar.py."""
 
 import os
+import stat
+import sys
 import re
 import test.support
 from test.support import os_helper
@@ -17,6 +19,7 @@
      reach, is_HDN, domain_match, user_domain_match, request_path,
      request_port, request_host)
 
+mswindows = (sys.platform == "win32")
 
 class DateTimeTests(unittest.TestCase):
 
@@ -364,10 +367,37 @@ def test_lwp_valueless_cookie(self):
             c = LWPCookieJar()
             c.load(filename, ignore_discard=True)
         finally:
-            try: os.unlink(filename)
-            except OSError: pass
+            os_helper.unlink(filename)
         self.assertEqual(c._cookies["www.acme.com"]["/"]["boo"].value, None)
 
+    @unittest.skipIf(mswindows, "windows file permissions are incompatible with file modes")
+    @os_helper.skip_unless_working_chmod
+    def test_lwp_filepermissions(self):
+        # Cookie file should only be readable by the creator
+        filename = os_helper.TESTFN
+        c = LWPCookieJar()
+        interact_netscape(c, "http://www.acme.com/", 'boo')
+        try:
+            c.save(filename, ignore_discard=True)
+            st = os.stat(filename)
+            self.assertEqual(stat.S_IMODE(st.st_mode), 0o600)
+        finally:
+            os_helper.unlink(filename)
+
+    @unittest.skipIf(mswindows, "windows file permissions are incompatible with file modes")
+    @os_helper.skip_unless_working_chmod
+    def test_mozilla_filepermissions(self):
+        # Cookie file should only be readable by the creator
+        filename = os_helper.TESTFN
+        c = MozillaCookieJar()
+        interact_netscape(c, "http://www.acme.com/", 'boo')
+        try:
+            c.save(filename, ignore_discard=True)
+            st = os.stat(filename)
+            self.assertEqual(stat.S_IMODE(st.st_mode), 0o600)
+        finally:
+            os_helper.unlink(filename)
+
     def test_bad_magic(self):
         # OSErrors (eg. file doesn't exist) are allowed to propagate
         filename = os_helper.TESTFN
@@ -391,8 +421,7 @@ def test_bad_magic(self):
                     c = cookiejar_class()
                     self.assertRaises(LoadError, c.load, filename)
         finally:
-            try: os.unlink(filename)
-            except OSError: pass
+            os_helper.unlink(filename)
 
 class CookieTests(unittest.TestCase):
     # XXX
@@ -496,7 +525,7 @@ def test_missing_value(self):
             c = MozillaCookieJar(filename)
             c.revert(ignore_expires=True, ignore_discard=True)
         finally:
-            os.unlink(c.filename)
+            os_helper.unlink(c.filename)
         # cookies unchanged apart from lost info re. whether path was specified
         self.assertEqual(
             repr(c),
@@ -1766,8 +1795,7 @@ def test_rejection(self):
             c = LWPCookieJar(policy=pol)
             c.load(filename, ignore_discard=True)
         finally:
-            try: os.unlink(filename)
-            except OSError: pass
+            os_helper.unlink(filename)
 
         self.assertEqual(old, repr(c))
 
@@ -1826,8 +1854,7 @@ def save_and_restore(cj, ignore_discard):
                                          DefaultCookiePolicy(rfc2965=True))
                 new_c.load(ignore_discard=ignore_discard)
             finally:
-                try: os.unlink(filename)
-                except OSError: pass
+                os_helper.unlink(filename)
             return new_c
 
         new_c = save_and_restore(c, True)

Misc/NEWS.d/next/Security/2022-06-03-12-52-53.gh-issue-79096.YVoxgC.rst

@@ -0,0 +1 @@
+LWPCookieJar and MozillaCookieJar create files with file mode 600 instead of 644 (Microsoft Windows is not affected)