gh-79096: Protect cookie file created by {LWP,Mozilla}CookieJar.save() (GH-93463)gentoo-3.8.15

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.

(backported to 3.8 by Michał Górny)

3 files changed, 34 insertions, 2 deletions

diff --git a/Lib/http/cookiejar.py b/Lib/http/cookiejar.py
index 47ed5c3d64a..a2c66aa5ffa 100644
--- a/Lib/http/cookiejar.py
+++ b/Lib/http/cookiejar.py
@@ -1887,7 +1887,7 @@ class LWPCookieJar(FileCookieJar):
             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.
@@ -2082,7 +2082,7 @@ class MozillaCookieJar(FileCookieJar):
             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(self.header)
             now = time.time()
             for cookie in self:
diff --git a/Lib/test/test_http_cookiejar.py b/Lib/test/test_http_cookiejar.py
index 2d7077af6da..f3b6a4ae132 100644
--- a/Lib/test/test_http_cookiejar.py
+++ b/Lib/test/test_http_cookiejar.py
@@ -1,6 +1,7 @@
 """Tests for http/cookiejar.py."""
 
 import os
+import sys
 import re
 import test.support
 import time
@@ -15,6 +16,7 @@ from http.cookiejar import (time2isoz, http2time, iso2time, time2netscape,
      reach, is_HDN, domain_match, user_domain_match, request_path,
      request_port, request_host)
 
+mswindows = (sys.platform == "win32")
 
 class DateTimeTests(unittest.TestCase):
 
@@ -366,6 +368,35 @@ class FileCookieJarTests(unittest.TestCase):
             except OSError: pass
         self.assertEqual(c._cookies["www.acme.com"]["/"]["boo"].value, None)
 
+    @unittest.skipIf(mswindows, "windows file permissions are incompatible with file modes")
+    def test_lwp_filepermissions(self):
+        # Cookie file should only be readable by the creator
+        filename = test.support.TESTFN
+        c = LWPCookieJar()
+        interact_netscape(c, "http://www.acme.com/", 'boo')
+        try:
+            c.save(filename, ignore_discard=True)
+            status = os.stat(filename)
+            print(status.st_mode)
+            self.assertEqual(oct(status.st_mode)[-3:], '600')
+        finally:
+            try: os.unlink(filename)
+            except OSError: pass
+
+    @unittest.skipIf(mswindows, "windows file permissions are incompatible with file modes")
+    def test_mozilla_filepermissions(self):
+        # Cookie file should only be readable by the creator
+        filename = test.support.TESTFN
+        c = MozillaCookieJar()
+        interact_netscape(c, "http://www.acme.com/", 'boo')
+        try:
+            c.save(filename, ignore_discard=True)
+            status = os.stat(filename)
+            self.assertEqual(oct(status.st_mode)[-3:], '600')
+        finally:
+            try: os.unlink(filename)
+            except OSError: pass
+
     def test_bad_magic(self):
         # OSErrors (eg. file doesn't exist) are allowed to propagate
         filename = test.support.TESTFN
diff --git a/Misc/NEWS.d/next/Security/2022-06-03-12-52-53.gh-issue-79096.YVoxgC.rst b/Misc/NEWS.d/next/Security/2022-06-03-12-52-53.gh-issue-79096.YVoxgC.rst
new file mode 100644
index 00000000000..9ec3335dc71
--- /dev/null
+++ b/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)