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gh-90309: Base64-encode cookie values embedded in JS #148848

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gh-90309: Base64-encode cookie values embedded in JS #148848

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8 changes: 6 additions & 2 deletions Lib/http/cookies.py
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Original file line number Diff line number Diff line change
Expand Up @@ -391,17 +391,21 @@ def __repr__(self):
return '<%s: %s>' % (self.__class__.__name__, self.OutputString())

def js_output(self, attrs=None):
import base64
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@StanFromIreland StanFromIreland Apr 21, 2026

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We can use lazy on the main branch.

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@sethmlarson sethmlarson Apr 21, 2026

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@StanFromIreland Is there a substantial performance difference for using lazy if the module is used immediately after it's imported? I am new to the lazy keyword! :)

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@serhiy-storchaka serhiy-storchaka Apr 21, 2026

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I think that @StanFromIreland meant that we can use lazy import at module level.

But this will make backporting more difficult.

# Print javascript
output_string = self.OutputString(attrs)
if _has_control_character(output_string):
raise CookieError("Control characters are not allowed in cookies")
# Base64-encode value to avoid template
# injection in cookie values.
output_encoded = base64.b64encode(output_string.encode('utf-8')).decode("ascii")
return """
<script type="text/javascript">
<!-- begin hiding
document.cookie = \"%s\";
document.cookie = atob(\"%s\");
// end hiding -->
</script>
""" % (output_string.replace('"', r'\"'))
""" % (output_encoded,)

def OutputString(self, attrs=None):
# Build up our result
Expand Down
29 changes: 18 additions & 11 deletions Lib/test/test_http_cookies.py
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
# Simple test suite for http/cookies.py

import base64
import copy
import unittest
import doctest
Expand Down Expand Up @@ -175,17 +175,19 @@ def test_load(self):

self.assertEqual(C.output(['path']),
'Set-Cookie: Customer="WILE_E_COYOTE"; Path=/acme')
self.assertEqual(C.js_output(), r"""
cookie_encoded = base64.b64encode(b'Customer="WILE_E_COYOTE"; Path=/acme; Version=1').decode('ascii')
self.assertEqual(C.js_output(), fr"""
<script type="text/javascript">
<!-- begin hiding
document.cookie = "Customer=\"WILE_E_COYOTE\"; Path=/acme; Version=1";
document.cookie = atob("{cookie_encoded}");
// end hiding -->
</script>
""")
self.assertEqual(C.js_output(['path']), r"""
cookie_encoded = base64.b64encode(b'Customer="WILE_E_COYOTE"; Path=/acme').decode('ascii')
self.assertEqual(C.js_output(['path']), fr"""
<script type="text/javascript">
<!-- begin hiding
document.cookie = "Customer=\"WILE_E_COYOTE\"; Path=/acme";
document.cookie = atob("{cookie_encoded}");
// end hiding -->
</script>
""")
Expand Down Expand Up @@ -290,17 +292,19 @@ def test_quoted_meta(self):

self.assertEqual(C.output(['path']),
'Set-Cookie: Customer="WILE_E_COYOTE"; Path=/acme')
self.assertEqual(C.js_output(), r"""
expected_encoded_cookie = base64.b64encode(b'Customer=\"WILE_E_COYOTE\"; Path=/acme; Version=1').decode('ascii')
self.assertEqual(C.js_output(), fr"""
<script type="text/javascript">
<!-- begin hiding
document.cookie = "Customer=\"WILE_E_COYOTE\"; Path=/acme; Version=1";
document.cookie = atob("{expected_encoded_cookie}");
// end hiding -->
</script>
""")
self.assertEqual(C.js_output(['path']), r"""
expected_encoded_cookie = base64.b64encode(b'Customer=\"WILE_E_COYOTE\"; Path=/acme').decode('ascii')
self.assertEqual(C.js_output(['path']), fr"""
<script type="text/javascript">
<!-- begin hiding
document.cookie = "Customer=\"WILE_E_COYOTE\"; Path=/acme";
document.cookie = atob("{expected_encoded_cookie}");
// end hiding -->
</script>
""")
Expand Down Expand Up @@ -391,13 +395,16 @@ def test_setter(self):
self.assertEqual(
M.output(),
"Set-Cookie: %s=%s; Path=/foo" % (i, "%s_coded_val" % i))
expected_encoded_cookie = base64.b64encode(
("%s=%s; Path=/foo" % (i, "%s_coded_val" % i)).encode("ascii")
).decode('ascii')
expected_js_output = """
<script type="text/javascript">
<!-- begin hiding
document.cookie = "%s=%s; Path=/foo";
document.cookie = atob("%s");
// end hiding -->
</script>
""" % (i, "%s_coded_val" % i)
""" % (expected_encoded_cookie,)
self.assertEqual(M.js_output(), expected_js_output)
for i in ["foo bar", "foo@bar"]:
# Try some illegal characters
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Security/2026-04-21-13-46-30.gh-issue-90309.srvj9q.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Base64-encode values when embedding cookies to JavaScript to avoid injection
and escaping.
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