--- /dev/null
+From e3110c3cfbb7daa690d54d0eff6c264c870a71bf Mon Sep 17 00:00:00 2001
+From: Senthil Kumaran <senthil@uthcode.com>
+Date: Mon, 15 Feb 2021 10:15:02 -0800
+Subject: [PATCH] [3.8] bpo-42967: only use '&' as a query string separator
+ (GH-24297) (#24529)
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+* bpo-42967: only use '&' as a query string separator (#24297)
+
+bpo-42967: [security] Address a web cache-poisoning issue reported in
+urllib.parse.parse_qsl().
+
+urllib.parse will only us "&" as query string separator by default
+instead of both ";" and "&" as allowed in earlier versions. An optional
+argument seperator with default value "&" is added to specify the
+separator.
+
+Co-authored-by: Éric Araujo <merwok@netwok.org>
+Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
+Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
+Co-authored-by: Éric Araujo <merwok@netwok.org>
+(cherry picked from commit fcbe0cb04d35189401c0c880ebfb4311e952d776)
+
+* [3.8] bpo-42967: only use '&' as a query string separator (GH-24297)
+
+bpo-42967: [security] Address a web cache-poisoning issue reported in urllib.parse.parse_qsl().
+
+urllib.parse will only us "&" as query string separator by default instead of both ";" and "&" as allowed in earlier versions. An optional argument seperator with default value "&" is added to specify the separator.
+
+Co-authored-by: Éric Araujo <merwok@netwok.org>
+Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
+Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
+Co-authored-by: Éric Araujo <merwok@netwok.org>.
+(cherry picked from commit fcbe0cb04d35189401c0c880ebfb4311e952d776)
+
+Co-authored-by: Adam Goldschmidt <adamgold7@gmail.com>
+
+* Update correct version information.
+
+* fix docs and make logic clearer
+
+Co-authored-by: Adam Goldschmidt <adamgold7@gmail.com>
+Co-authored-by: Fidget-Spinner <28750310+Fidget-Spinner@users.noreply.github.com>
+
+Upstream-Status: Backport [https://github.com/python/cpython/commit/e3110c3cfbb7daa690d54d0eff6c264c870a71bf]
+CVE: CVE-2020-23336
+Signed-off-by: Chee Yang Lee <chee.yang.lee@intel.com>
+
+---
+ Doc/library/cgi.rst | 11 ++-
+ Doc/library/urllib.parse.rst | 22 +++++-
+ Doc/whatsnew/3.6.rst | 13 ++++
+ Doc/whatsnew/3.7.rst | 13 ++++
+ Doc/whatsnew/3.8.rst | 13 ++++
+ Lib/cgi.py | 23 ++++---
+ Lib/test/test_cgi.py | 29 ++++++--
+ Lib/test/test_urlparse.py | 68 +++++++++++++------
+ Lib/urllib/parse.py | 19 ++++--
+ .../2021-02-14-15-59-16.bpo-42967.YApqDS.rst | 1 +
+ 10 files changed, 166 insertions(+), 46 deletions(-)
+ create mode 100644 Misc/NEWS.d/next/Security/2021-02-14-15-59-16.bpo-42967.YApqDS.rst
+
+diff --git a/Doc/library/cgi.rst b/Doc/library/cgi.rst
+index 4048592e7361f..880074bed6026 100644
+--- a/Doc/library/cgi.rst
++++ b/Doc/library/cgi.rst
+@@ -277,14 +277,16 @@ These are useful if you want more control, or if you want to employ some of the
+ algorithms implemented in this module in other circumstances.
+
+
+-.. function:: parse(fp=None, environ=os.environ, keep_blank_values=False, strict_parsing=False)
++.. function:: parse(fp=None, environ=os.environ, keep_blank_values=False, strict_parsing=False, separator="&")
+
+ Parse a query in the environment or from a file (the file defaults to
+- ``sys.stdin``). The *keep_blank_values* and *strict_parsing* parameters are
++ ``sys.stdin``). The *keep_blank_values*, *strict_parsing* and *separator* parameters are
+ passed to :func:`urllib.parse.parse_qs` unchanged.
+
++ .. versionchanged:: 3.8.8
++ Added the *separator* parameter.
+
+-.. function:: parse_multipart(fp, pdict, encoding="utf-8", errors="replace")
++.. function:: parse_multipart(fp, pdict, encoding="utf-8", errors="replace", separator="&")
+
+ Parse input of type :mimetype:`multipart/form-data` (for file uploads).
+ Arguments are *fp* for the input file, *pdict* for a dictionary containing
+@@ -303,6 +305,9 @@ algorithms implemented in this module in other circumstances.
+ Added the *encoding* and *errors* parameters. For non-file fields, the
+ value is now a list of strings, not bytes.
+
++ .. versionchanged:: 3.8.8
++ Added the *separator* parameter.
++
+
+ .. function:: parse_header(string)
+
+diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
+index 25e5cc1a6ce0b..fcad7076e6c77 100644
+--- a/Doc/library/urllib.parse.rst
++++ b/Doc/library/urllib.parse.rst
+@@ -165,7 +165,7 @@ or on combining URL components into a URL string.
+ now raise :exc:`ValueError`.
+
+
+-.. function:: parse_qs(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None)
++.. function:: parse_qs(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None, separator='&')
+
+ Parse a query string given as a string argument (data of type
+ :mimetype:`application/x-www-form-urlencoded`). Data are returned as a
+@@ -190,6 +190,9 @@ or on combining URL components into a URL string.
+ read. If set, then throws a :exc:`ValueError` if there are more than
+ *max_num_fields* fields read.
+
++ The optional argument *separator* is the symbol to use for separating the
++ query arguments. It defaults to ``&``.
++
+ Use the :func:`urllib.parse.urlencode` function (with the ``doseq``
+ parameter set to ``True``) to convert such dictionaries into query
+ strings.
+@@ -201,8 +204,14 @@ or on combining URL components into a URL string.
+ .. versionchanged:: 3.8
+ Added *max_num_fields* parameter.
+
++ .. versionchanged:: 3.8.8
++ Added *separator* parameter with the default value of ``&``. Python
++ versions earlier than Python 3.8.8 allowed using both ``;`` and ``&`` as
++ query parameter separator. This has been changed to allow only a single
++ separator key, with ``&`` as the default separator.
++
+
+-.. function:: parse_qsl(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None)
++.. function:: parse_qsl(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None, separator='&')
+
+ Parse a query string given as a string argument (data of type
+ :mimetype:`application/x-www-form-urlencoded`). Data are returned as a list of
+@@ -226,6 +235,9 @@ or on combining URL components into a URL string.
+ read. If set, then throws a :exc:`ValueError` if there are more than
+ *max_num_fields* fields read.
+
++ The optional argument *separator* is the symbol to use for separating the
++ query arguments. It defaults to ``&``.
++
+ Use the :func:`urllib.parse.urlencode` function to convert such lists of pairs into
+ query strings.
+
+@@ -235,6 +247,12 @@ or on combining URL components into a URL string.
+ .. versionchanged:: 3.8
+ Added *max_num_fields* parameter.
+
++ .. versionchanged:: 3.8.8
++ Added *separator* parameter with the default value of ``&``. Python
++ versions earlier than Python 3.8.8 allowed using both ``;`` and ``&`` as
++ query parameter separator. This has been changed to allow only a single
++ separator key, with ``&`` as the default separator.
++
+
+ .. function:: urlunparse(parts)
+
+diff --git a/Doc/whatsnew/3.6.rst b/Doc/whatsnew/3.6.rst
+index 85a6657fdfbda..03a877a3d9178 100644
+--- a/Doc/whatsnew/3.6.rst
++++ b/Doc/whatsnew/3.6.rst
+@@ -2443,3 +2443,16 @@ because of the behavior of the socket option ``SO_REUSEADDR`` in UDP. For more
+ details, see the documentation for ``loop.create_datagram_endpoint()``.
+ (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
+ :issue:`37228`.)
++
++Notable changes in Python 3.6.13
++================================
++
++Earlier Python versions allowed using both ``;`` and ``&`` as
++query parameter separators in :func:`urllib.parse.parse_qs` and
++:func:`urllib.parse.parse_qsl`. Due to security concerns, and to conform with
++newer W3C recommendations, this has been changed to allow only a single
++separator key, with ``&`` as the default. This change also affects
++:func:`cgi.parse` and :func:`cgi.parse_multipart` as they use the affected
++functions internally. For more details, please see their respective
++documentation.
++(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)
+diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
+index 4933cba3990b1..824dc13e0c6fd 100644
+--- a/Doc/whatsnew/3.7.rst
++++ b/Doc/whatsnew/3.7.rst
+@@ -2556,3 +2556,16 @@ because of the behavior of the socket option ``SO_REUSEADDR`` in UDP. For more
+ details, see the documentation for ``loop.create_datagram_endpoint()``.
+ (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
+ :issue:`37228`.)
++
++Notable changes in Python 3.7.10
++================================
++
++Earlier Python versions allowed using both ``;`` and ``&`` as
++query parameter separators in :func:`urllib.parse.parse_qs` and
++:func:`urllib.parse.parse_qsl`. Due to security concerns, and to conform with
++newer W3C recommendations, this has been changed to allow only a single
++separator key, with ``&`` as the default. This change also affects
++:func:`cgi.parse` and :func:`cgi.parse_multipart` as they use the affected
++functions internally. For more details, please see their respective
++documentation.
++(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)
+diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
+index 1a192800b2f02..632ccc1f2c40a 100644
+--- a/Doc/whatsnew/3.8.rst
++++ b/Doc/whatsnew/3.8.rst
+@@ -2251,3 +2251,16 @@ The constant values of future flags in the :mod:`__future__` module
+ are updated in order to prevent collision with compiler flags. Previously
+ ``PyCF_ALLOW_TOP_LEVEL_AWAIT`` was clashing with ``CO_FUTURE_DIVISION``.
+ (Contributed by Batuhan Taskaya in :issue:`39562`)
++
++Notable changes in Python 3.8.8
++===============================
++
++Earlier Python versions allowed using both ``;`` and ``&`` as
++query parameter separators in :func:`urllib.parse.parse_qs` and
++:func:`urllib.parse.parse_qsl`. Due to security concerns, and to conform with
++newer W3C recommendations, this has been changed to allow only a single
++separator key, with ``&`` as the default. This change also affects
++:func:`cgi.parse` and :func:`cgi.parse_multipart` as they use the affected
++functions internally. For more details, please see their respective
++documentation.
++(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)
+diff --git a/Lib/cgi.py b/Lib/cgi.py
+index 77ab703cc0360..1e880e51848af 100755
+--- a/Lib/cgi.py
++++ b/Lib/cgi.py
+@@ -115,7 +115,8 @@ def closelog():
+ # 0 ==> unlimited input
+ maxlen = 0
+
+-def parse(fp=None, environ=os.environ, keep_blank_values=0, strict_parsing=0):
++def parse(fp=None, environ=os.environ, keep_blank_values=0,
++ strict_parsing=0, separator='&'):
+ """Parse a query in the environment or from a file (default stdin)
+
+ Arguments, all optional:
+@@ -134,6 +135,9 @@ def parse(fp=None, environ=os.environ, keep_blank_values=0, strict_parsing=0):
+ strict_parsing: flag indicating what to do with parsing errors.
+ If false (the default), errors are silently ignored.
+ If true, errors raise a ValueError exception.
++
++ separator: str. The symbol to use for separating the query arguments.
++ Defaults to &.
+ """
+ if fp is None:
+ fp = sys.stdin
+@@ -154,7 +158,7 @@ def parse(fp=None, environ=os.environ, keep_blank_values=0, strict_parsing=0):
+ if environ['REQUEST_METHOD'] == 'POST':
+ ctype, pdict = parse_header(environ['CONTENT_TYPE'])
+ if ctype == 'multipart/form-data':
+- return parse_multipart(fp, pdict)
++ return parse_multipart(fp, pdict, separator=separator)
+ elif ctype == 'application/x-www-form-urlencoded':
+ clength = int(environ['CONTENT_LENGTH'])
+ if maxlen and clength > maxlen:
+@@ -178,10 +182,10 @@ def parse(fp=None, environ=os.environ, keep_blank_values=0, strict_parsing=0):
+ qs = ""
+ environ['QUERY_STRING'] = qs # XXX Shouldn't, really
+ return urllib.parse.parse_qs(qs, keep_blank_values, strict_parsing,
+- encoding=encoding)
++ encoding=encoding, separator=separator)
+
+
+-def parse_multipart(fp, pdict, encoding="utf-8", errors="replace"):
++def parse_multipart(fp, pdict, encoding="utf-8", errors="replace", separator='&'):
+ """Parse multipart input.
+
+ Arguments:
+@@ -205,7 +209,7 @@ def parse_multipart(fp, pdict, encoding="utf-8", errors="replace"):
+ except KeyError:
+ pass
+ fs = FieldStorage(fp, headers=headers, encoding=encoding, errors=errors,
+- environ={'REQUEST_METHOD': 'POST'})
++ environ={'REQUEST_METHOD': 'POST'}, separator=separator)
+ return {k: fs.getlist(k) for k in fs}
+
+ def _parseparam(s):
+@@ -315,7 +319,7 @@ class FieldStorage:
+ def __init__(self, fp=None, headers=None, outerboundary=b'',
+ environ=os.environ, keep_blank_values=0, strict_parsing=0,
+ limit=None, encoding='utf-8', errors='replace',
+- max_num_fields=None):
++ max_num_fields=None, separator='&'):
+ """Constructor. Read multipart/* until last part.
+
+ Arguments, all optional:
+@@ -363,6 +367,7 @@ def __init__(self, fp=None, headers=None, outerboundary=b'',
+ self.keep_blank_values = keep_blank_values
+ self.strict_parsing = strict_parsing
+ self.max_num_fields = max_num_fields
++ self.separator = separator
+ if 'REQUEST_METHOD' in environ:
+ method = environ['REQUEST_METHOD'].upper()
+ self.qs_on_post = None
+@@ -589,7 +594,7 @@ def read_urlencoded(self):
+ query = urllib.parse.parse_qsl(
+ qs, self.keep_blank_values, self.strict_parsing,
+ encoding=self.encoding, errors=self.errors,
+- max_num_fields=self.max_num_fields)
++ max_num_fields=self.max_num_fields, separator=self.separator)
+ self.list = [MiniFieldStorage(key, value) for key, value in query]
+ self.skip_lines()
+
+@@ -605,7 +610,7 @@ def read_multi(self, environ, keep_blank_values, strict_parsing):
+ query = urllib.parse.parse_qsl(
+ self.qs_on_post, self.keep_blank_values, self.strict_parsing,
+ encoding=self.encoding, errors=self.errors,
+- max_num_fields=self.max_num_fields)
++ max_num_fields=self.max_num_fields, separator=self.separator)
+ self.list.extend(MiniFieldStorage(key, value) for key, value in query)
+
+ klass = self.FieldStorageClass or self.__class__
+@@ -649,7 +654,7 @@ def read_multi(self, environ, keep_blank_values, strict_parsing):
+ else self.limit - self.bytes_read
+ part = klass(self.fp, headers, ib, environ, keep_blank_values,
+ strict_parsing, limit,
+- self.encoding, self.errors, max_num_fields)
++ self.encoding, self.errors, max_num_fields, self.separator)
+
+ if max_num_fields is not None:
+ max_num_fields -= 1
+diff --git a/Lib/test/test_cgi.py b/Lib/test/test_cgi.py
+index 101942de947fb..4e1506a6468b9 100644
+--- a/Lib/test/test_cgi.py
++++ b/Lib/test/test_cgi.py
+@@ -53,12 +53,9 @@ def do_test(buf, method):
+ ("", ValueError("bad query field: ''")),
+ ("&", ValueError("bad query field: ''")),
+ ("&&", ValueError("bad query field: ''")),
+- (";", ValueError("bad query field: ''")),
+- (";&;", ValueError("bad query field: ''")),
+ # Should the next few really be valid?
+ ("=", {}),
+ ("=&=", {}),
+- ("=;=", {}),
+ # This rest seem to make sense
+ ("=a", {'': ['a']}),
+ ("&=a", ValueError("bad query field: ''")),
+@@ -73,8 +70,6 @@ def do_test(buf, method):
+ ("a=a+b&b=b+c", {'a': ['a b'], 'b': ['b c']}),
+ ("a=a+b&a=b+a", {'a': ['a b', 'b a']}),
+ ("x=1&y=2.0&z=2-3.%2b0", {'x': ['1'], 'y': ['2.0'], 'z': ['2-3.+0']}),
+- ("x=1;y=2.0&z=2-3.%2b0", {'x': ['1'], 'y': ['2.0'], 'z': ['2-3.+0']}),
+- ("x=1;y=2.0;z=2-3.%2b0", {'x': ['1'], 'y': ['2.0'], 'z': ['2-3.+0']}),
+ ("Hbc5161168c542333633315dee1182227:key_store_seqid=400006&cuyer=r&view=bustomer&order_id=0bb2e248638833d48cb7fed300000f1b&expire=964546263&lobale=en-US&kid=130003.300038&ss=env",
+ {'Hbc5161168c542333633315dee1182227:key_store_seqid': ['400006'],
+ 'cuyer': ['r'],
+@@ -201,6 +196,30 @@ def test_strict(self):
+ else:
+ self.assertEqual(fs.getvalue(key), expect_val[0])
+
++ def test_separator(self):
++ parse_semicolon = [
++ ("x=1;y=2.0", {'x': ['1'], 'y': ['2.0']}),
++ ("x=1;y=2.0;z=2-3.%2b0", {'x': ['1'], 'y': ['2.0'], 'z': ['2-3.+0']}),
++ (";", ValueError("bad query field: ''")),
++ (";;", ValueError("bad query field: ''")),
++ ("=;a", ValueError("bad query field: 'a'")),
++ (";b=a", ValueError("bad query field: ''")),
++ ("b;=a", ValueError("bad query field: 'b'")),
++ ("a=a+b;b=b+c", {'a': ['a b'], 'b': ['b c']}),
++ ("a=a+b;a=b+a", {'a': ['a b', 'b a']}),
++ ]
++ for orig, expect in parse_semicolon:
++ env = {'QUERY_STRING': orig}
++ fs = cgi.FieldStorage(separator=';', environ=env)
++ if isinstance(expect, dict):
++ for key in expect.keys():
++ expect_val = expect[key]
++ self.assertIn(key, fs)
++ if len(expect_val) > 1:
++ self.assertEqual(fs.getvalue(key), expect_val)
++ else:
++ self.assertEqual(fs.getvalue(key), expect_val[0])
++
+ def test_log(self):
+ cgi.log("Testing")
+
+diff --git a/Lib/test/test_urlparse.py b/Lib/test/test_urlparse.py
+index 4ae6ed33858ce..90c8d6922629e 100644
+--- a/Lib/test/test_urlparse.py
++++ b/Lib/test/test_urlparse.py
+@@ -32,16 +32,10 @@
+ (b"&a=b", [(b'a', b'b')]),
+ (b"a=a+b&b=b+c", [(b'a', b'a b'), (b'b', b'b c')]),
+ (b"a=1&a=2", [(b'a', b'1'), (b'a', b'2')]),
+- (";", []),
+- (";;", []),
+- (";a=b", [('a', 'b')]),
+- ("a=a+b;b=b+c", [('a', 'a b'), ('b', 'b c')]),
+- ("a=1;a=2", [('a', '1'), ('a', '2')]),
+- (b";", []),
+- (b";;", []),
+- (b";a=b", [(b'a', b'b')]),
+- (b"a=a+b;b=b+c", [(b'a', b'a b'), (b'b', b'b c')]),
+- (b"a=1;a=2", [(b'a', b'1'), (b'a', b'2')]),
++ (";a=b", [(';a', 'b')]),
++ ("a=a+b;b=b+c", [('a', 'a b;b=b c')]),
++ (b";a=b", [(b';a', b'b')]),
++ (b"a=a+b;b=b+c", [(b'a', b'a b;b=b c')]),
+ ]
+
+ # Each parse_qs testcase is a two-tuple that contains
+@@ -68,16 +62,10 @@
+ (b"&a=b", {b'a': [b'b']}),
+ (b"a=a+b&b=b+c", {b'a': [b'a b'], b'b': [b'b c']}),
+ (b"a=1&a=2", {b'a': [b'1', b'2']}),
+- (";", {}),
+- (";;", {}),
+- (";a=b", {'a': ['b']}),
+- ("a=a+b;b=b+c", {'a': ['a b'], 'b': ['b c']}),
+- ("a=1;a=2", {'a': ['1', '2']}),
+- (b";", {}),
+- (b";;", {}),
+- (b";a=b", {b'a': [b'b']}),
+- (b"a=a+b;b=b+c", {b'a': [b'a b'], b'b': [b'b c']}),
+- (b"a=1;a=2", {b'a': [b'1', b'2']}),
++ (";a=b", {';a': ['b']}),
++ ("a=a+b;b=b+c", {'a': ['a b;b=b c']}),
++ (b";a=b", {b';a': [b'b']}),
++ (b"a=a+b;b=b+c", {b'a':[ b'a b;b=b c']}),
+ ]
+
+ class UrlParseTestCase(unittest.TestCase):
+@@ -884,10 +872,46 @@ def test_parse_qsl_encoding(self):
+ def test_parse_qsl_max_num_fields(self):
+ with self.assertRaises(ValueError):
+ urllib.parse.parse_qs('&'.join(['a=a']*11), max_num_fields=10)
+- with self.assertRaises(ValueError):
+- urllib.parse.parse_qs(';'.join(['a=a']*11), max_num_fields=10)
+ urllib.parse.parse_qs('&'.join(['a=a']*10), max_num_fields=10)
+
++ def test_parse_qs_separator(self):
++ parse_qs_semicolon_cases = [
++ (";", {}),
++ (";;", {}),
++ (";a=b", {'a': ['b']}),
++ ("a=a+b;b=b+c", {'a': ['a b'], 'b': ['b c']}),
++ ("a=1;a=2", {'a': ['1', '2']}),
++ (b";", {}),
++ (b";;", {}),
++ (b";a=b", {b'a': [b'b']}),
++ (b"a=a+b;b=b+c", {b'a': [b'a b'], b'b': [b'b c']}),
++ (b"a=1;a=2", {b'a': [b'1', b'2']}),
++ ]
++ for orig, expect in parse_qs_semicolon_cases:
++ with self.subTest(f"Original: {orig!r}, Expected: {expect!r}"):
++ result = urllib.parse.parse_qs(orig, separator=';')
++ self.assertEqual(result, expect, "Error parsing %r" % orig)
++
++
++ def test_parse_qsl_separator(self):
++ parse_qsl_semicolon_cases = [
++ (";", []),
++ (";;", []),
++ (";a=b", [('a', 'b')]),
++ ("a=a+b;b=b+c", [('a', 'a b'), ('b', 'b c')]),
++ ("a=1;a=2", [('a', '1'), ('a', '2')]),
++ (b";", []),
++ (b";;", []),
++ (b";a=b", [(b'a', b'b')]),
++ (b"a=a+b;b=b+c", [(b'a', b'a b'), (b'b', b'b c')]),
++ (b"a=1;a=2", [(b'a', b'1'), (b'a', b'2')]),
++ ]
++ for orig, expect in parse_qsl_semicolon_cases:
++ with self.subTest(f"Original: {orig!r}, Expected: {expect!r}"):
++ result = urllib.parse.parse_qsl(orig, separator=';')
++ self.assertEqual(result, expect, "Error parsing %r" % orig)
++
++
+ def test_urlencode_sequences(self):
+ # Other tests incidentally urlencode things; test non-covered cases:
+ # Sequence and object values.
+diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
+index 95be7181133b4..0c1c94f5fc986 100644
+--- a/Lib/urllib/parse.py
++++ b/Lib/urllib/parse.py
+@@ -650,7 +650,7 @@ def unquote(string, encoding='utf-8', errors='replace'):
+
+
+ def parse_qs(qs, keep_blank_values=False, strict_parsing=False,
+- encoding='utf-8', errors='replace', max_num_fields=None):
++ encoding='utf-8', errors='replace', max_num_fields=None, separator='&'):
+ """Parse a query given as a string argument.
+
+ Arguments:
+@@ -674,12 +674,15 @@ def parse_qs(qs, keep_blank_values=False, strict_parsing=False,
+ max_num_fields: int. If set, then throws a ValueError if there
+ are more than n fields read by parse_qsl().
+
++ separator: str. The symbol to use for separating the query arguments.
++ Defaults to &.
++
+ Returns a dictionary.
+ """
+ parsed_result = {}
+ pairs = parse_qsl(qs, keep_blank_values, strict_parsing,
+ encoding=encoding, errors=errors,
+- max_num_fields=max_num_fields)
++ max_num_fields=max_num_fields, separator=separator)
+ for name, value in pairs:
+ if name in parsed_result:
+ parsed_result[name].append(value)
+@@ -689,7 +692,7 @@ def parse_qs(qs, keep_blank_values=False, strict_parsing=False,
+
+
+ def parse_qsl(qs, keep_blank_values=False, strict_parsing=False,
+- encoding='utf-8', errors='replace', max_num_fields=None):
++ encoding='utf-8', errors='replace', max_num_fields=None, separator='&'):
+ """Parse a query given as a string argument.
+
+ Arguments:
+@@ -712,19 +715,25 @@ def parse_qsl(qs, keep_blank_values=False, strict_parsing=False,
+ max_num_fields: int. If set, then throws a ValueError
+ if there are more than n fields read by parse_qsl().
+
++ separator: str. The symbol to use for separating the query arguments.
++ Defaults to &.
++
+ Returns a list, as G-d intended.
+ """
+ qs, _coerce_result = _coerce_args(qs)
+
++ if not separator or (not isinstance(separator, (str, bytes))):
++ raise ValueError("Separator must be of type string or bytes.")
++
+ # If max_num_fields is defined then check that the number of fields
+ # is less than max_num_fields. This prevents a memory exhaustion DOS
+ # attack via post bodies with many fields.
+ if max_num_fields is not None:
+- num_fields = 1 + qs.count('&') + qs.count(';')
++ num_fields = 1 + qs.count(separator)
+ if max_num_fields < num_fields:
+ raise ValueError('Max number of fields exceeded')
+
+- pairs = [s2 for s1 in qs.split('&') for s2 in s1.split(';')]
++ pairs = [s1 for s1 in qs.split(separator)]
+ r = []
+ for name_value in pairs:
+ if not name_value and not strict_parsing:
+diff --git a/Misc/NEWS.d/next/Security/2021-02-14-15-59-16.bpo-42967.YApqDS.rst b/Misc/NEWS.d/next/Security/2021-02-14-15-59-16.bpo-42967.YApqDS.rst
+new file mode 100644
+index 0000000000000..f08489b41494e
+--- /dev/null
++++ b/Misc/NEWS.d/next/Security/2021-02-14-15-59-16.bpo-42967.YApqDS.rst
+@@ -0,0 +1 @@
++Fix web cache poisoning vulnerability by defaulting the query args separator to ``&``, and allowing the user to choose a custom separator.
