From 273e15fee950a71842eae45173c3fec480b41f54 Mon Sep 17 00:00:00 2001
From: Adam Goldschmidt <adamgold7@gmail.com>
Date: Mon, 15 Feb 2021 00:41:57 +0200
Subject: [PATCH 01/11] bpo-42967: only use '&' as a query string separator
 (#24297)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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)
---
 Doc/library/cgi.rst                           |  9 ++-
 Doc/library/urllib.parse.rst                  | 16 ++++-
 Doc/whatsnew/3.6.rst                          | 13 ++++
 Doc/whatsnew/3.7.rst                          | 13 ++++
 Doc/whatsnew/3.8.rst                          | 13 ++++
 Doc/whatsnew/3.9.rst                          | 13 ++++
 Lib/cgi.py                                    | 23 ++++---
 Lib/test/test_cgi.py                          | 29 ++++++--
 Lib/test/test_urlparse.py                     | 68 +++++++++++++------
 Lib/urllib/parse.py                           | 20 ++++--
 .../2021-02-14-15-59-16.bpo-42967.YApqDS.rst  |  1 +
 11 files changed, 172 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 4048592e7361f7..05d9cdf424073f 100644
--- a/Doc/library/cgi.rst
+++ b/Doc/library/cgi.rst
@@ -277,14 +277,14 @@ 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.
 
 
-.. 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 +303,9 @@ 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:: 3.10
+      Added the *separator* parameter.
+
 
 .. function:: parse_header(string)
 
diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
index 536cf952bda434..0e520371320f9d 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 +190,8 @@ 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 +203,12 @@ or on combining URL components into a URL string.
    .. versionchanged:: 3.8
       Added *max_num_fields* parameter.
 
+   .. versionchanged:: 3.10
+      Added *separator* parameter with the default value of `&`. Python versions earlier than Python 3.10 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 +232,8 @@ 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 +243,10 @@ or on combining URL components into a URL string.
    .. versionchanged:: 3.8
       Added *max_num_fields* parameter.
 
+   .. versionchanged:: 3.10
+      Added *separator* parameter with the default value of `&`. Python versions earlier than Python 3.10 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 85a6657fdfbdac..8a64da1b249d7d 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 25b1e1e33e325c..9204cc7fbf8c47 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 0b4820f3333e13..d21921d3dd51e7 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -2234,3 +2234,16 @@ 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 3.8.8
+===============================
+
+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`.)
\ No newline at end of file
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 68b1e504da89ef..5f4f8ba211b180 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -1516,3 +1516,16 @@ invalid forms of parameterizing :class:`collections.abc.Callable` which may have
 passed silently in Python 3.9.1.  This :exc:`DeprecationWarning` will
 become a :exc:`TypeError` in Python 3.10.
 (Contributed by Ken Jin in :issue:`42195`.)
+
+urllib.parse
+------------
+
+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 77ab703cc03600..1e880e51848af2 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 101942de947fb4..4e1506a6468b93 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 762500789f73ac..3b1c360625b5a6 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):
@@ -886,10 +874,46 @@ 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 ea897c3032257b..5bd067895bfa3d 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -662,7 +662,7 @@ 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:
@@ -686,12 +686,15 @@ 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)
@@ -701,7 +704,7 @@ 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:
@@ -724,19 +727,26 @@ 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)
+        and not isinstance(separator, 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 00000000000000..f08489b41494ea
--- /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.

From c920ee5a3e124ef08c13213491c4327006505f92 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <senthil@uthcode.com>
Date: Sun, 14 Feb 2021 17:36:37 -0800
Subject: [PATCH 02/11] [3.9] bpo-42967: only use '&' as a query string
 separator (GH-24297)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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>

From daef72bd6038a5dbff3eed84e1c817509be2535f Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <senthil@uthcode.com>
Date: Mon, 15 Feb 2021 05:10:42 -0800
Subject: [PATCH 03/11] Update version information.

---
 Doc/library/cgi.rst          |  2 +-
 Doc/library/urllib.parse.rst | 19 ++++++++++++-------
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/Doc/library/cgi.rst b/Doc/library/cgi.rst
index 05d9cdf424073f..e60a3f13595cf0 100644
--- a/Doc/library/cgi.rst
+++ b/Doc/library/cgi.rst
@@ -303,7 +303,7 @@ 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:: 3.10
+   .. versionchanged:: 3.9.2
       Added the *separator* parameter.
 
 
diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
index 0e520371320f9d..aa992619604dea 100644
--- a/Doc/library/urllib.parse.rst
+++ b/Doc/library/urllib.parse.rst
@@ -190,7 +190,8 @@ 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 `&`.
+   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
@@ -203,9 +204,11 @@ or on combining URL components into a URL string.
    .. versionchanged:: 3.8
       Added *max_num_fields* parameter.
 
-   .. versionchanged:: 3.10
-      Added *separator* parameter with the default value of `&`. Python versions earlier than Python 3.10 allowed using both ";" and "&" as
-      query parameter separator. This has been changed to allow only a single separator key, with "&" as the default separator.
+   .. versionchanged:: 3.9.2
+      Added *separator* parameter with the default value of `&`. Python
+      versions earlier than Python 3.9.2 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, separator='&')
@@ -243,9 +246,11 @@ or on combining URL components into a URL string.
    .. versionchanged:: 3.8
       Added *max_num_fields* parameter.
 
-   .. versionchanged:: 3.10
-      Added *separator* parameter with the default value of `&`. Python versions earlier than Python 3.10 allowed using both ";" and "&" as
-      query parameter separator. This has been changed to allow only a single separator key, with "&" as the default separator.
+   .. versionchanged:: 3.9.2
+      Added *separator* parameter with the default value of `&`. Python
+      versions earlier than Python 3.9.2 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)

From 95c9ce7aafe237516d3f475535ef990ab50317d6 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:04 -0800
Subject: [PATCH 04/11] Update Doc/library/urllib.parse.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/library/urllib.parse.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
index aa992619604dea..462412d2df1d91 100644
--- a/Doc/library/urllib.parse.rst
+++ b/Doc/library/urllib.parse.rst
@@ -191,7 +191,7 @@ or on combining URL components into a URL string.
    *max_num_fields* fields read.
 
    The optional argument *separator* is the symbol to use for separating the
-   query arguments. It defaults to `&`.
+   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

From 81ce22d0d9509b58219029aedce19f911384584d Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:15 -0800
Subject: [PATCH 05/11] Update Doc/library/urllib.parse.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/library/urllib.parse.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
index 462412d2df1d91..8be06252869d7b 100644
--- a/Doc/library/urllib.parse.rst
+++ b/Doc/library/urllib.parse.rst
@@ -205,10 +205,10 @@ or on combining URL components into a URL string.
       Added *max_num_fields* parameter.
 
    .. versionchanged:: 3.9.2
-      Added *separator* parameter with the default value of `&`. Python
-      versions earlier than Python 3.9.2 allowed using both ";" and "&" as
+      Added *separator* parameter with the default value of ``&``. Python
+      versions earlier than Python 3.9.2 allowed using both ``;`` and ``&`` as
       query parameter separator. This has been changed to allow only a single
-      separator key, with "&" as the default separator.
+      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, separator='&')

From 8d08c605af69f7b94516eb0d8e8d8fbca7d5a206 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:22 -0800
Subject: [PATCH 06/11] Update Doc/library/urllib.parse.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/library/urllib.parse.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
index 8be06252869d7b..0e2e992b94da83 100644
--- a/Doc/library/urllib.parse.rst
+++ b/Doc/library/urllib.parse.rst
@@ -235,7 +235,7 @@ 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 `&`.
+   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.

From 6b53c442ffb6bfc94156a545f7c5225e07545579 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:29 -0800
Subject: [PATCH 07/11] Update Doc/library/urllib.parse.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/library/urllib.parse.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst
index 0e2e992b94da83..38e2986334c807 100644
--- a/Doc/library/urllib.parse.rst
+++ b/Doc/library/urllib.parse.rst
@@ -247,10 +247,10 @@ or on combining URL components into a URL string.
       Added *max_num_fields* parameter.
 
    .. versionchanged:: 3.9.2
-      Added *separator* parameter with the default value of `&`. Python
-      versions earlier than Python 3.9.2 allowed using both ";" and "&" as
+      Added *separator* parameter with the default value of ``&``. Python
+      versions earlier than Python 3.9.2 allowed using both ``;`` and ``&`` as
       query parameter separator. This has been changed to allow only a single
-      separator key, with "&" as the default separator.
+      separator key, with ``&`` as the default separator.
 
 
 .. function:: urlunparse(parts)

From 10e4828075b6f986317274c2ea8af29b0c38f026 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:36 -0800
Subject: [PATCH 08/11] Update Doc/whatsnew/3.6.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/whatsnew/3.6.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Doc/whatsnew/3.6.rst b/Doc/whatsnew/3.6.rst
index 8a64da1b249d7d..03a877a3d91785 100644
--- a/Doc/whatsnew/3.6.rst
+++ b/Doc/whatsnew/3.6.rst
@@ -2447,11 +2447,11 @@ details, see the documentation for ``loop.create_datagram_endpoint()``.
 Notable changes in Python 3.6.13
 ================================
 
-Earlier Python versions allowed using both ";" and "&" as
+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
+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.

From 21a2822d3ad912ffe5088b7884cf66aadd25b566 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:43 -0800
Subject: [PATCH 09/11] Update Doc/whatsnew/3.8.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/whatsnew/3.8.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index d21921d3dd51e7..dbc3875aae61b3 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -2238,12 +2238,12 @@ details, see the documentation for ``loop.create_datagram_endpoint()``.
 Notable changes in Python 3.8.8
 ===============================
 
-Earlier Python versions allowed using both ";" and "&" as
+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
+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`.)
\ No newline at end of file
+(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in :issue:`42967`.)

From 23792f66825dc8a03c336fcf2dbef9eb1b4062df Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:05:50 -0800
Subject: [PATCH 10/11] Update Doc/whatsnew/3.9.rst

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Doc/whatsnew/3.9.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 5f4f8ba211b180..3086930569dc98 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -1520,11 +1520,11 @@ become a :exc:`TypeError` in Python 3.10.
 urllib.parse
 ------------
 
-Earlier Python versions allowed using both ";" and "&" as
+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
+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.

From eb39e4567cc67b8b6f70bb7510a8e96cd41ffa02 Mon Sep 17 00:00:00 2001
From: Senthil Kumaran <skumaran@gatech.edu>
Date: Mon, 15 Feb 2021 08:07:05 -0800
Subject: [PATCH 11/11] Update Lib/urllib/parse.py

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
---
 Lib/urllib/parse.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
index 5bd067895bfa3d..335e183498d8bd 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -734,8 +734,7 @@ def parse_qsl(qs, keep_blank_values=False, strict_parsing=False,
     """
     qs, _coerce_result = _coerce_args(qs)
 
-    if not separator or (not isinstance(separator, str)
-        and not isinstance(separator, bytes)):
+    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