diff --git a/Lib/cgi.py b/Lib/cgi.py
index 6018c3608697af..4e227aa05bd554 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,
+          separators=('&', ';')):
     """Parse a query in the environment or from a file (default stdin)
 
         Arguments, all optional:
@@ -134,6 +135,8 @@ 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.
+
+        separators: valid separators in the query string, e.g. ('&',)
     """
     if fp is None:
         fp = sys.stdin
@@ -154,7 +157,7 @@ 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, separators=separators)
         elif ctype == 'application/x-www-form-urlencoded':
             clength = int(environ['CONTENT_LENGTH'])
             if maxlen and clength > maxlen:
@@ -178,10 +181,11 @@ 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, separators=separators)
 
 
-def parse_multipart(fp, pdict, encoding="utf-8", errors="replace"):
+def parse_multipart(fp, pdict, encoding="utf-8", errors="replace",
+                    separators=('&', ';')):
     """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'}, separators=separators)
     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, separators=('&', ';')):
         """Constructor.  Read multipart/* until last part.
 
         Arguments, all optional:
@@ -358,11 +362,15 @@ def __init__(self, fp=None, headers=None, outerboundary=b'',
         max_num_fields: int. If set, then __init__ throws a ValueError
             if there are more than n fields read by parse_qsl().
 
+        separators: valid separators in the query string, e.g. ('&',).
+            Passed directly to urllib.parse.parse_qsl internally.
+
         """
         method = 'GET'
         self.keep_blank_values = keep_blank_values
         self.strict_parsing = strict_parsing
         self.max_num_fields = max_num_fields
+        self.separators = separators
         if 'REQUEST_METHOD' in environ:
             method = environ['REQUEST_METHOD'].upper()
         self.qs_on_post = None
@@ -472,7 +480,7 @@ def __init__(self, fp=None, headers=None, outerboundary=b'',
         if ctype == 'application/x-www-form-urlencoded':
             self.read_urlencoded()
         elif ctype[:10] == 'multipart/':
-            self.read_multi(environ, keep_blank_values, strict_parsing)
+            self.read_multi(environ, keep_blank_values, strict_parsing, separators)
         else:
             self.read_single()
 
@@ -589,13 +597,15 @@ 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,
+            separators=self.separators)
         self.list = [MiniFieldStorage(key, value) for key, value in query]
         self.skip_lines()
 
     FieldStorageClass = None
 
-    def read_multi(self, environ, keep_blank_values, strict_parsing):
+    def read_multi(self, environ, keep_blank_values, strict_parsing,
+                   separators=('&', ';')):
         """Internal: read a part that is itself multipart."""
         ib = self.innerboundary
         if not valid_boundary(ib):
@@ -605,7 +615,8 @@ 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,
+                separators=self.separators)
             self.list.extend(MiniFieldStorage(key, value) for key, value in query)
 
         klass = self.FieldStorageClass or self.__class__
@@ -649,7 +660,8 @@ 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,
+                         separators)
 
             if max_num_fields is not None:
                 max_num_fields -= 1
diff --git a/Lib/test/test_urlparse.py b/Lib/test/test_urlparse.py
index 762500789f73ac..9c3ab771714a06 100644
--- a/Lib/test/test_urlparse.py
+++ b/Lib/test/test_urlparse.py
@@ -11,7 +11,33 @@
 # Each parse_qsl testcase is a two-tuple that contains
 # a string with the query and a list with the expected result.
 
-parse_qsl_test_cases = [
+parse_qsl_test_cases_allow_semicolon = [
+    (";", []),
+    (";;", []),
+    (";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')]),
+]
+
+parse_qsl_test_cases_deny_semicolon = [
+    (";", [(';', '')]),
+    (";;", [(';;', '')]),
+    (";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'')]),
+    (b";;", [(b';;', b'')]),
+    (b";a=b", [(b';a', b'b')]),
+    (b"a=a+b;b=b+c", [(b'a', b'a b;b=b c')]),
+    (b"a=1;a=2", [(b'a', b'1;a=2')]),
+]
+
+parse_qsl_test_cases_ampersand = [
     ("", []),
     ("&", []),
     ("&&", []),
@@ -32,22 +58,43 @@
     (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')]),
 ]
 
+parse_qsl_test_cases = (parse_qsl_test_cases_ampersand
+                        + parse_qsl_test_cases_allow_semicolon)
+
+parse_qsl_test_cases_semicolon = (parse_qsl_test_cases_ampersand
+                                  + parse_qsl_test_cases_deny_semicolon)
+
 # Each parse_qs testcase is a two-tuple that contains
 # a string with the query and a dictionary with the expected result.
+parse_qs_test_cases_allow_semicolon = [
+    (";", {}),
+    (";;", {}),
+    (";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']}),
+]
 
-parse_qs_test_cases = [
+parse_qs_test_cases_deny_semicolon = [
+    (";", {';': ['']}),
+    (";;", {';;': ['']}),
+    (";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'']}),
+    (b";;", {b';;': [b'']}),
+    (b";a=b", {b';a': [b'b']}),
+    (b"a=a+b;b=b+c", {b'a': [b'a b;b=b c']}),
+    (b"a=1;a=2", {b'a': [b'1;a=2']}),
+]
+
+parse_qs_test_cases_ampersand = [
     ("", {}),
     ("&", {}),
     ("&&", {}),
@@ -68,18 +115,15 @@
     (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']}),
 ]
 
+parse_qs_test_cases = (parse_qs_test_cases_ampersand
+                       + parse_qs_test_cases_allow_semicolon)
+
+parse_qs_test_cases_semicolon = (parse_qs_test_cases_ampersand
+                                 + parse_qs_test_cases_deny_semicolon)
+
+
 class UrlParseTestCase(unittest.TestCase):
 
     def checkRoundtrips(self, url, parsed, split):
@@ -135,23 +179,55 @@ def checkRoundtrips(self, url, parsed, split):
 
     def test_qsl(self):
         for orig, expect in parse_qsl_test_cases:
-            result = urllib.parse.parse_qsl(orig, keep_blank_values=True)
+            result = urllib.parse.parse_qsl(orig, keep_blank_values=True,
+                                            separators=('&', ';'))
             self.assertEqual(result, expect, "Error parsing %r" % orig)
             expect_without_blanks = [v for v in expect if len(v[1])]
-            result = urllib.parse.parse_qsl(orig, keep_blank_values=False)
+            result = urllib.parse.parse_qsl(orig, keep_blank_values=False,
+                                            separators=('&', ';'))
             self.assertEqual(result, expect_without_blanks,
                             "Error parsing %r" % orig)
 
     def test_qs(self):
         for orig, expect in parse_qs_test_cases:
-            result = urllib.parse.parse_qs(orig, keep_blank_values=True)
+            result = urllib.parse.parse_qs(orig, keep_blank_values=True,
+                                           separators=('&', ';'))
             self.assertEqual(result, expect, "Error parsing %r" % orig)
             expect_without_blanks = {v: expect[v]
                                      for v in expect if len(expect[v][0])}
-            result = urllib.parse.parse_qs(orig, keep_blank_values=False)
+            result = urllib.parse.parse_qs(orig, keep_blank_values=False,
+                                           separators=('&', ';'))
             self.assertEqual(result, expect_without_blanks,
                             "Error parsing %r" % orig)
 
+    def test_qsl_no_semicolon(self):
+        # See bpo-42967 for more information.
+        for orig, expect in parse_qsl_test_cases_semicolon:
+            with self.subTest(f"Original: {orig!r}, Expected: {expect!r}"):
+                result = urllib.parse.parse_qsl(orig, keep_blank_values=True,
+                                                separators=('&',))
+                self.assertEqual(result, expect,
+                                "Error parsing %r" % orig)
+                expect_without_blanks = [v for v in expect if len(v[1])]
+                result = urllib.parse.parse_qsl(orig, keep_blank_values=False,
+                                                separators=('&',))
+                self.assertEqual(result, expect_without_blanks,
+                                 "Error parsing %r" % orig)
+
+    def test_qs_no_semicolon(self):
+        # See bpo-42967 for more information.
+        for orig, expect in parse_qs_test_cases_semicolon:
+            with self.subTest(f"Original: {orig!r}, Expected: {expect!r}"):
+                result = urllib.parse.parse_qs(orig, keep_blank_values=True,
+                                               separators=('&',))
+                self.assertEqual(result, expect, "Error parsing %r" % orig)
+                expect_without_blanks = {v: expect[v]
+                                         for v in expect if len(expect[v][0])}
+                result = urllib.parse.parse_qs(orig, keep_blank_values=False,
+                                               separators=('&',))
+                self.assertEqual(result, expect_without_blanks,
+                             "Error parsing %r" % orig)
+
     def test_roundtrips(self):
         str_cases = [
             ('file:///tmp/junk.txt',
@@ -887,7 +963,8 @@ 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']*11), max_num_fields=10,
+                                  separators=('&', ';'))
         urllib.parse.parse_qs('&'.join(['a=a']*10), max_num_fields=10)
 
     def test_urlencode_sequences(self):
diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
index ea897c3032257b..745312cbef1a33 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -662,7 +662,8 @@ 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,
+             separators=('&', ';')):
     """Parse a query given as a string argument.
 
         Arguments:
@@ -686,12 +687,15 @@ 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().
 
+        separators: valid separators in the query string, e.g. ('&',)
+
         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,
+                      separators=separators)
     for name, value in pairs:
         if name in parsed_result:
             parsed_result[name].append(value)
@@ -701,7 +705,8 @@ 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,
+              separators=('&', ';')):
     """Parse a query given as a string argument.
 
         Arguments:
@@ -724,6 +729,8 @@ 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().
 
+        separators: valid separators in the query string, e.g. ('&',)
+
         Returns a list, as G-d intended.
     """
     qs, _coerce_result = _coerce_args(qs)
@@ -732,11 +739,16 @@ def parse_qsl(qs, keep_blank_values=False, strict_parsing=False,
     # 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 + sum(qs.count(sep) for sep in separators)
         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(';')]
+    if not separators:
+        raise ValueError('Separators must be a tuple or list of strings')
+    for sep in separators[1:]:
+        qs = qs.replace(sep, separators[0])
+    pairs = qs.split(separators[0])
+
     r = []
     for name_value in pairs:
         if not name_value and not strict_parsing:
diff --git a/Misc/ACKS b/Misc/ACKS
index 136266965a869b..41d65a6f7712a9 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -613,6 +613,7 @@ Karan Goel
 Jeroen Van Goey
 Christoph Gohlke
 Tim Golden
+Adam Goldschmidt
 Yonatan Goldschmidt
 Mark Gollahon
 Mikhail Golubev