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>

Author: 4 people

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)
 

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)
 

Doc/whatsnew/3.10.rst

@@ -546,6 +546,19 @@ Add new method :meth:`~unittest.TestCase.assertNoLogs` to complement the
 existing :meth:`~unittest.TestCase.assertLogs`. (Contributed by Kit Yan Choi
 in :issue:`39385`.)
 
+urllib.parse
+------------
+
+Python versions earlier than Python 3.10 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`.)
+
 xml
 ---
 

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`.)

Doc/whatsnew/3.7.rst

@@ -2557,3 +2557,16 @@ 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`.)

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`.)

Doc/whatsnew/3.9.rst

@@ -1515,4 +1515,17 @@ need to account for this change.  A :exc:`DeprecationWarning` may be emitted for
 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`.)
+(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`.)

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

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")