python
@@ -65,6 +65,11 @@ of the new API.
    *email address* parts.  Returns a tuple of that information, unless the parse
    fails, in which case a 2-tuple of ``('', '')`` is returned.
 
+   .. versionchanged:: 3.12
+      For security reasons, addresses that were ambiguous and could parse into
+      multiple different addresses now cause ``('', '')`` to be returned
+      instead of only one of the *potential* addresses.
+
 
 .. function:: formataddr(pair, charset='utf-8')
 
@@ -87,7 +92,7 @@ of the new API.
    This method returns a list of 2-tuples of the form returned by ``parseaddr()``.
    *fieldvalues* is a sequence of header field values as might be returned by
    :meth:`Message.get_all <email.message.Message.get_all>`.  Here's a simple
-   example that gets all the recipients of a message::
+   example that gets all the recipients of a message:
 
       from email.utils import getaddresses
 
@@ -97,6 +102,25 @@ of the new API.
       resent_ccs = msg.get_all('resent-cc', [])
       all_recipients = getaddresses(tos + ccs + resent_tos + resent_ccs)
 
+   When parsing fails for a single fieldvalue, a 2-tuple of ``('', '')``
+   is returned in its place.  Other errors in parsing the list of
+   addresses such as a fieldvalue seemingly parsing into multiple
+   addresses may result in a list containing a single empty 2-tuple
+   ``[('', '')]`` being returned rather than returning potentially
+   invalid output.
+
+   Example malformed input parsing:
+
+   .. doctest::
+
+      >>> from email.utils import getaddresses
+      >>> getaddresses(['alice@example.com <bob@example.com>', 'me@example.com'])
+      [('', '')]
+
+   .. versionchanged:: 3.12
+      The 2-tuple of ``('', '')`` in the returned values when parsing
+      fails were added as to address a security issue.
+
 
 .. function:: parsedate(date)
 

@@ -564,6 +564,14 @@ dis
   :data:`~dis.hasarg` collection instead.
   (Contributed by Irit Katriel in :gh:`94216`.)
 
+email
+-----
+
+* :func:`email.utils.getaddresses` and :func:`email.utils.parseaddr` now return
+  ``('', '')`` 2-tuples in more situations where invalid email addresses are
+  encountered instead of potentially inaccurate values.
+  (Contributed by Thomas Dwyer for :gh:`102988` to ameliorate CVE-2023-27043.)
+
 fractions
 ---------
 

@@ -106,12 +106,54 @@ def formataddr(pair, charset='utf-8'):
     return address
 
 
+def _pre_parse_validation(email_header_fields):
+    accepted_values = []
+    for v in email_header_fields:
+        s = v.replace('\\(', '').replace('\\)', '')
+        if s.count('(') != s.count(')'):
+            v = "('', '')"
+        accepted_values.append(v)
+
+    return accepted_values
+
+
+def _post_parse_validation(parsed_email_header_tuples):
+    accepted_values = []
+    # The parser would have parsed a correctly formatted domain-literal
+    # The existence of an [ after parsing indicates a parsing failure
+    for v in parsed_email_header_tuples:
+        if '[' in v[1]:
+            v = ('', '')
+        accepted_values.append(v)
+
+    return accepted_values
+
 
 def getaddresses(fieldvalues):
-    """Return a list of (REALNAME, EMAIL) for each fieldvalue."""
-    all = COMMASPACE.join(str(v) for v in fieldvalues)
+    """Return a list of (REALNAME, EMAIL) or ('','') for each fieldvalue.
+
+    When parsing fails for a fieldvalue, a 2-tuple of ('', '') is returned in
+    its place.
+
+    If the resulting list of parsed address is not the same as the number of
+    fieldvalues in the input list a parsing error has occurred.  A list
+    containing a single empty 2-tuple [('', '')] is returned in its place.
+    This is done to avoid invalid output.
+    """
+    fieldvalues = [str(v) for v in fieldvalues]
+    fieldvalues = _pre_parse_validation(fieldvalues)
+    all = COMMASPACE.join(v for v in fieldvalues)
     a = _AddressList(all)
-    return a.addresslist
+    result = _post_parse_validation(a.addresslist)
+
+    n = 0
+    for v in fieldvalues:
+        n += v.count(',') + 1
+
+    if len(result) != n:
+        return [('', '')]
+
+    return result
 
 
 def _format_timetuple_and_zone(timetuple, zone):
@@ -212,9 +254,18 @@ def parseaddr(addr):
     Return a tuple of realname and email address, unless the parse fails, in
     which case return a 2-tuple of ('', '').
     """
-    addrs = _AddressList(addr).addresslist
-    if not addrs:
-        return '', ''
+    if isinstance(addr, list):
+        addr = addr[0]
+
+    if not isinstance(addr, str):
+        return ('', '')
+
+    addr = _pre_parse_validation([addr])[0]
+    addrs = _post_parse_validation(_AddressList(addr).addresslist)
+
+    if not addrs or len(addrs) > 1:
+        return ('', '')
+
     return addrs[0]
 
 

@@ -3319,15 +3319,90 @@ def test_getaddresses(self):
            [('Al Person', 'aperson@dom.ain'),
             ('Bud Person', 'bperson@dom.ain')])
 
+    def test_getaddresses_parsing_errors(self):
+        """Test for parsing errors from CVE-2023-27043"""
+        eq = self.assertEqual
+        eq(utils.getaddresses(['alice@example.org(<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org)<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org<<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org><bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org@<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org,<bob@example.com>']),
+           [('', 'alice@example.org'), ('', 'bob@example.com')])
+        eq(utils.getaddresses(['alice@example.org;<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org:<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org.<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org"<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org[<bob@example.com>']),
+           [('', '')])
+        eq(utils.getaddresses(['alice@example.org]<bob@example.com>']),
+           [('', '')])
+
+    def test_parseaddr_parsing_errors(self):
+        """Test for parsing errors from CVE-2023-27043"""
+        eq = self.assertEqual
+        eq(utils.parseaddr(['alice@example.org(<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org)<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org<<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org><bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org@<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org,<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org;<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org:<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org.<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org"<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org[<bob@example.com>']),
+           ('', ''))
+        eq(utils.parseaddr(['alice@example.org]<bob@example.com>']),
+           ('', ''))
+
     def test_getaddresses_nasty(self):
         eq = self.assertEqual
         eq(utils.getaddresses(['foo: ;']), [('', '')])
-        eq(utils.getaddresses(
-           ['[]*-- =~$']),
-           [('', ''), ('', ''), ('', '*--')])
+        eq(utils.getaddresses(['[]*-- =~$']), [('', '')])
         eq(utils.getaddresses(
            ['foo: ;', '"Jason R. Mastaler" <jason@dom.ain>']),
            [('', ''), ('Jason R. Mastaler', 'jason@dom.ain')])
+        eq(utils.getaddresses(
+           [r'Pete(A nice \) chap) <pete(his account)@silly.test(his host)>']),
+           [('Pete (A nice ) chap his account his host)', 'pete@silly.test')])
+        eq(utils.getaddresses(
+           ['(Empty list)(start)Undisclosed recipients  :(nobody(I know))']),
+           [('', '')])
+        eq(utils.getaddresses(
+           ['Mary <@machine.tld:mary@example.net>, , jdoe@test   . example']),
+           [('Mary', 'mary@example.net'), ('', ''), ('', 'jdoe@test.example')])
+        eq(utils.getaddresses(
+           ['John Doe <jdoe@machine(comment).  example>']),
+           [('John Doe (comment)', 'jdoe@machine.example')])
+        eq(utils.getaddresses(
+           ['"Mary Smith: Personal Account" <smith@home.example>']),
+           [('Mary Smith: Personal Account', 'smith@home.example')])
+        eq(utils.getaddresses(
+           ['Undisclosed recipients:;']),
+           [('', '')])
+        eq(utils.getaddresses(
+           [r'<boss@nil.test>, "Giant; \"Big\" Box" <bob@example.net>']),
+           [('', 'boss@nil.test'), ('Giant; "Big" Box', 'bob@example.net')])
 
     def test_getaddresses_embedded_comment(self):
         """Test proper handling of a nested comment"""

diff --git a/Misc/NEWS.d/next/Security/2023-06-13-20-52-24.gh-issue-102988.Kei7Vf.rst b/Misc/NEWS.d/next/Security/2023-06-13-20-52-24.gh-issue-102988.Kei7Vf.rst
@@ -0,0 +1,4 @@
+CVE-2023-27043: Prevent :func:`email.utils.parseaddr`
+and :func:`email.utils.getaddresses` from returning the realname portion of an
+invalid RFC2822 email header in the email address portion of the 2-tuple
+returned after being parsed by :class:`email._parseaddr.AddressList`.