diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
index f396f4f9b32a9a8..cb07d05943f6101 100644
--- a/Lib/email/_header_value_parser.py
+++ b/Lib/email/_header_value_parser.py
@@ -3199,14 +3199,23 @@ def _fold_mime_parameters(part, lines, maxlen, encoding):
                 # have that, we'd be stuck, so in that case fall back to
                 # the RFC standard width.
                 maxlen = 78
-            splitpoint = maxchars = maxlen - chrome_len - 2
-            while True:
+            maxchars = maxlen - chrome_len - 2
+            splitpoint = maxchars = max(1, maxchars)
+            while splitpoint > 1:
                 partial = value[:splitpoint]
                 encoded_value = urllib.parse.quote(
                     partial, safe='', errors=error_handler)
                 if len(encoded_value) <= maxchars:
                     break
                 splitpoint -= 1
+            else:
+                # Even a single (encoded) character does not fit in the
+                # remaining room.  Write it anyway, exceeding maxlen, so that
+                # the value always gets shorter and the loop terminates.
+                splitpoint = 1
+                partial = value[:splitpoint]
+                encoded_value = urllib.parse.quote(
+                    partial, safe='', errors=error_handler)
             lines.append(" {}*{}*={}{}".format(
                 name, section, extra_chrome, encoded_value))
             extra_chrome = ''
diff --git a/Lib/test/test_email/test_message.py b/Lib/test/test_email/test_message.py
index 974b0ceaafbde4e..5c9bdadfc7c37da 100644
--- a/Lib/test/test_email/test_message.py
+++ b/Lib/test/test_email/test_message.py
@@ -1132,6 +1132,45 @@ def test_string_payload_with_multipart_content_type(self):
         attachments = msg.iter_attachments()
         self.assertEqual(list(attachments), [])
 
+    def test_fold_mime_parameters_long_name(self):
+        # gh-138223: _fold_mime_parameters() used to loop forever when the
+        # parameter name left no room for even one (encoded) character of
+        # the value.  How long the name must be depends on the value.
+        name = 'x' * 62
+        m = self._make_message()
+        m.set_content('hi')
+        m.set_param(name, '\U0001F600')
+        self.assertEqual(
+            m['Content-Type'].fold(policy=m.policy),
+            'Content-Type: text/plain; charset="utf-8";\n'
+            f" {name}*0*=utf-8''%F0%9F%98%80\n")
+
+        name = 'x' * 66
+        m = self._make_message()
+        m.set_content('hi')
+        m.set_param(name, 'abcdefgh')
+        self.assertEqual(
+            m['Content-Type'].fold(policy=m.policy),
+            'Content-Type: text/plain; charset="utf-8";\n'
+            f" {name}*0*=us-ascii''a;\n"
+            f" {name}*1*=bcdefg;\n"
+            f" {name}*2*=h\n")
+
+    def test_fold_mime_parameters_long_name_round_trip(self):
+        # gh-138223
+        values = ['abcdefgh', 'abc def 123', 'b' * 100,
+                  '\xe9', '\u65e5', '\U0001F600',
+                  '\u65e5' * 20, '\U0001F600' * 20]
+        for name_len in (40, 60, 61, 62, 63, 64, 66, 70, 80, 100, 120):
+            for value in values:
+                with self.subTest(name_len=name_len, value=value):
+                    name = 'x' * name_len
+                    m = self._make_message()
+                    m.set_content('hi')
+                    m.set_param(name, value)
+                    m2 = message_from_string(
+                        m.as_string(), policy=policy.default)
+                    self.assertEqual(m2['Content-Type'].params[name], value)
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Security/2025-08-29-14-04-17.gh-issue-138223.gX_GJv.rst b/Misc/NEWS.d/next/Security/2025-08-29-14-04-17.gh-issue-138223.gX_GJv.rst
new file mode 100644
index 000000000000000..7e3ab4f90e54cf3
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2025-08-29-14-04-17.gh-issue-138223.gX_GJv.rst
@@ -0,0 +1,6 @@
+:mod:`email.message`: fix an infinite loop in :meth:`EmailMessage.as_string
+<email.message.EmailMessage.as_string>` when :meth:`~email.message.EmailMessage.add_attachment`
+is called with very long parameter keys.
+
+This could cause denial of service by hanging the application indefinitely
+during MIME parameter folding and :rfc:`2231` encoding.