gh-138223: Fix Infinite loop in email._header_value_parser._fold_mime_parameters when parameter names are too long

Lib/email/_header_value_parser.py

@@ -3080,14 +3080,25 @@ 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
+            # Ensure maxchars is at least 1 to prevent negative values
+            if maxchars <= 0:
+                maxchars = 1
+            splitpoint = maxchars
+            splitpoint = max(1, splitpoint)  # Ensure splitpoint is always at least 1
+            while splitpoint > 1:
                 partial = value[:splitpoint]
                 encoded_value = urllib.parse.quote(
                     partial, safe='', errors=error_handler)
                 if len(encoded_value) <= maxchars:
                     break
                 splitpoint -= 1
+            # If we still can't fit, force a minimal split
+            if splitpoint <= 1:
+                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 = ''

Lib/test/test_email/test_infinite_loop_fix.py

@@ -0,0 +1,150 @@
+"""Test for infinite loop fix in email MIME parameter folding.
+
+This test verifies that the fix for GitHub issue #138223 prevents infinite loops
+when processing MIME parameters with very long keys during RFC 2231 encoding.
+"""
+
+import unittest
+import email.message
+
+
+class TestInfiniteLoopFix(unittest.TestCase):
+    """Test that infinite loops are prevented in MIME parameter folding."""
+
+    def test_long_parameter_key_no_infinite_loop(self):
+        """Test that very long parameter keys don't cause infinite loops."""
+        msg = email.message.EmailMessage()
+
+        # Create a parameter key that's 64 characters long (the problematic length)
+        long_key = "a" * 64
+
+        # Add attachment first
+        msg.add_attachment(
+            b"test content",
+            maintype="text",
+            subtype="plain",
+            filename="test.txt"
+        )
+
+        # Set the long parameter using set_param
+        msg.set_param(long_key, "test_value")
+
+        # This should not hang - it should complete in reasonable time
+        try:
+            result = msg.as_string()
+            # If we get here, no infinite loop occurred
+            self.assertIsInstance(result, str)
+            self.assertIn(long_key, result)
+        except Exception as e:
+            self.fail(f"as_string() failed with exception: {e}")
+
+    def test_extremely_long_parameter_key(self):
+        """Test with extremely long parameter keys that could cause maxchars < 0."""
+        msg = email.message.EmailMessage()
+
+        # Create an extremely long parameter key (100+ characters)
+        extremely_long_key = "b" * 100
+
+        # Add attachment first
+        msg.add_attachment(
+            b"test content",
+            maintype="text",
+            subtype="plain",
+            filename="test.txt"
+        )
+
+        # Set the extremely long parameter
+        msg.set_param(extremely_long_key, "test_value")
+
+        # This should not hang even with extremely long keys
+        try:
+            result = msg.as_string()
+            self.assertIsInstance(result, str)
+            self.assertIn(extremely_long_key, result)
+        except Exception as e:
+            self.fail(f"as_string() failed with exception: {e}")
+
+    def test_multiple_long_parameters(self):
+        """Test with multiple long parameters to ensure robust handling."""
+        msg = email.message.EmailMessage()
+
+        # Add attachment first
+        msg.add_attachment(
+            b"test content",
+            maintype="text",
+            subtype="plain",
+            filename="test.txt"
+        )
+
+        # Add multiple long parameters
+        long_params = {
+            "a" * 64: "value1",
+            "b" * 80: "value2",
+            "c" * 90: "value3"
+        }
+
+        for key, value in long_params.items():
+            msg.set_param(key, value)
+
+        try:
+            result = msg.as_string()
+            self.assertIsInstance(result, str)
+            # Check that all parameters are present
+            for key in long_params:
+                self.assertIn(key, result)
+        except Exception as e:
+            self.fail(f"as_string() failed with exception: {e}")
+
+    def test_edge_case_parameter_lengths(self):
+        """Test edge cases around the problematic parameter lengths."""
+        # Test parameter keys of various lengths around the problematic 64-char mark
+        test_lengths = [60, 61, 62, 63, 64, 65, 66, 67, 68]
+
+        for length in test_lengths:
+            key = "x" * length
+            msg = email.message.EmailMessage()
+
+            # Add attachment first
+            msg.add_attachment(
+                b"test content",
+                maintype="text",
+                subtype="plain",
+                filename="test.txt"
+            )
+
+            # Set the parameter
+            msg.set_param(key, "test_value")
+
+            try:
+                result = msg.as_string()
+                self.assertIsInstance(result, str)
+                self.assertIn(key, result)
+            except Exception as e:
+                self.fail(f"as_string() failed with length {length}: {e}")
+
+    def test_rfc_2231_compliance(self):
+        """Test that the fix maintains RFC 2231 compliance."""
+        msg = email.message.EmailMessage()
+
+        # Add attachment first
+        msg.add_attachment(
+            b"test content",
+            maintype="text",
+            subtype="plain",
+            filename="test.txt"
+        )
+
+        # Set a parameter with special characters that should trigger RFC 2231 encoding
+        msg.set_param("long_param", "value_with_special_chars_ñáéíóú")
+
+        try:
+            result = msg.as_string()
+            self.assertIsInstance(result, str)
+            # The parameter should be properly encoded
+            self.assertIn("long_param", result)
+        except Exception as e:
+            self.fail(f"as_string() failed with exception: {e}")
+
+
+if __name__ == '__main__':
+    unittest.main()

Misc/NEWS.d/next/Library/2025-01-27-12-00-00.gh-issue-138223.email-infinite-loop.rst

@@ -0,0 +1 @@
+Fix infinite loop in email.message.EmailMessage.as_string() when add_attachment() is called with very long parameter keys.

Misc/NEWS.d/next/Security/2025-01-27-12-00-00.gh-issue-138223.email-infinite-loop.rst

@@ -0,0 +1 @@
+Fix infinite loop in email.message.EmailMessage.as_string() when 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.