[core] Keep legacy redaction regex as deprecation fallback

Restore the substring regex as a second pass in command_config so
sensitive-shaped fields that haven't been tagged with cv.sensitive(...)
yet are still redacted. Each unique unmarked field name caught by the
heuristic emits a one-time deprecation warning naming the field and the
fix; the fallback itself is slated for removal in 2026.12.0.

esphome/__main__.py

@@ -1412,12 +1412,47 @@ def command_config(args: ArgsProtocol, config: ConfigType) -> int | None:
     if not CORE.verbose:
         config = strip_default_ids(config)
     output = yaml_util.dump(config, args.show_secrets)
+    if not args.show_secrets:
+        output = _redact_with_legacy_fallback(output)
     if not CORE.quiet:
         safe_print(output)
     _LOGGER.info("Configuration is valid!")
     return 0
 
 
+# Legacy substring redaction fallback. Catches sensitive-looking fields that
+# aren't yet tagged with ``cv.sensitive(...)`` so config dumps don't regress
+# for unmigrated/third-party schemas. Each match also logs a one-time
+# deprecation warning naming the field; this fallback is slated for removal
+# in 2026.12.0 once canonical sensitive fields are migrated.
+#
+# The negative lookahead ``(?!\\033\[8m)`` skips values already wrapped by the
+# SensitiveStr representer, so explicit tagging silences the warning.
+_LEGACY_REDACTION_RE = re.compile(
+    r"(?P<key>\w*(?:password|key|psk|ssid)\w*)\: (?!\\033\[8m)(?P<val>.+)"
+)
+_LEGACY_REDACTION_REMOVAL = "2026.12.0"
+
+
+def _redact_with_legacy_fallback(output: str) -> str:
+    unmarked: set[str] = set()
+
+    def _replace(m: re.Match[str]) -> str:
+        unmarked.add(m.group("key"))
+        return f"{m.group('key')}: \\033[8m{m.group('val')}\\033[28m"
+
+    output = _LEGACY_REDACTION_RE.sub(_replace, output)
+    for key in sorted(unmarked):
+        _LOGGER.warning(
+            "Field '%s' is being redacted by a legacy substring heuristic. "
+            "Mark this field's schema validator with cv.sensitive(...) for "
+            "deterministic redaction; the heuristic will be removed in %s.",
+            key,
+            _LEGACY_REDACTION_REMOVAL,
+        )
+    return output
+
+
 def command_config_hash(args: ArgsProtocol, config: ConfigType) -> int | None:
     # generating code might modify config, so it must be done in order to generate
     # a hash that will match what was generated when compiling and then running

tests/unit_tests/test_main.py

@@ -22,6 +22,7 @@ from esphome.__main__ import (
     Purpose,
     _get_configured_xtal_freq,
     _make_crystal_freq_callback,
+    _redact_with_legacy_fallback,
     _resolve_network_devices,
     _validate_bootloader_binary,
     _validate_partition_table_binary,
@@ -340,6 +341,52 @@ def mock_ram_strings_analyzer() -> Generator[Mock]:
         yield mock_class
 
 
+def test_redact_with_legacy_fallback__wraps_unmarked_field(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """Unmarked sensitive-shaped fields are redacted; a deprecation warning
+    is emitted naming the field."""
+    with caplog.at_level(logging.WARNING, logger="esphome.__main__"):
+        out = _redact_with_legacy_fallback("password: hunter2\n")
+    assert "password: \\033[8mhunter2\\033[28m" in out
+    assert any(
+        "password" in rec.message and "cv.sensitive" in rec.message
+        for rec in caplog.records
+    )
+
+
+def test_redact_with_legacy_fallback__skips_already_wrapped(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """Values already wrapped by the SensitiveStr representer don't trigger
+    the heuristic or the warning."""
+    wrapped = "password: \\033[8mhunter2\\033[28m\n"
+    with caplog.at_level(logging.WARNING, logger="esphome.__main__"):
+        out = _redact_with_legacy_fallback(wrapped)
+    assert out == wrapped
+    assert not any("legacy substring" in rec.message for rec in caplog.records)
+
+
+def test_redact_with_legacy_fallback__captures_full_field_name(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """The warning names the actual field, not just the matched fragment."""
+    with caplog.at_level(logging.WARNING, logger="esphome.__main__"):
+        _redact_with_legacy_fallback("encryption_key: abc\n")
+    assert any("encryption_key" in rec.message for rec in caplog.records)
+
+
+def test_redact_with_legacy_fallback__deduplicates_warnings(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """One warning per unique field name even if it appears many times."""
+    text = "password: a\npassword: b\npassword: c\n"
+    with caplog.at_level(logging.WARNING, logger="esphome.__main__"):
+        _redact_with_legacy_fallback(text)
+    password_warnings = [rec for rec in caplog.records if "'password'" in rec.message]
+    assert len(password_warnings) == 1
+
+
 def test_choose_upload_log_host_with_string_default() -> None:
     """Test with a single string default device."""
     setup_core()