Redact denylisted fields before a log line is rendered

A processor in the shared chain masks sensitive field names (credential
fragments like password/token/authorization, plus exact names such as
card_number and holder_name) at any depth, for structlog and stdlib
records alike. It sees field names only: values interpolated into the
event string still leak until call sites pass them as fields.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

common/logging_config.py

@@ -36,6 +36,41 @@ def _add_trace_context(logger, method_name, event_dict):
     return event_dict
 
 
+# Field names whose values never reach a log line. Keys are compared
+# lowercased with "-" folded to "_", so header names (X-Api-Key) match too.
+# Fragments match anywhere in the key (db_password, refresh_token); the exact
+# names are too short or too specific to match as fragments ("pan" would
+# catch "company").
+_REDACT_FRAGMENTS = ("password", "passwd", "secret", "token", "authorization", "cookie", "api_key")
+_REDACT_EXACT = {"pan", "card_number", "cvv", "cvc", "holder_name"}
+_REDACTED = "[REDACTED]"
+
+
+def _is_sensitive(key) -> bool:
+    k = str(key).lower().replace("-", "_")
+    return k in _REDACT_EXACT or any(f in k for f in _REDACT_FRAGMENTS)
+
+
+def _scrub(value):
+    if isinstance(value, dict):
+        return {k: _REDACTED if _is_sensitive(k) else _scrub(v) for k, v in value.items()}
+    if isinstance(value, list):
+        return [_scrub(v) for v in value]
+    if isinstance(value, tuple):
+        # Stays a tuple: format_exc_info only trusts an exc_info that is one.
+        return tuple(_scrub(v) for v in value)
+    return value
+
+
+def _redact(logger, method_name, event_dict):
+    """Mask denylisted fields, at any depth (a logged headers dict included).
+
+    This works on field names only, so it is only as good as the call sites:
+    a value interpolated into the `event` string is invisible to it.
+    """
+    return _scrub(event_dict)
+
+
 def configure_logging(service_name: str) -> structlog.stdlib.BoundLogger:
     """One JSON object per line on stdout, for structlog and stdlib loggers alike.
 
@@ -50,6 +85,7 @@ def configure_logging(service_name: str) -> structlog.stdlib.BoundLogger:
         structlog.processors.TimeStamper(fmt="iso", utc=True),
         _add_service(service_name),
         _add_trace_context,
+        _redact,
     ]
 
     structlog.configure(