The class that will handle filtering the log statements

LogMaskingConverter.groovy

@Plugin(name = 'logmask', category = 'Converter')
@ConverterKeys(['cm'])
class LogMaskingConverter extends LogEventPatternConverter {
    private static final String NAME = 'cm'
    private static final String JSON_REPLACEMENT_REGEX = "\"\$1\": \"****\""
    private static final String JSON_KEYS = ['ssn', 'private', 'creditCard'].join('|')
    private static final Pattern JSON_PATTERN = Pattern.compile(/"(${JSON_KEYS})": "([^"]+)"/)
    
    LogMaskingConverter(String[] options) {
        super(NAME, NAME)
    }
    
    static LogMaskingConverter newInstance(final String[] options) {
        return new LogMaskingConverter(options)
    }
    
    @Override
    void format(LogEvent event, StringBuilder outputMessage) {
        String message = event.message.formattedMessage
        String maskedMessage = message
        
        if (event.marker?.name == LoggingMarkers.JSON.name) {
            try {
                maskedMessage = mask(message)
                
            } catch (Exception e) {
                maskedMessage = message // Although if this fails, it may be better to not log the message
            }
        }
        
        outputMessage.append(maskedMessage)
    }
    
    private String mask(String message) {
        StringBuffer buffer = new StringBuffer()
        Matcher matcher = JSON_PATTERN.matcher(message)
        
        while (matcher.find()) {
            matcher.appendReplacement(buffer, JSON_REPLACEMENT_REGEX)
        }
        
        matcher.appendTail(buffer)
        
        return buffer.toString()
    }
}