Mask json values using logback

I'm using below logstash encoder in logback config of my spring boot project.

<encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
        <providers>
            <pattern>
                <pattern>
                    {
                        "creation_timestamp": "%d{yyyy-MM-dd'T'HH:mm:ss, UTC}",
                        "msg": "%msg",
                        "log_type": "%level",
                     }   
                </pattern>
                <omitEmptyFields>true</omitEmptyFields>
            </pattern>
        </providers>
    </encoder>

I want to mask sensitive data in the json log message. I tried the below to mask the value of key "Code". But it doesn't work.

"msg": "%replace(%msg){'\"Code\":'.*'', '\"Code\":'xxx''}",

How can I mask json value within the logstash appender ?

I was able to mask sensitive data by writing a custom implementation of ValueMasker

Two components are required:

1. A class that implements ValueMasker
2. Declaring the jsonGeneratorDecorator in your logback.xml

package com.mask;

import com.fasterxml.jackson.core.JsonStreamContext;
import net.logstash.logback.mask.ValueMasker;

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.IntStream;

public class SensitiveMaskingPatternLayout implements ValueMasker {
    private Pattern multilinePattern = Pattern.compile("\\\"msg\\\"\\s*:\\s*\\\"(.*?)\\\"", Pattern.MULTILINE);;
    private List<String> maskPatterns = new ArrayList<>();

    private String maskMessage(String message) {

        StringBuilder sb = new StringBuilder(message);
        Matcher matcher = multilinePattern.matcher(sb);
        while (matcher.find()) {
            IntStream.rangeClosed(1, matcher.groupCount()).forEach(group -> {
                if (matcher.group(group) != null) {
                    IntStream.range(matcher.start(group), matcher.end(group)).forEach(i -> sb.setCharAt(i, 'x'));
                }
            });
        }
        return sb.toString();
    }

    @Override
    public Object mask(JsonStreamContext context, Object value) {
        if (value instanceof CharSequence) {
            return maskMessage((String) value);
        }
        return value;
    }
}

<configuration>

    <appender name="mask" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
            <providers>
                <pattern>
                    <pattern>
                        {
                            "msg": "%msg",
                            "log_type": "%level"
                        }
                    </pattern>
                    <omitEmptyFields>true</omitEmptyFields>
                </pattern>
            </providers>
            <logLevel>
                INFO
            </logLevel>
            <jsonGeneratorDecorator class="net.logstash.logback.mask.MaskingJsonGeneratorDecorator">
                <valueMasker class="com.mask.SensitiveMaskingPatternLayout"/>
            </jsonGeneratorDecorator>
        </encoder>
    </appender>

    <root level="INFO">
        <appender-ref ref="mask" />
    </root>

</configuration>

Example:

package com.example;

import java.util.HashMap;
import java.util.Map;
import org.json.JSONObject;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class App {

       private static final Logger logger = LoggerFactory.getLogger(App.class);

    public static void main(String[] args) {
        Map<String, String> user = new HashMap<>();
        user.put("user_id", "97588");
        user.put("email_id", "random@email.com");
        user.put("msg", "hello world");
        JSONObject userDetails = new JSONObject(user);

        logger.info("MaskingPatternExample log from {}" + userDetails);
    }

}

{"msg":"MaskingPatternExample log from {}{\"email_id\":\"random@email.com\",\"msg\":\"xxxxxxxxxxx\",\"user_id\":\"97588\"}","log_type":"INFO"}

Further documentation.