SerialKiller

SerialKiller is an easy-to-use look-ahead Java deserialization library to secure application from untrusted input.

When Java serialization is used to exchange information between a client and a server, attackers can replace the legitimate serialized stream with malicious data. Inspired by this article, SerialKiller inspects Java classes during naming resolution and allows a combination of blacklisting/whitelisting to secure your application.

Disclaimer: This library may (or may not) be 100% production ready yet. Use at your own risk!

How to protect your application with SerialKiller

1. Download the latest version of the SerialKiller's Jar. Alternatively, this library is also available on Maven Central
2. Import SerialKiller's Jar in your project
3. Replace your deserialization ObjectInputStream with SerialKiller
4. Tune the configuration file, based on your application requirements

Easy, isn't it? Let's look at a few details...

Changes required in your code (step 3)

In your original code, you'll probably have something similar to:

ObjectInputStream ois = new ObjectInputStream(is);
String msg = (String) ois.readObject();

In order to detect malicious payloads or allow your application's classes only, we need to use SerialKiller instead of the standard java.io.ObjectInputStream. This can be done with a one-line change:

ObjectInputStream ois = new SerialKiller(is, "/etc/serialkiller.conf");
String msg = (String) ois.readObject();

The second argument is the location of SerialKiller's configuration file.

Finally, you may want to catch InvalidClassException exceptions to gracefully handle insecure object deserializations. Please note that this library does require Java 8.

Tuning SerialKiller's configuration file (step 4)

SerialKiller config supports the following settings:

• Refresh: The refresh delay in milliseconds, used to hot-reload the configuration file. Good news! You don't need to restart your application if you change the config file
• BlackList: A Java regex to define malicious classes. The default configuration file already includes several known payloads so that your application is protected by default against known attacks
• WhiteList: A Java regex to define classes used by your application. If you can quickly identify a list of trusted classes, this is the best way to secure your application. For instance, you could allow classes in your own package only
• Profiling: Starting from v0.4, SerialKiller introduces a profiling mode to enumerate classes deserialized by the application. In this mode, the deserialization is not blocked. To protect your application, make sure to use 'false' for this setting in production (default value)
• Logging: Logging support compatible to native LogManager using the java.util.logging.config.file system property or lib/logging.properties. See Java8 LogManager for more details.

Example of serialkiller.conf

<?xml version="1.0" encoding="UTF-8"?>
<!-- serialkiller.conf -->
<config>
  <refresh>6000</refresh>
  <mode>
    <!-- set to 'false' for blocking mode -->
    <profiling>false</profiling>
  </mode>
  <blacklist>
  <!--Section for Regular Expressions-->
    <regexps>
        <!-- ysoserial's BeanShell1 payload  -->
        <regexp>bsh\.XThis$</regexp>
        <regexp>bsh\.Interpreter$</regexp>
        <!-- ysoserial's C3P0 payload  -->
        <regexp>com\.mchange\.v2\.c3p0\.impl\.PoolBackedDataSourceBase$</regexp>
	    <!-- ysoserial's MozillaRhino1 payload -->
	    <regexp>org\.mozilla\.javascript\..*$</regexp>
        [...]
    </regexps>
    <!--Section for full-package name-->
    <list>
        <!-- ysoserial's CommonsCollections1,3,5,6 payload  -->
        <name>org.apache.commons.collections.functors.InstantiateTransformer</name>
        <name>org.apache.commons.collections.functors.ConstantTransformer</name>
        <name>org.apache.commons.collections.functors.ChainedTransformer</name>
        <name>org.apache.commons.collections.functors.InvokerTransformer</name>
        [...]
    </list>
  </blacklist>
  <whitelist>
    <regexps>
        <regexp>.*</regexp>
    </regexps>
  </whitelist>
</config>

Credits

• Ironically, SerialKiller uses some Apache Commons libraries (configuration, logging, lang, collections)
• Thanks to @frohoff and @gebl for their work on unsafe Java object deserialization payloads. Ysoserial is awesome!
• Pierre Ernst for the original idea around look-ahead java deserialization filters

License

This library has been dual-licensed to Apache License, Version 2.0 and GNU General Public License.

Contributing

• If you've discovered a bug, please open an issue in Github.
• Submit a new RB, especially if you're aware of Java gadgets that can be abused by vulnerable applications. Providing a safe default configuration is extremely useful for less security-oriented users.