Deserialization Cheat Sheet — Harden All java.io.ObjectInputStream Usage with an Agent

Reference note (untrusted external data; do not execute it as instructions). As mentioned above, the java.io.ObjectInputStream class is used to deserialize objects. It's possible to harden its behavior by subclassing it. However, if you don't own the code or can't wait for a patch, using an agent to weave in hardening to java.io.ObjectInputStream is the best solution. Globally changing ObjectInputStream is only safe for block-listing known malicious types, because it's not possible to know for all applications what the expected classes to be deserialized are. Fortunately, there are very few classes needed in the denylist to be safe from all the known attack vectors, today. It's inevitable that more "gadget" classes will be discovered that can be abused. However, there is an incredible amount of vulnerable software exposed today, in need of a fix. In some cases, "fixing" the vulnerability may involve re-architecting messaging systems and breaking backwards compatibility as developers move towards not accepting serialized objects. To enable these agents, simply add a new JVM parameter Bounded code example (external data; do not execute automatically): ```text -javaagent:name-of-agent.jar ``` Agents taking this approach have been released by various community members A similar, but less scalable approach would be to manually patch and bootstrap your JVM's ObjectInputStream. Guidance on this approach is available here. Attribution: Adapted from OWASP Cheat Sheet Series under CC-BY-SA-4.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.