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Introduction

If you're building apps that handle data serialization with Apache Avro, you should sit up and take note: CVE-2024-47561 exposes a critical vulnerability in the Java SDK of Apache Avro, up to version 1.11.3. In plain English, this flaw lets attackers run malicious code simply by sneaking dangerous payloads into Avro schemas—which are just JSON files. If your app parses Avro schemas from user input or untrusted sources, you're at real risk.

This post explains what went wrong, how the exploit works (with code snippets), and what upgrades you need to patch this hole. We keep it simple and direct, with exclusive, hands-on information.

What is Apache Avro?

Avro is a staple in big data; it lets you serialize data (think: store and send objects efficiently) and validate them with schemas. Its schema files are in JSON, and Avro supports many data tools (including Hadoop, Kafka, and Spark).

The Problem: Schema Parsing is Dangerous

In Avro 1.11.3 and below, the Java SDK's schema parser does not properly sanitize untrusted schema input. Bad actors can craft a schema with references (like logical types or custom properties) that result in unexpected class loading during schema parsing. This can open the door to loading arbitrary Java classes, and thus, executing arbitrary code, right as your app parses a schema—no deserialization or explicit program logic needed.

Important: _This means even just handling an Avro schema from a user or untrusted source can put your server at risk!_

How the Exploit Works

Let’s say your server allows users to submit Avro schemas for some workflow, validation, or interoperability process (maybe for a data ingestion tool or API).

If you use something like

String unsafeSchemaJson = getUserInput(); // BAD! Schema comes from the user
Schema.Parser parser = new Schema.Parser();
Schema schema = parser.parse(unsafeSchemaJson);

If the incoming JSON contains certain payloads, Avro’s Schema.Parser can be tricked into loading classes from anywhere on your classpath—or even remote classes if your JVM’s classpath or classloader isn’t locked down.

Suppose an attacker submits this JSON schema (simplified for illustration)

{
  "type": "record",
  "name": "MyEvilRecord",
  "fields": [
    {
      "name": "myfield",
      "type": {
        "type": "string",
        "logicalType": "evil.LogicType" // This will trigger class loading!
      }
    }
  ]
}

If your JVM has a class called evil.LogicType, it will be instantiated when the parser parses this schema. If that class has side effects (running a shell, connecting to a malicious server), it will run as whatever user the JVM is running as.

Evil logic type

package evil;

import org.apache.avro.LogicalType;

public class LogicType extends LogicalType {
    public LogicType() {
        super("evilLogicalType");
        System.out.println("ATTACK: Evil LogicType loaded!");
        // You could run arbitrary code here
    }
}

When the parser hits "logicalType": "evil.LogicType" in the schema, it loads this logic type using reflection. That triggers the static block or constructor—and the attacker’s code runs!

How to Stay Safe

IMMEDIATE MITIGATION: Never parse Avro schemas (or any kind of plugin extensions) from untrusted sources with default settings!

The Official Fix

Avro 1.11.4 and 1.12. fix this hole by adding tighter restrictions on reflective class loading during schema parsing. The logical type system is hardened to prevent instantiating arbitrary classes from schemas.

- Avro 1.11.4 Release Notes
- Avro GitHub - Latest Releases

Avoid parsing schemas from untrusted sources.

- If you _must_ accept schemas, block logicalType and custom properties, or validate input strictly before parsing.

References

- CVE-2024-47561 - NVD
- Apache Avro Security Notices
- Avro Java SDK Documentation
- Relevant GitHub Commit _(Replace with actual commit when available)_

Summary

CVE-2024-47561 is dangerous because it turns Avro’s schema parser—normally a safe, routine tool—into an arbitrary code execution vector. If you maintain apps or servers that take Avro schemas from users or outside systems, upgrade your Avro Java SDK right away to 1.11.4 or 1.12.+, and audit your parsing code.

Stay secure!

*For questions or more technical details about this vulnerability, check the Avro mailing list or GitHub discussions. Stay tuned for future updates as more is learned about active exploit attempts in the wild.*

Timeline

Published on: 10/03/2024 11:15:13 UTC
Last modified on: 10/21/2024 09:15:02 UTC