CVE-2025-27533 is a memory allocation vulnerability classified under CWE-789 that affects Apache ActiveMQ, a widely used open-source message broker. The issue occurs during the unmarshalling process of OpenWire protocol commands, where the size values of buffers are not properly validated. This flaw allows an attacker with authenticated access and high privileges to send specially crafted messages that specify excessively large buffer sizes, causing the broker to allocate an excessive amount of memory. This can lead to depletion of process memory resources, resulting in a denial of service (DoS) condition that disrupts the availability of the ActiveMQ service and any dependent applications. The vulnerability affects multiple ActiveMQ versions: 5.16.0 through 5.18.x (before 5.18.7), 5.17.x (before 5.17.7), 6.0.0 through 6.1.5, with version 5.19.0 and 6.1.6 or later containing fixes. Exploitation requires authentication with high privileges but does not require user interaction or mutual TLS connections, which if enabled, can mitigate the risk by restricting access. The CVSS 4.0 base score is 6.9, reflecting a medium severity level due to the impact on availability and the requirement for authenticated access. No known exploits have been reported in the wild as of the publication date. The vulnerability highlights the importance of validating input sizes in protocol handling to prevent resource exhaustion attacks.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on Apache ActiveMQ for critical messaging infrastructure in financial services, telecommunications, manufacturing, and government sectors. A successful exploitation could lead to denial of service, causing disruption of message delivery, delayed processing, and potential cascading failures in dependent systems. This could affect business continuity, operational efficiency, and service-level agreements. Organizations using affected versions without mutual TLS are at higher risk. Given the requirement for authenticated access with high privileges, insider threats or compromised credentials could be leveraged to exploit this vulnerability. The impact is primarily on availability, but indirect effects on integrity and confidentiality could occur if system disruptions lead to fallback on less secure communication methods or loss of auditability. The absence of known exploits provides a window for proactive mitigation before active attacks emerge.

1. Upgrade Apache ActiveMQ to the latest patched versions: 6.1.6 or later, 5.19.0 or later, 5.18.7 or later, 5.17.7 or later, or 5.16.8 or later to ensure the vulnerability is fixed. 2. Implement mutual TLS (mTLS) on ActiveMQ brokers to enforce strong authentication and encryption, reducing the attack surface by limiting access to trusted clients only. 3. Restrict administrative and high-privilege access to the ActiveMQ broker to trusted personnel and systems, employing strict access controls and monitoring. 4. Monitor broker logs and network traffic for unusual or oversized OpenWire commands that could indicate exploitation attempts. 5. Employ network segmentation to isolate ActiveMQ brokers from untrusted networks and limit exposure. 6. Regularly audit and rotate credentials used for broker access to reduce risk of credential compromise. 7. Conduct security testing and validation of message handling to detect potential malformed inputs. 8. Prepare incident response plans to quickly address potential DoS events affecting messaging infrastructure.