CVE-2025-27533 is a high-severity vulnerability in Apache ActiveMQ affecting versions before 5.16.8, between 5.17.0 and 5.17.7, between 5.18.0 and 5.18.7, and between 6.0.0 and 6.1.6. It involves improper validation of buffer size values during unmarshalling of OpenWire commands, leading to excessive memory allocation. This can be exploited to cause denial of service by depleting process memory, impacting the availability of the ActiveMQ broker. The issue does not affect ActiveMQ 5.19.0 and later fixed versions. Mitigation includes upgrading to fixed versions or implementing mutual TLS to reduce risk on affected brokers.