CVE-2025-27533 is a vulnerability classified under CWE-789 (Memory Allocation with Excessive Size Value) affecting Apache ActiveMQ, a widely used open-source message broker from the Apache Software Foundation. The flaw arises during the unmarshalling process of OpenWire commands, where the size values of buffers are not properly validated. This improper validation allows an attacker to specify an excessively large buffer size, leading to excessive memory allocation. The consequence is a potential denial of service (DoS) attack by exhausting the memory resources of the ActiveMQ broker process. This can disrupt the availability of messaging services that rely on ActiveMQ, impacting dependent applications and services. The vulnerability affects multiple versions of ActiveMQ: from 6.0.0 before 6.1.6, from 5.18.0 before 5.18.7, from 5.17.0 before 5.17.7, and versions before 5.16.8. Notably, version 5.19.0 and later releases have addressed this issue. Exploitation requires network access to the broker and privileges to authenticate (PR:H), but no user interaction is needed. The vulnerability does not affect brokers configured with mutual TLS, which can mitigate the risk by restricting unauthorized access. The CVSS v4.0 base score is 6.9 (medium severity), reflecting the moderate impact on availability and the requirement for privileged authentication. No known exploits are currently reported in the wild, but the vulnerability poses a significant risk to service continuity if exploited.

For European organizations, the impact of this vulnerability can be substantial, especially for enterprises and public sector entities relying on Apache ActiveMQ for critical messaging infrastructure. A successful DoS attack could lead to service outages, disrupting business operations, automated workflows, and inter-system communications. This is particularly critical for sectors such as finance, telecommunications, healthcare, and government services where message brokers facilitate real-time data exchange and transaction processing. The depletion of memory resources could cause broker crashes or severe performance degradation, impacting availability and potentially causing cascading failures in dependent systems. Additionally, organizations with compliance obligations around service availability and incident response (e.g., GDPR, NIS Directive) may face regulatory scrutiny if the vulnerability is exploited and leads to service disruption. The requirement for privileged authentication reduces the attack surface but does not eliminate risk, especially in environments with weak internal access controls or compromised credentials.

To mitigate this vulnerability, European organizations should prioritize upgrading Apache ActiveMQ to fixed 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. Patch management processes should be accelerated to deploy these updates promptly. For environments where immediate patching is not feasible, implementing mutual TLS (mTLS) authentication on ActiveMQ brokers is a strong compensating control that restricts access to trusted clients only, thereby reducing the risk of exploitation. Network segmentation and firewall rules should be applied to limit broker access to authorized systems. Monitoring and alerting on unusual memory usage patterns or broker restarts can provide early detection of exploitation attempts. Additionally, reviewing and tightening authentication and authorization policies to minimize privileged access reduces the likelihood of an attacker gaining the necessary permissions to exploit this vulnerability. Regular security audits and penetration testing focusing on messaging infrastructure can help identify and remediate related weaknesses.