CVE-2025-6515 identifies a security weakness in the oatpp-mcp component of the oatpp framework, specifically in the MCP Server-Sent Events (SSE) endpoint. The vulnerability arises because the session ID returned by the endpoint is derived from an instance pointer, which is inherently predictable and not unique or cryptographically random. This design flaw violates secure session management principles (CWE-330), as session identifiers must be unpredictable to prevent session hijacking. An attacker with network access to the oatpp-mcp server can exploit this by guessing or predicting future session IDs, thereby hijacking active client sessions. Once hijacked, the attacker can inject malicious responses, potentially disrupting service integrity and availability. The vulnerability has a CVSS 3.1 base score of 6.8, reflecting a medium severity level, with attack vector being network-based, requiring no privileges but some user interaction, and having high impact on integrity and availability but no confidentiality impact. The affected version is 0 of oatpp-mcp, and no official patches or fixes have been released as of the publication date. The vulnerability was reserved in June 2025 and published in October 2025, with no known exploits in the wild to date.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on oatpp-mcp for real-time communication or microservice coordination. Successful exploitation could lead to session hijacking, allowing attackers to impersonate legitimate clients and inject malicious data or commands. This compromises the integrity and availability of services, potentially disrupting business operations or critical infrastructure. Industries such as finance, healthcare, telecommunications, and government services that use oatpp-mcp in their backend systems are at higher risk. Additionally, the ability to hijack sessions without requiring authentication or elevated privileges increases the threat surface. Although confidentiality is not directly impacted, the integrity and availability losses can lead to cascading effects, including data corruption, service downtime, and loss of trust. The absence of known exploits currently provides a window for mitigation, but the medium severity score indicates that proactive measures are essential to prevent exploitation.

To mitigate CVE-2025-6515, organizations should first assess their deployment of oatpp-mcp and identify exposed MCP SSE endpoints. Immediate mitigation includes restricting network access to the oatpp-mcp server to trusted hosts only, using network segmentation and firewall rules to limit attacker access. Implementing Transport Layer Security (TLS) can help protect session data in transit, although it does not fix the session ID predictability itself. Developers should avoid using instance pointers or any predictable values as session identifiers and instead implement cryptographically secure random session IDs with sufficient entropy. Until an official patch is released, consider deploying application-layer filters or proxies that validate session IDs against a secure session management system. Monitoring and logging MCP session activity for anomalies can help detect attempted hijacking. Finally, coordinate with the oatpp project maintainers for updates and apply patches promptly once available.