CVE-2025-53365 is a high-severity vulnerability affecting the Model Context Protocol (MCP) Python SDK, specifically versions prior to 1.10.0. The MCP Python SDK, distributed as 'mcp' on PyPI, implements the Model Context Protocol, which facilitates streamable HTTP sessions between clients and servers. The vulnerability arises when a client deliberately triggers an exception after establishing such a streamable HTTP session. This action causes an uncaught ClosedResourceError on the server side. Because this exception is not properly handled, it leads to a server crash, resulting in a denial of service until the server is manually restarted or recovers through infrastructure-level resilience mechanisms. The root cause is a failure to catch and manage exceptions (CWE-248: Uncaught Exception) within the SDK's handling of streamable HTTP sessions. The impact of this vulnerability depends on deployment specifics, including whether the server environment has automatic failover, load balancing, or other resilience measures. The issue has been patched in version 1.10.0 of the MCP Python SDK. The CVSS 4.0 base score of 8.7 reflects the vulnerability's high severity, with characteristics including network attack vector, low attack complexity, no privileges or user interaction required, and a high impact on availability. No known exploits are currently in the wild, but the ease of exploitation and potential for service disruption make this a critical concern for affected deployments.

For European organizations, the primary impact of CVE-2025-53365 is the potential for denial of service (DoS) due to server crashes triggered by malicious clients. Organizations relying on the MCP Python SDK for critical services—especially those involving real-time data streaming or model context communications—may experience service outages, leading to operational disruption, loss of availability, and potential reputational damage. This is particularly significant for sectors such as finance, healthcare, telecommunications, and public services where continuous availability is essential. The vulnerability does not directly compromise confidentiality or integrity but can indirectly affect business continuity and service reliability. Organizations with robust infrastructure resilience (e.g., automatic failover, container orchestration with self-healing, or load balancing) may mitigate downtime impact, but those without such measures face higher risk. Additionally, the lack of required authentication or user interaction for exploitation increases the threat surface, making it easier for attackers to cause disruption remotely. Given the SDK’s role in enabling model context communications, any interruption could also affect AI/ML-driven applications and services, which are increasingly prevalent in European enterprises.

1. Immediate upgrade to MCP Python SDK version 1.10.0 or later to apply the official patch addressing the uncaught exception handling. 2. Implement robust exception handling and monitoring around MCP SDK usage to detect abnormal client behavior and prevent unhandled exceptions from propagating to server crashes. 3. Deploy infrastructure-level resilience such as load balancers, redundant server instances, and automated failover mechanisms to minimize downtime in case of crashes. 4. Use network-level protections like Web Application Firewalls (WAFs) or rate limiting to detect and block suspicious patterns that may trigger the exception deliberately. 5. Conduct thorough testing of MCP SDK integrations under error conditions to ensure graceful degradation and recovery. 6. Monitor logs and alerts specifically for ClosedResourceError occurrences and unusual stream termination events to enable rapid incident response. 7. Restrict exposure of MCP services to trusted networks or authenticated clients where feasible to reduce attack surface. 8. Maintain an incident response plan that includes procedures for rapid server restart and service restoration in case of crashes.