CVE-2026-59279: Unbounded persistent session allocation via repeated initialize requests
The MCP Streamable HTTP server transport (WebFlux and WebMvc variants) does not place any limit on the number of sessions it retains, and by default does not require clients to be authenticated. As a result, a remote attacker can cause the server to accumulate an unbounded number of sessions over time, gradually exhausting available memory and ultimately causing a Denial of Service that affects all legitimate clients. Affected versions: Spring AI: 2.0.0
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed to unauthenticated denial-of-service attempts?
MCP Streamable HTTP server deployments using either the WebFlux or WebMvc transport are exposed because session retention is unbounded and clients are not required to authenticate by default.
What does an attacker need to do to exploit this issue?
An attacker only needs network access to repeatedly send initialize requests. No authentication, user interaction, or special privileges are required under the default configuration.
What is the practical impact on an affected server?
Each repeated initialize request can cause another retained session to accumulate. Over time, this can exhaust available memory and deny service to all legitimate clients.