CVE-2026-63128 RMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transport leads to remote denial-of-service
Unauthenticated attackers can exhaust server memory and cause denial of service by triggering unclean session creation in the RMCP Rust SDK.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63128?
CVE-2026-63128 is a high-severity denial of service vulnerability in modelcontextprotocol rust-sdk, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.0.0, the rmcp crate's stateful Streamable HTTP server in crates/rmcp/src/transport/streamable_http_server/tower.rs allows an unauthenticated client to send a well-formed JSON-RPC POST that is not an initialization request, or an initialization request with a mismatched protocol header, causing StreamableHttpService::handle_post to call LocalSessionManager.create_session before validating the message. An early validation failure returns without removing the inserted LocalSessionHandle from LocalSessionManager.sessions, permanently retaining session and channel state for the server process lifetime. Repeated requests can grow the shared session table without bound, degrade legitimate-client latency through lock contention, exhaust memory, and terminate the server. This issue is fixed in version 2.0.0.
Which versions are affected by CVE-2026-63128?
2 open source packages ship an affected version, including rmcp.
| Package | Affected | Fixed in |
|---|---|---|
rmcp crates.io | All versions | 2.0.0 |
rmcp crates.io | All versions | 2.0.0 |
How do I fix CVE-2026-63128?
Upgrade rmcp to 2.0.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-63128 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63128 exists.
Sources
Also known as GHSA-9pj6-vhgr-3mwh.