Vulnerability details

CVE-2026-63128

modelcontextprotocol rust-sdk

RMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transport leads to remote denial-of-service

CVSS 3.1

7.5High

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.

CVSS scores

Score	Version	Source	Vector
7.5High	3.1	CVEProject cvelistV5 (CNA) (GitHub_M)	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`

Affected products

Vendor	Product	Versions	Source
modelcontextprotocol	rust-sdk	< 2.0.0	CVEProject cvelistV5 (CNA)

References and advisories

https://github.com/modelcontextprotocol/rust-sdk/pull/934 x_refsource_MISC
https://github.com/modelcontextprotocol/rust-sdk/commit/dfa7fd6f9309deab60bea230b041be9a3fcda846 x_refsource_MISC
https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0 x_refsource_MISC
https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9pj6-vhgr-3mwh x_refsource_CONFIRM

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Details

Published: 2026-09-16
Last updated: 2026-09-16
State: Published
Assigner (CNA): GitHub_M

Exploit prediction (EPSS)

0.53%

Probability of exploitation in 30 days

45.3%

Percentile (among all CVEs)

SSVC assessment (CISA)

Automatable: No
Exploitation: Proof of concept
Technical impact: Partial

Weaknesses (CWE)

CWE-400
Uncontrolled Resource Consumption
CWE-401
Missing Release of Memory after Effective Lifetime
CWE-772
Missing Release of Resource after Effective Lifetime