# 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.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 16 Sep 2026
- **Also known as:** GHSA-9pj6-vhgr-3mwh

## 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.

Weakness: CWE-772 (Missing Release of Resource after Effective Lifetime), CWE-401 (Missing Release of Memory after Effective Lifetime), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-63128?

2 open source packages ship an affected version, including rmcp.


| Package | Ecosystem | 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.

## Is CVE-2026-63128 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63128 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63128 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63128 exists.

Affected packages and fixed versions:
- rmcp (crates.io): affected all versions; fixed in 2.0.0
- rmcp (crates.io): affected all versions; fixed in 2.0.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63128/ (Markdown: https://insights.sbomify.com/CVE-2026-63128/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63128
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63128
- OSV: https://osv.dev/vulnerability/CVE-2026-63128

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63128/
Summaries are generated and may contain mistakes; the official description above is the record.