# CVE-2026-63127: RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

> A malicious MCP server can trick clients into sending valid access tokens to it, allowing impersonation of the victim against legitimate resources.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 16 Sep 2026
- **Also known as:** GHSA-33f5-2c5q-wgwj

## What is CVE-2026-63127?

CVE-2026-63127 is a high-severity authentication bypass vulnerability in modelcontextprotocol rust-sdk, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.0.0, the rmcp crate's OAuth implementation in crates/rmcp/src/transport/auth.rs omits the RFC 9728 resource field from ResourceServerMetadata and allows discover_oauth_server_via_resource_metadata to use protected-resource metadata without confirming that the returned resource identifier exactly matches the configured MCP server. A malicious MCP server can publish metadata for a different legitimate MCP resource and its authorization server, causing a victim who connects and completes the authorization flow to obtain a legitimate access token that the client subsequently sends to the malicious server. The attacker can capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes. This issue is fixed in version 2.0.0.

Weakness: CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-63127?

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-2026-63127?

Upgrade rmcp to 2.0.0 or later.

## Is CVE-2026-63127 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63127 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63127 and fix it if it is affected.

What it is: CVE-2026-63127 is a high-severity authentication bypass vulnerability in modelcontextprotocol rust-sdk, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63127 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-2026-63127/ (Markdown: https://insights.sbomify.com/CVE-2026-63127/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63127
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63127
- OSV: https://osv.dev/vulnerability/CVE-2026-63127

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63127/
Summaries are generated and may contain mistakes; the official description above is the record.