Vulnerability-Lookup

CVE-2026-63127 (GCVE-0-2026-63127)

Vulnerability from cvelistv5 – Published: 2026-09-16 14:50 – Updated: 2026-09-16 15:39

Title

RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

Summary

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.

Severity

8.2 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-09-16 15:38 UTC

CWE

CWE-345 - Insufficient Verification of Data Authenticity

Assigner

GitHub_M mitre root

References

4 references

URL	Tags
https://github.com/modelcontextprotocol/rust-sdk/…	x_refsource_CONFIRM
https://github.com/modelcontextprotocol/rust-sdk/…	x_refsource_MISC
https://github.com/modelcontextprotocol/rust-sdk/…	x_refsource_MISC
https://github.com/modelcontextprotocol/rust-sdk/…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
modelcontextprotocol	rust-sdk	Affected: < 2.0.0	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-63127

Vulnerability from fkie_nvd - Published: 2026-09-16 15:17 - Updated: 2026-09-16 16:17

Severity

8.2 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec
	security-advisories@github.com	https://github.com/modelcontextprotocol/rust-sdk/pull/937
	security-advisories@github.com	https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0
	security-advisories@github.com	https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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      "affectedData": [
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          "product": "rust-sdk",
          "vendor": "modelcontextprotocol",
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GHSA-33F5-2C5Q-WGWJ

Vulnerability from github – Published: 2026-09-16 22:13 – Updated: 2026-09-16 22:13

Summary

RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

Details

Summary

The rmcp library does not validate the resource parameter in OAuth Protected Resource metadata (RFC 9728), allowing a malicious MCP server to redirect OAuth flows to a legitimate authorization server and steal the resulting access tokens.

Details

RFC 9728 specifies two MUST requirements for resource parameter validation: - Section 7.3: the client MUST ensure that the resource identifier URL it is using as the prefix for the metadata request exactly matches the resource value in the returned metadata document. - Section 3.3: if the resource value returned is not identical to the URL the client used, the data MUST NOT be used.

In the current implementation (crates/rmcp/src/transport/auth.rs), the ResourceServerMetadata struct (lines 390–394) does not include a resource field:

struct ResourceServerMetadata {
    authorization_server: Option<String>,
    authorization_servers: Option<Vec<String>>,
    scopes_supported: Option<Vec<String>>,
}

And discover_oauth_server_via_resource_metadata() (lines 1446–1465) proceeds without any resource URL validation.

Recommended fix

Add the resource field to the struct: rust struct ResourceServerMetadata { resource: Option<String>, // RFC 9728 REQUIRED field authorization_server: Option<String>, authorization_servers: Option<Vec<String>>, scopes_supported: Option<Vec<String>>, }
Add validation logic after fetching metadata: ```rust let Some(resource_metadata) = self .fetch_resource_metadata_from_url(&resource_metadata_url) .await? else { return Ok(None); };

// RFC 9728: validate that the resource identifier matches our target server if let Some(resource) = &resource_metadata.resource { if resource.trim_end_matches('/') != self.base_url.as_str().trim_end_matches('/') { return Err(AuthError::MetadataError(format!( "Resource metadata mismatch: expected '{}', got '{}'", self.base_url, resource ))); } } ```

PoC

Attacker sets up a malicious MCP server at fake-mcp.com/mcp.
At fake-mcp.com/mcp/.well-known/oauth-protected-resource, the attacker serves metadata declaring:
- resource: real-mcp.com/mcp (the legitimate server)
- authorization_servers: the legitimate authorization server(s) of real-mcp.com/mcp
Victim configures any MCP client using rmcp to connect to fake-mcp.com/mcp.
rmcp fetches the protected resource metadata and, without validating that the resource field (real-mcp.com/mcp) differs from the configured server (fake-mcp.com/mcp), initiates an OAuth flow with the legitimate authorization server.
The victim sees a legitimate authorization prompt and completes the flow.
The resulting access token — valid for real-mcp.com/mcp — is sent to fake-mcp.com/mcp in subsequent requests.
The attacker captures the token and can impersonate the victim on real-mcp.com/mcp.

Impact

This is an access token theft vulnerability via OAuth resource metadata spoofing. All MCP clients built on rmcp that rely on OAuth-protected MCP servers are affected. An attacker who tricks a user into connecting to a malicious MCP server can steal valid access tokens for any legitimate MCP server, enabling full impersonation of the victim.

Credit

Jian Cui, Minsun Shim, Zhou Li, Xiaojing Liao University of Illinois Urbana-Champaign (UIUC) University of California, Irvine (UCI)

Severity

8.2 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

Show details on source website

JSON

To clipboard

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        "name": "rmcp"
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        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-63127"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-345"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-16T22:13:26Z",
    "nvd_published_at": "2026-09-16T15:17:39Z",
    "severity": "HIGH"
  },
  "details": "### Summary\nThe `rmcp` library does not validate the `resource` parameter in OAuth Protected Resource metadata (RFC 9728), allowing a malicious MCP server to redirect OAuth flows to a legitimate authorization server and steal the resulting access tokens.\n\n### Details\nRFC 9728 specifies two MUST requirements for resource parameter validation:\n- Section 7.3: the client MUST ensure that the resource identifier URL it is using as the prefix for the metadata request exactly matches the `resource` value in the returned metadata document.\n- Section 3.3: if the `resource` value returned is not identical to the URL the client used, the data MUST NOT be used.\n\nIn the current implementation (`crates/rmcp/src/transport/auth.rs`), the `ResourceServerMetadata` struct (lines 390\u2013394) does not include a resource field:\n```rust\nstruct ResourceServerMetadata {\n    authorization_server: Option\u003cString\u003e,\n    authorization_servers: Option\u003cVec\u003cString\u003e\u003e,\n    scopes_supported: Option\u003cVec\u003cString\u003e\u003e,\n}\n```\nAnd discover_oauth_server_via_resource_metadata() (lines 1446\u20131465) proceeds without any resource URL validation.\n\n#### Recommended fix\n1. Add the `resource` field to the struct:\n    ```rust\n    struct ResourceServerMetadata {\n        resource: Option\u003cString\u003e,  // RFC 9728 REQUIRED field\n        authorization_server: Option\u003cString\u003e,\n        authorization_servers: Option\u003cVec\u003cString\u003e\u003e,\n        scopes_supported: Option\u003cVec\u003cString\u003e\u003e,\n    }\n    ```\n2. Add validation logic after fetching metadata:\n    ```rust\n    let Some(resource_metadata) = self\n        .fetch_resource_metadata_from_url(\u0026resource_metadata_url)\n        .await?\n    else {\n        return Ok(None);\n    };\n \n    // RFC 9728: validate that the resource identifier matches our target server\n    if let Some(resource) = \u0026resource_metadata.resource {\n        if resource.trim_end_matches(\u0027/\u0027) != self.base_url.as_str().trim_end_matches(\u0027/\u0027) {\n            return Err(AuthError::MetadataError(format!(\n                \"Resource metadata mismatch: expected \u0027{}\u0027, got \u0027{}\u0027\",\n                self.base_url, resource\n            )));\n        }\n    }\n    ```\n\n### PoC\n\n1. Attacker sets up a malicious MCP server at `fake-mcp.com/mcp`. \n2. At `fake-mcp.com/mcp/.well-known/oauth-protected-resource`, the attacker serves metadata declaring:\n    - resource: `real-mcp.com/mcp` (the legitimate server)\n    - authorization_servers: the legitimate authorization server(s) of `real-mcp.com/mcp`\n\n\n3. Victim configures any MCP client using `rmcp` to connect to `fake-mcp.com/mcp`.\n\n4. `rmcp` fetches the protected resource metadata and, without validating that the `resource` field (`real-mcp.com/mcp`) differs from the configured server (`fake-mcp.com/mcp`), initiates an OAuth flow with the legitimate authorization server.\n5. The victim sees a legitimate authorization prompt and completes the flow.\n6. The resulting access token \u2014 valid for `real-mcp.com/mcp` \u2014 is sent to `fake-mcp.com/mcp` in subsequent requests.\n7. The attacker captures the token and can impersonate the victim on `real-mcp.com/mcp`.\n\n### Impact\nThis is an access token theft vulnerability via OAuth resource metadata spoofing. All MCP clients built on `rmcp` that rely on OAuth-protected MCP servers are affected. An attacker who tricks a user into connecting to a malicious MCP server can steal valid access tokens for any legitimate MCP server, enabling full impersonation of the victim.\n\n#### Credit\nJian Cui, Minsun Shim, Zhou Li, Xiaojing Liao\nUniversity of Illinois Urbana-Champaign (UIUC)\nUniversity of California, Irvine (UCI)",
  "id": "GHSA-33f5-2c5q-wgwj",
  "modified": "2026-09-16T22:13:26Z",
  "published": "2026-09-16T22:13:26Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63127"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/pull/937"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/rust-sdk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery"
}

Sightings

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Action not permitted

CVE-2026-63127 (GCVE-0-2026-63127)

FKIE_CVE-2026-63127

GHSA-33F5-2C5Q-WGWJ

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