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

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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.

Source published

16 Sep 2026 14:50:54 UTC

Source modified

16 Sep 2026 15:39:26 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

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.

Source published

Sep 16, 2026, 2:50:54 PM (timezone not stated)

Source modified

Sep 16, 2026, 3:39:26 PM (timezone not stated)

Observed here

11 Nov 2026 15:05:36 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

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.

Source published

16 Sep 2026 15:17:39 UTC

Source modified

30 Sep 2026 17:51:56 UTC

Observed here

25 Oct 2026 14:19:28 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

### 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: ```rust 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 1. 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>>, } ``` 2. 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 1. Attacker sets up a malicious MCP server at `fake-mcp.com/mcp`. 2. 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` 3. Victim configures any MCP client using `rmcp` to connect to `fake-mcp.com/mcp`. 4. `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. 5. The victim sees a legitimate authorization prompt and completes the flow. 6. The resulting access token — valid for `real-mcp.com/mcp` — is sent to `fake-mcp.com/mcp` in subsequent requests. 7. 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)

Source published

16 Sep 2026 22:13:26 UTC

Source modified

16 Sep 2026 22:30:08 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

Publisher rating · HIGH