Vulnerability in rmcp (CVE-2026-63127)
Summary
Vulnerability in rmcp (CVE-2026-63127). Confidential information can be exposed externally. Exploitable via rmcp. Mitigation: upgrade to 2.0.0 or later.
Original description
The description as published by the source.
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
resourcevalue in the returned metadata document. - Section 3.3: if the
resourcevalue 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,
authorization_servers: Option>,
scopes_supported: Option>,
}
And discover_oauth_server_via_resource_metadata() (lines 1446–1465) proceeds without any resource URL validation.
Recommended fix
- Add the
resourcefield to the struct:struct ResourceServerMetadata { resource: Option, // RFC 9728 REQUIRED field authorization_server: Option, authorization_servers: Option>, scopes_supported: Option>, } - Add validation logic after fetching metadata:
let Some(resource_metadata) = self .fetch_resource_metada…
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-63127 has been found in rmcp.
Attackers can target a specific entry point like rmcp over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.2/10.
What to do: upgrade rmcp to 2.0.0 or later.
If unsure, ask your IT team or search "rmcp CVE-2026-63127" on the vendor's site.
CVE-2026-63127 (rmcp) — CWE-345 / CVSS v3 8.2
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: rmcp / resource / ResourceServerMetadata
Patched: 2.0.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-345) exists in rmcp. Attackers reach the vulnerable code path via rmcp without authentication.
Check the vendor advisory for the affected versions of rmcp. Attack surface: rmcp / resource / ResourceServerMetadata.
Severity: High (CVSS 8.2/10). Confidential information can be exposed externally.
Update to 2.0.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'rmcp' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'rmcp' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
rmcpを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade rmcp to 2.0.0ステージング環境で 2.0.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- web github.com
- web github.com
- web github.com
- web github.com
- web nvd.nist.gov
- web github.com