CVE-2026-63127

HIGHPre-NVD 8.28.2—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 8.2 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.2%, top 90% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
Trending — 3 sources updated this weekElevated
8.2EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 8.2Exploit: Elevated riskExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
8.2
EG Score
8.2(medium)
EG Risk
53(Track)
EG Risk 53/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity82% × 45%
Exploitation40% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
10%
KEV
Not listed

Published

September 16, 2026

Last Modified

September 16, 2026

Advisory Details (4)

Auto-updated Sep 16, 2026
🔬 Proof of concept available. Patch available. Sources: github, github_pr, github_commit, github_release.
github_release Patch Available

rmcp-v2.0.0

Patch available: modelcontextprotocol/rust-sdk rmcp-v2.0.0

https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0
github_commit Patch Available

commit c1a8b29ff2cc (modelcontextprotocol/rust-sdk)

Patch available: modelcontextprotocol/rust-sdk rmcp-macros-v3.0.0-beta.1 (contains commit c1a8b29ff2cc)

https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec
github_pr Patch Available

fix: prevent OAuth resource spoofing

Patch available: modelcontextprotocol/rust-sdk rmcp-macros-v3.0.0-beta.1 (PR #937 merged 2026-06-27)

https://github.com/modelcontextprotocol/rust-sdk/pull/937
github Patch Available🟡 PoC Available

Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery · Advisory · modelcontextprotocol/rust-sdk · GitHub

https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj

Vendor Advisories for CVE-2026-63127(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(1 across 1 ecosystem)
crates.io(1)
PackageVulnerable rangeFixed inDependents
rmcp—2.0.0—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 8× in last 7d / 8× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-09-19 01:48 UTCEG score recompute
  2. 2026-09-18 19:28 UTCEPSS rescore
  3. 2026-09-18 02:56 UTCEG score recompute
  4. 2026-09-16 16:32 UTCEG score recompute
  5. 2026-09-16 15:45 UTCEG score recompute
  6. 2026-09-16 15:20 UTCEG score recompute
  7. 2026-09-16 15:01 UTCEG score recompute
  8. 2026-09-16 15:00 UTCMITRE cvelistV5first tracked

Frequently asked(5)

What is CVE-2026-63127?
CVE-2026-63127 is a high vulnerability published on September 16, 2026. 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 discoveroauthserverviaresource_metadata to use protected-resource metadata…
When was CVE-2026-63127 disclosed?
CVE-2026-63127 was first published in the National Vulnerability Database on September 16, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-63127 actively exploited?
CVE-2026-63127 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 90.5% of all scored CVEs.
What is the CVSS score of CVE-2026-63127?
CVE-2026-63127 has a CVSS v3 base score of 8.2 (NVD).
How do I remediate CVE-2026-63127?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-63127, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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