HighCVE-2026-63127

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

    Published Last modified Sources: Red Hat, BSI, CVE.org, OSV, Hersteller

    What is affected, what to do

    A flaw was found in rmcp, a Rust SDK for the Model Context Protocol. The OAuth implementation in rmcp omits the RFC 9728 resource field from ResourceServerMetadata.

    Update Modelcontextprotocol Rust-sdk to version 2.0.0.

    Affected
    • Modelcontextprotocol Rust-sdk lt *..2.0.0
    Fixed in
    • Modelcontextprotocol Rust-sdk 2.0.0

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.2

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    8.2

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: Red Hat, BSI, CVE.org, OSV, Hersteller

    0.2 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 9.2

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    69.09 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.2 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.2
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.2 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    9.2
    Low — this CVE sits in the lower 10% of all CVEs scored today.
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: proof of concept · not automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 16 September 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-345

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A flaw was found in rmcp, a Rust SDK for the Model Context Protocol. The OAuth implementation in rmcp omits the RFC 9728 resource field from ResourceServerMetadata. This allows a malicious Model Context Protocol (MCP) server to publish metadata for a different legitimate MCP resource. Consequently, a victim connecting to the malicious server may obtain a legitimate access token, which the client then sends to the malicious server. This enables the attacker to capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes, leading to unauthorized access and information disclosure.

    Source: redhat_csaf, BSI WID Portal, BSI CSAF, NVD (NIST), OSV (Open Source Vulnerabilities), GitHub Security Advisories, cvelistv5

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    modelcontextprotocolrust-sdk
    • < 2.0.0
    • 2.0.0
    Red HatEnterprise Linux
    • 10
    • 9
    • —
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-345Insufficient Verification of Data Authenticity

      Class

      The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.

      Entry at MITRE CWE-345

    References & sources

    • github.commodelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5…x_refsource_CONFIRM
    • github.commodelcontextprotocol/rust-sdk/pull/937x_refsource_MISC
    • github.commodelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae…x_refsource_MISC
    • github.commodelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0x_refsource_MISC
    • NVD (NIST)vuln/detail/CVE-2026-63127advisory
    • github.commodelcontextprotocol/rust-sdkpackage

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: proof of concept, automatable: no

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj
      SSVC assessmentadded
      —
      {"id":"CVE-2026-63127","role":"CISA Coordinator","options":[{"exploitation":"poc"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-16T15:38:50.771703Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    2. ReceivedGitHub

      CVSS V3.1: AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

      Weakness type: CWE-345

      Description added or changed

      Show 8 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      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.
      CVSSadded
      —
      AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
      Weakness type (CWE)added
      —
      CWE-345
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/63xxx/CVE-2026-63127.json">CVE-2026-63127</a>
      Referenceadded
      —
      https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec
      Referenceadded
      —
      https://github.com/modelcontextprotocol/rust-sdk/pull/937
      Referenceadded
      —
      https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0
      Referenceadded
      —
      https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj

      Source identifier: security-advisories@github.com

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