HighCVE-2026-109527

    Modelcontextprotocol Python-sdk: Allocation of Resources Without Limits or Throttling (CVE-2026-109527)

    Published Last modified Sources: Red Hat, CVE.org

    What is affected, what to do

    A flaw was found in mcp. This vulnerability allows an unauthenticated remote attacker to cause a Denial of Service (DoS) by exhausting system memory.

    Update Modelcontextprotocol Python-sdk to version 1.29.1.

    Affected
    • Modelcontextprotocol Python-sdk gte 2.0.0a1, < 2.1.0..*
    • Modelcontextprotocol Python-sdk lt *..1.29.1
    Fixed in
    • Modelcontextprotocol Python-sdk 1.29.1

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

    Metrics

    7.5

    Priority

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

    Source: Vendor (CNA), CVSS v3

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: Red Hat, CVE.org

    0.7 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 53.2

    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
    90.13 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
    7.5 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
    7.5
    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
    0.7 %
    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
    53.2
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    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: none known · automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 20 October 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-770

    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 mcp. This vulnerability allows an unauthenticated remote attacker to cause a Denial of Service (DoS) by exhausting system memory. Affected HTTP endpoints buffer incoming request bodies in memory without enforcing a maximum size limit. By sending excessively large or streamed request bodies, an attacker can consume significant process memory and terminate the server.

    Source: redhat_csaf, cvelistv5, NVD (NIST)

    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
    modelcontextprotocolpython-sdk
    • < 1.29.1
    • ≥ 2.0.0a1, < 2.1.0
    • 1.29.1
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-770Allocation of Resources Without Limits or Throttling

      Base

      The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

      Entry at MITRE CWE-770

    References & sources

    • github.commodelcontextprotocol/python-sdk/security/advisories/GHSA-fmmv-w…x_refsource_CONFIRM
    • github.commodelcontextprotocol/python-sdk/commit/03aaebd3aadb2567878dd811…x_refsource_MISC
    • github.commodelcontextprotocol/python-sdk/commit/4dc224fab201bdef2fd5a065…x_refsource_MISC
    • github.commodelcontextprotocol/python-sdk/commit/57394b0548d1e2dc2dce8d67…x_refsource_MISC
    • github.commodelcontextprotocol/python-sdk/commit/5f0b6af9f2b0aad3be3ba5cb…x_refsource_MISC
    • github.commodelcontextprotocol/python-sdk/releases/tag/v1.29.1x_refsource_MISC
    • github.commodelcontextprotocol/python-sdk/releases/tag/v2.1.0x_refsource_MISC

    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: none known, automatable: yes

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentadded
      —
      {"id":"CVE-2026-109527","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-20T01:46:33.833865Z"}

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

    2. ReceivedGitHub

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

      Weakness type: CWE-770

      Description added or changed

      Show 11 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. In mcp versions before 1.29.1 and from 2.0.0a1 through 2.0.1, the Streamable HTTP session manager, SSE message endpoint, and OAuth authorization-server routes buffer complete HTTP request bodies without enforcing a maximum size. An unauthenticated remote client able to reach an affected endpoint can send a large declared or streamed request body that consumes multiple times the body size in process memory and can repeatedly exhaust memory or terminate the server. Stdio-only servers and deployments with an effective upstream request-size limit are not affected, and applications that use StreamableHTTPServerTransport directly remain responsible for enforcing their own limit. The standard SDK server applications and session managers are fixed in versions 1.29.1 and 2.1.0.
      CVSSadded
      —
      AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
      Weakness type (CWE)added
      —
      CWE-770
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/109xxx/CVE-2026-109527.json">CVE-2026-109527</a>
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/commit/03aaebd3aadb2567878dd811cb1f141454e8a080
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/commit/4dc224fab201bdef2fd5a065976b7ca31420187c
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/commit/57394b0548d1e2dc2dce8d67d84985769df3b8bb
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/commit/5f0b6af9f2b0aad3be3ba5cbfe80a9e4ea32e189
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.29.1
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.1.0
      Referenceadded
      —
      https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-fmmv-w9g8-j3gc

      Source identifier: security-advisories@github.com

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