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Fencer Intel
high 8.0

CVE-2026-123272 json

mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata

published 4d ago modified yesterday also EUVD-2026-117725

cvss
8.0
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 9%
public exploits
0
none known

Summary

Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in OAuthHandler.fetchServerMetadata that allows malicious MCP servers to obtain tokens for other resources by declaring arbitrary resources in well-known metadata. Attackers can declare a different MCP server as the RFC 8707 resource indicator, causing the authorization server to mint tokens the attacker replays against that resource.

Patch / fix

  • mark3labs mcp-go v1.2.1

Is CVE-2026-123272 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-123272 updated cvss 8.0

github.com/mark3labs/mcp-go: mcp-go: Unauthorized access via improper OAuth metadata validation

  • Red Hat Hardened Images tempo3.0 3.0.4-0.3.hum1, tempo2.10 2.10.9-0.1.hum1
    affected: workaround, no fix available
  • Red Hat Connectivity Link 1 rhcl-tech-preview/mcp-gateway-rhel9, rhcl-tech-preview/mcp-gateway-rhel9-operator Red Hat OpenShift AI (RHOAI) rhoai/rhai-cli-rhel9 Red Hat OpenShift distributed tracing 3 rhosdt/tempo-rhel9, rhosdt/tempo-query-rhel9
    under investigation

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
total
Assessed
4d ago

Exploit prediction

0.20% chance of exploitation in the next 30 days, higher than 9% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 4.0 5.1 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
CNA 3.1 8.0 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N
CNA 4.0 5.1 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected products

  • mark3labs mcp-go

Classification

Weakness

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.