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Back to active CVEs
Medium · CVSS 5.1CVE-2026-123272

mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource MetadataCVE-2026-123272

Answer in brief

CVE-2026-123272 records a Medium severity (CVSS 5.1) vulnerability in mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata. The current sources do not mark it as known exploited. The current feed maps mark3labs/mcp-go (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Nov 5, 2026Updated Nov 5, 2026Source checked Nov 5, 2026First seen by HOL Nov 5, 2026Material review Nov 5, 2026
Upstream Advisory

Key facts

Risk
Medium · CVSS 5.1
Exploitation
Not marked as known exploited
Affected software
1 mapped package or product
Fix availability
Available

Why this deserves its current priority

CVSS is 5.1. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps mark3labs/mcp-go (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
mark3labs/mcp-gogeneric>=v0.48.0 <v1.2.1v1.2.1

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for mark3labs/mcp-go.
  2. 2Review the reported fix. Update mark3labs/mcp-go to v1.2.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

  1. Published upstream

    Nov 5, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Nov 5, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Nov 5, 2026

Sources and claim methodology

  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • Source referencevulncheck.com
Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.

Record context

Vulnerability class
Tool Poisoning
EPSS
Not reported
CWE IDs
CWE-346
Source
CVE List V5
Source checked
Nov 5, 2026
References
6 linked sources
Open source record