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


- **Severity:** Medium 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`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 5 Nov 2026
- **Updated:** 8 Nov 2026
- **Also known as:** GHSA-h3x9-8c36-m89w, GHSA-xf6j-693p-wqxx

## What is CVE-2026-123272?

CVE-2026-123272 is a medium-severity vulnerability in mark3labs mcp-go, published on 5 Nov 2026. 

### Official 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.

Weakness: CWE-346 (Origin Validation Error)

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


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

No fix has been published for CVE-2026-123272 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-123272 being exploited?

There is no known exploitation of CVE-2026-123272 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-123272 and fix it if it is affected.

What it is: CVE-2026-123272 is a medium-severity vulnerability in mark3labs mcp-go, published on 5 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-123272 in the wild and no public exploit code.

Affected product: mark3labs mcp-go. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-123272/ (Markdown: https://insights.sbomify.com/CVE-2026-123272/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-123272
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-123272
- OSV: https://osv.dev/vulnerability/CVE-2026-123272

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-123272/
Summaries are generated and may contain mistakes; the official description above is the record.