# Root.Mkdir(All) can follow junctions out of the root on Windows in os

Advisory record · `GO-2026-6604`

- **Identifiers:** `GO-2026-6604` · `BIT-golang-2026-56857` · `CVE-2026-56857`
- **State:** Confirmed
- **Severity:** CVSS 9.8 Critical
- **Critical:** Yes: CVSS 9+
- **Weakness (CWE):** [CWE-1386](https://cwe.mitre.org/data/definitions/1386.html)
- **Tags:** [CWE-1386](https://agentprime.io/advisories/tag/cwe-1386.md)
- **Published:** 8 Oct 2026
- **Updated:** 16 Oct 2026
- **Reviewed by a person:** No

Published by OSV at 8 Oct 2026, 22:31 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Critical

Critical, because:

- CVSS score 9.8: 9.0 or more.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected packages

- `stdlib` (Go): affected `<1.26.9 || >=1.27.0-0, <1.27.2`; fixed in `1.26.9`, `1.27.2`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Go standard library (os package) versions before 1.26.9, and 1.27.0 before 1.27.2, are vulnerable on Windows. When the target of Root.Mkdir or Root.MkdirAll is a junction pointing to an empty location, a directory can be created at the junction target even when it lies outside the root. Upgrade to Go 1.26.9 or 1.27.2.

Written 1 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 9.8 Critical, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`, from ADP (CISA-ADP)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0055, the 46th percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:12 UTC: First seen by agentprime
- 31 Oct 2026, 14:12 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GO-2026-6604)

## Sources

- [OSV: GO-2026-6604](https://osv.dev/vulnerability/GO-2026-6604)
- <https://go.dev/cl/847305>
- <https://groups.google.com/g/golang-announce/c/U2fTuyDJznI>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GO-2026-6604>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GO-2026-6604/>. Everything for agents: <https://agentprime.io/llms.txt>.