---
title: "CVE-2026-56857: Root.Mkdir(All) can follow junctions out of the root on Windows in os"
url: https://erroneo.us/security/cve/CVE-2026-56857/
description: "CVE-2026-56857 (Go standard library os): On Windows, when the target of Root.Mkdir or Root.MkdirAll is a junction pointing to an empty location, the…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-10-08
date_modified: 2026-10-21
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-56857"
kev: false
cvss: 9.8
epss: 0.00493
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-56857: Root.Mkdir(All) can follow junctions out of the root on Windows in os

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.5% , percentile 41: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 41% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Editorial verdict: Patch this week. Go October 2026 Security Releases: 16 Stdlib Fixes — Rebuild and redeploy Go services with Go 1.27.3 or 1.26.10 this week, starting with internet-facing HTTP/2 servers and reverse proxies; none of these bugs is known to be exploited.

## What it is

On Windows, when the target of Root.Mkdir or Root.MkdirAll is a junction pointing to an empty location, the operation can create a directory at the junction target even when that target is located outside the root. This only applies to operations where the last path component is a junction (path/to/junction, but not path/junction/target).

## Affected products (CVE record)

- Go standard library os: < 1.26.9, 1.27.0-0 < 1.27.2
- Go standard library internal/syscall/windows: < 1.26.9, 1.27.0-0 < 1.27.2

## Severity

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

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.49% probability of exploitation activity in the next 30 days; percentile 41.

## Sources

- [CVE.org record CVE-2026-56857](https://www.cve.org/CVERecord?id=CVE-2026-56857)
- [NVD entry CVE-2026-56857](https://nvd.nist.gov/vuln/detail/CVE-2026-56857)
- [go.dev](https://go.dev/cl/847305)
- [go.dev](https://go.dev/issue/81739)
- [groups.google.com](https://groups.google.com/g/golang-announce/c/U2fTuyDJznI)
- [pkg.go.dev](https://pkg.go.dev/vuln/GO-2026-6604)

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Source page: https://erroneo.us/security/cve/CVE-2026-56857/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt