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

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-08 · updated 2026-10-09
Weakness: CWE-1386

Description

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

Go Stdlib: before 1.26.9; from 1.27.0-0 before 1.27.2 — fixed in 1.26.9, 1.27.2
Go standard library internal/syscall/windows: before 1.26.9; from 1.27.0-0 before 1.27.2 — fixed in 1.26.9, 1.27.2
Go standard library os: before 1.26.9; from 1.27.0-0 before 1.27.2 — fixed in 1.26.9, 1.27.2

How to fix

Upgrade to a fixed version: Go Stdlib 1.26.9, 1.27.2; Go standard library internal/syscall/windows 1.26.9, 1.27.2; Go standard library os 1.26.9, 1.27.2. Check the vendor advisory for mitigations.

References

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