All vulnerabilities
CVE-2026-56857

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

Go standard library os
Open in ThreatlineRecord JSON
CVSS
9.8Critical
ExploitationNo known exploitationNot in CISA KEV, which does not prove it is unexploitedCISA KEV
Exploit odds (EPSS)0.49%Chance of exploitation activity in the next 30 days, higher than 41% of CVEs. Scored 23 Oct 2026FIRST EPSS
Fixed in1.26.9Also 1.27.2CVE List
Published8 Oct 2026Updated 9 Oct 2026CVE List

What fixes it

A fixed version on one release line is not the upgrade target for every line. Check your exact version and any vendor backports.

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).

Text from CVE List

Affected versions 4

  • osaffected: all versions before 1.26.9CVE List
  • osaffected: 1.27.0-0 before 1.27.2CVE List
  • internal/syscall/windowsaffected: all versions before 1.26.9CVE List
  • internal/syscall/windowsaffected: 1.27.0-0 before 1.27.2CVE List

References 4