os: Insecure Operation on Windows Junction / Mount Point
CVE-2026-56857 is a critical-severity insecure operation on windows junction / mount point vulnerability in os from Go standard library, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 1.26.9, 1.27.2 as the first without the flaw, so updating os to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.55% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update os to 1.26.9 or 1.27.2, whichever matches your release line, or a later version.
- Update internal/syscall/windows to 1.26.9 or 1.27.2, whichever matches your release line, or a later version.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Go standard library | os | before 1.26.9; 1.27.0-0 up to, but not including, 1.27.2 | 1.26.9, 1.27.2 |
| Go standard library | internal/syscall/windows | before 1.26.9; 1.27.0-0 up to, but not including, 1.27.2 | 1.26.9, 1.27.2 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by CISA (ADP).
Official 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).
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Insecure Operation on Windows Junction / Mount Point
The product opens a file or directory, but it does not properly prevent the name from being associated with a junction or mount point to a destination that is outside of the intended control sphere.
How developers prevent this class of flaw
- When designing software that will have different rights than the executer, the software should check that files that it is interacting with are not improper hard links or mount points. One way to do this in Windows is to use the functionality embedded in the following command: dir /al /s /b or, in PowerShell, use LinkType as a filter. In addition, some software uses authentication via signing to ensure that the file is the correct one to use. Make checks atomic with the file action, otherwise a TOCTOU weakness (CWE-367) can be introduced.
More Insecure Operation on Windows Junction / Mount Point vulnerabilities (CWE-1386)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Other references
Other vulnerabilities in os
- 7.8 High severity
- 7.5 High severity
Recent Insecure Operation on Windows Junction / Mount Point vulnerabilities
- 8.8 High severity
- 8.5 High severity
- 7.1 High severity
- 7.1 High severity
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