CWE-1386 Base Incomplete

Insecure Operation on Windows Junction / Mount Point

This vulnerability occurs when a Windows application opens a file or directory without properly validating that the path is not a symbolic link (junction or mount point) pointing to a location…

Definition

What is CWE-1386?

This vulnerability occurs when a Windows application opens a file or directory without properly validating that the path is not a symbolic link (junction or mount point) pointing to a location outside the application's intended security boundary.
On Windows NTFS file systems, junction points and mount points are types of symbolic links (reparse points) that can redirect file operations from one directory to another, potentially to a completely different drive or privileged location. If an application with elevated privileges opens a file without checking for these links, an attacker can replace the expected file with a link to a sensitive system file, registry key, or named object, tricking the application into performing unauthorized read, write, or delete operations. This allows privilege escalation or data manipulation because the privileged process unknowingly operates on the attacker's target. For example, an attacker could redirect a file operation to AUTOEXEC.BAT, critical configuration files, or security databases, causing the application to corrupt, expose, or modify sensitive data with its own high-level permissions. Developers must explicitly validate paths and handle reparse points to confine operations within the intended directory scope.
Real-world impact

Real-world CVEs caused by CWE-1386

  • CVE-2021-26426

    Privileged service allows attackers to delete unauthorized files using a directory junction, leading to arbitrary code execution as SYSTEM.

  • CVE-2020-0863

    By creating a mount point and hard links, an attacker can abuse a service to allow users arbitrary file read permissions.

  • CVE-2019-1161

    Chain: race condition (CWE-362) in anti-malware product allows deletion of files by creating a junction (CWE-1386) and using hard links during the time window in which a temporary file is created and deleted.

  • CVE-2014-0568

    Escape from sandbox for document reader by using a mountpoint [REF-1264]

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-1386

  • Architecture and Design 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.
Detection signals

How to detect CWE-1386

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-1386 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

Get a demo Try Plexicus free
Frequently asked questions

Frequently asked questions

What is CWE-1386?

This vulnerability occurs when a Windows application opens a file or directory without properly validating that the path is not a symbolic link (junction or mount point) pointing to a location outside the application's intended security boundary.

How serious is CWE-1386?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-1386?

MITRE lists the following affected platforms: Windows.

How can I prevent CWE-1386?

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,…

How does Plexicus detect and fix CWE-1386?

Plexicus's SAST engine matches the data-flow signature for CWE-1386 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-1386?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1386.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-1386

CWE-59 Parent

Improper Link Resolution Before File Access ('Link Following')

This vulnerability occurs when an application uses a filename to access a file but fails to properly check if that name points to a…

CWE-61 Sibling

UNIX Symbolic Link (Symlink) Following

This vulnerability occurs when a software application opens files or directories without properly checking if they are symbolic links…

CWE-62 Sibling

UNIX Hard Link

This vulnerability occurs when an application opens a file or directory without verifying if the name points to a hard link that leads…

CWE-64 Sibling

Windows Shortcut Following (.LNK)

This vulnerability occurs when an application opens a file or directory without properly validating that it is a Windows shortcut (.LNK).…

CWE-65 Sibling

Windows Hard Link

This vulnerability occurs when a Windows application opens a file or directory without properly verifying if the path points to a hard…

Resources

Further reading

Ready when you are

Don't Let Security
Weigh You Down.

Stop choosing between AI velocity and security debt. Plexicus is the only platform that runs Vibe Coding Security and ASPM in parallel — one workflow, every codebase.

Get started free Book a demo