In early 2026, a serious vulnerability was uncovered in the Windows App for Mac, tracked as CVE-2026-21517. This bug falls under the category of "improper link resolution before file access"—sometimes called a *link following* or *symlink attack*. In simple terms, the app doesn't correctly check if a file is a symbolic link or shortcut before it opens or modifies it. This oversight can let a local attacker get more power on your Mac than they should.

In this post, we'll break down what this flaw means, show some example code, and discuss how it could be exploited. We’ll also link you to the original references so you can dig deeper if you'd like.

A *symbolic link* (symlink) is like a shortcut—a pointer to another file or folder. If an app doesn’t check whether it’s following a real file or just a link, attackers might trick the app into accessing sensitive files.

Imagine you have a program that copies data into a file you control. If you can change that file to be a symlink to, say, a system file that only administrators should touch, the program could overwrite it, even if normally you wouldn't have permission.

Explaining CVE-2026-21517

Summary:
*In Windows App for Mac, certain file operations are performed by following user-controlled links without verifying their target location. An attacker with local access can use symlinks to trick the app into writing or reading sensitive files, escalating their privileges from their user account up to full administrator on the Mac.*

Impact:

Exploit Scenario

Suppose Windows App for Mac lets you install or update virtual machines, and you (a regular user) can control where certain process logs or configs are stored. Here’s how an attack might go:

1. Find a Weak Spot: The attacker identifies a location where the Windows App for Mac writes files as a privileged user (like root or admin), but lets users pick the filename or folder.

2. Create a Symlink: The attacker deletes the expected log file and creates a symbolic link with the same name, pointing it to a sensitive system file (like /etc/sudoers).

3. Trigger the Action: The attacker performs an action in the Windows App for Mac that causes it to write to the log file, but now it’s actually writing to /etc/sudoers.

Example Attack Code

Here’s a simplified bash snippet of how such an attack might look on macOS. (Assume the Windows App for Mac writes a log file to /tmp/app.log as root, but lets users delete it.)

# Step 1: Remove the old log file, if it exists
rm -f /tmp/app.log

# Step 2: Create a symlink to a sensitive file
ln -s /etc/sudoers /tmp/app.log

# Step 3: Trigger the app to write to 'app.log'
open --args WindowsAppForMac --do-privileged-operation

# Step 4: If the app overwrites /etc/sudoers, attacker gets sudo rights

*Note: Don’t try this on any system you care about—this is for educational purposes only!*

Original Sources

- NVD entry for CVE-2026-21517
- Microsoft Security Response Center (MSRC)
- Explaining Symlink Attacks (OWASP)

If you use Windows App for Mac

- Update: Always install the latest updates from Microsoft—vulnerabilities like these get patched fast.

Conclusion

CVE-2026-21517 is a good reminder that following links without thinking can let attackers slip through security checks. If you’re a developer, always double-check the real target before accessing files, especially if the action is privileged. If you’re a user, keep your apps and system up to date to stay protected from these kinds of attacks.

Timeline

Published on: 02/10/2026 17:51:15 UTC
Last modified on: 02/11/2026 02:16:00 UTC