CVE-2022-30935 is a critical security vulnerability found in b2evolution, a popular open-source content management system (CMS). This flaw allows attackers anywhere on the internet to take over any user account—including admins—by exploiting predictable password reset tokens, all thanks to a weak randomness function.

In this article, I’ll break down what this vulnerability is, how it works, how to exploit it, and how you can protect yourself and your site. No prior security knowledge required—just some curiosity and a bit of technical patience.

Understanding the Bug: What Went Wrong in b2evolution

b2evolution implemented its password reset feature with a function to generate unique reset tokens. These tokens are supposed to be random and unguessable. Unfortunately, in version 7.2.3 (and possibly earlier ones), the randomness function is predictable.

Here’s what this means:  
If an attacker can predict the password reset token for a user, they can submit a password reset request and get instant, unauthorized access—even without the real user’s help or knowledge.

The vulnerability is due to use of weak or predictable random value generation, for example

// Vulnerable code snippet (simplified)
$token = md5( uniqid( mt_rand(), true ) );

The uniqid() and mt_rand() functions are not cryptographically secure. Given how PHP’s mt_rand() works, attackers can often guess or bruteforce the generated values in practical time.

1. Request a Password Reset for Any User

Go to the password reset page and submit a target username or email. The server will generate a password reset token using the weak random function.

2. Predict the Token

Because the randomness is weak, and PHP mt_rand() seeds are often predictable (especially with knowledge of timing or being able to observe token patterns), an attacker can replicate or brute-force the token generation locally until they find the correct reset URL.

Tools like PHP mt_rand cracker make this even easier.

Example:

Let’s say a reset token looks like this

e3f3a1b9e5f1f29c73bfc5e7f4d795db

With some information (timing, server timestamp, etc.), an attacker can mass-produce likely tokens and visit:

https://victimsite.com/login/resetpwd?token=e3f3a1b9e5f1f29c73bfc5e7f4d795db

3. Reset the Password and Take Over

Once the token is predicted, the attacker can set a new password for the target account. From here, they can:

Proof of Concept (PoC) Exploit

A simple Python code that attempts to predict a token based on known PHP timestamp and mt_rand sequence:

import hashlib
import time

# Simulate the target token generation time window
for ts in range(int(time.time()) - 60, int(time.time()) + 60):
    for rand in range(, 100):  # Assume mt_rand() output range
        uniq = str(ts) + str(rand)
        candidate = hashlib.md5(uniq.encode('utf-8')).hexdigest()
        print(candidate)
        # Optionally, test against live site or known samples

Note: Actual exploitation benefits from insight into your target’s system time, typical token pattern, or, ideally, sample tokens.

References & Full Disclosure

- CVE-2022-30935 NIST entry
- Original b2evolution report & fix
- PHP mt_rand predictability explained
- PHP mt_rand cracker tool

Older versions are likely at risk—including earlier major versions.

All user accounts are vulnerable: admins, editors, and regular users.

Upgrade to a patched version:

Ensure you’re running the latest version of b2evolution. Watch the official download page.

Review access logs for suspicious password resets.

For developers:  
Never use predictable random functions (like mt_rand() or uniqid()) to generate secrets. Use random_bytes() or secure libraries instead.

Final Thoughts

CVE-2022-30935 shows just how dangerous bad randomness can be. If you run b2evolution, patch right now. If you use or develop PHP apps—even simple ones—never generate secrets with weak random generators. Always choose cryptographically secure functions.

Timeline

Published on: 09/28/2022 11:15:00 UTC
Last modified on: 09/30/2022 13:35:00 UTC