CVE-2025-3889: WordPress Simple PayPal Cart IDOR Flaw

CVE-2025-3889 Overview

CVE-2025-3889 affects the WordPress Simple Shopping Cart plugin developed by tipsandtricks-hq. The vulnerability is an Insecure Direct Object Reference [CWE-639] in the process_payment_data function. The plugin fails to validate a user-controlled key that governs product quantity during checkout. Unauthenticated attackers can submit a negative quantity value, causing the plugin to subtract the product cost from the order total. All versions up to and including 5.1.3 are affected. Exploitation is limited to sites configured with Manual Checkout mode, since PayPal and Stripe reject negative-quantity transactions.

Critical Impact
Unauthenticated attackers can manipulate order totals downward — potentially to zero or negative amounts — on WordPress stores using Manual Checkout, resulting in direct financial loss to merchants.

Affected Products

Tips and Tricks HQ WordPress Simple PayPal Shopping Cart plugin, versions up to and including 5.1.3
WordPress sites running the plugin in Manual Checkout mode
Deployments identified by CPE cpe:2.3:a:tipsandtricks-hq:wordpress_simple_paypal_shopping_cart

Discovery Timeline

2025-05-01 - CVE-2025-3889 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-3889

Vulnerability Analysis

The flaw resides in the process_payment_data handler within wp_shopping_cart.php. The plugin accepts a client-supplied quantity parameter without enforcing that the value must be a positive integer. When the cart total is computed, the price is multiplied by this attacker-controlled quantity. Supplying a negative integer causes the multiplication to produce a negative subtotal, which is then added to the running order total. The result reduces the amount the customer owes at checkout.

Because the endpoint requires no authentication, any anonymous visitor can trigger the condition. PayPal and Stripe payment processors reject negative quantities during their own validation, which contains the impact to stores where the merchant has enabled Manual Checkout mode for offline or cash-on-delivery workflows.

Root Cause

The root cause is missing server-side validation of a user-controlled key that maps to product quantity. The plugin trusts a client-supplied reference and performs no bounds checking or sanity checking before it is used in a financial calculation. This matches the [CWE-639] pattern of Authorization Bypass Through User-Controlled Key, where an application relies on a request parameter that the attacker can freely modify.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker adds legitimate items to the cart and then submits a modified checkout request in which the quantity field for one or more line items is set to a negative integer. The plugin recalculates the order total using the negative multiplier and generates an invoice for a reduced or zero-dollar amount. If Manual Checkout is enabled, the malicious order proceeds to the merchant's manual fulfillment queue at the attacker-defined price.

See the WordPress Plugin Code Reference for the vulnerable code path and the Wordfence Vulnerability Report for the full technical writeup.

Detection Methods for CVE-2025-3889

Indicators of Compromise

Orders in the WordPress admin queue containing line items with negative quantity values
Order totals that are lower than the sum of individual item prices, or totals equal to zero or negative amounts
POST requests to the plugin's checkout endpoint containing quantity parameters with a leading minus sign
Manual Checkout orders received from unauthenticated sessions with mismatched cart contents

Detection Strategies

Enable verbose HTTP request logging on the WordPress site and review POST bodies submitted to process_payment_data for negative numeric values in quantity fields
Query the WordPress orders table for records where any line-item quantity is less than one
Compare the sum of price * quantity per line against the stored order total to identify arithmetic anomalies

Monitoring Recommendations

Alert on any completed order with a total below a defined minimum threshold
Monitor WordPress plugin version inventory to flag installations still running 5.1.3 or earlier
Correlate web application firewall logs with order records to identify checkout requests originating from anonymous IPs immediately preceding low-total orders

How to Mitigate CVE-2025-3889

Immediate Actions Required

Update the WordPress Simple Shopping Cart plugin to the version released after 5.1.3 that contains the fix from changeset 3284572
Audit recent Manual Checkout orders for negative-quantity line items and reconcile any fraudulent transactions
Temporarily disable Manual Checkout mode until patching is complete if the plugin cannot be updated immediately

Patch Information

The vendor addressed the vulnerability in the plugin repository via WordPress Change Set 3284572, which introduces validation on the quantity parameter within process_payment_data. Site administrators should apply the update through the WordPress plugin management interface or by downloading the patched release from the plugin's WordPress.org page.

Workarounds

Disable the Simple Shopping Cart plugin entirely until it can be updated to the patched release
Switch the store configuration from Manual Checkout to PayPal or Stripe, since those processors reject negative-quantity transactions
Deploy a web application firewall rule that blocks POST requests to checkout endpoints containing negative integer values in quantity parameters

bash

# Example ModSecurity rule to block negative quantity values at checkout
SecRule REQUEST_URI "@contains wp-admin/admin-ajax.php" \
    "chain,phase:2,deny,status:403,id:1003889,msg:'CVE-2025-3889 negative quantity blocked'"
    SecRule ARGS_NAMES "@rx (?i)quantity" \
        "chain"
        SecRule ARGS "@rx ^-[0-9]+$" "t:none"