CVE-2025-3889 is a medium-severity vulnerability affecting the WordPress Simple Shopping Cart plugin developed by mra13, present in all versions up to and including 5.1.3. The vulnerability is classified as CWE-639: Authorization Bypass Through User-Controlled Key, specifically an Insecure Direct Object Reference (IDOR) flaw. The root cause lies in the 'process_payment_data' function, which fails to properly validate a user-controlled key parameter. This lack of validation allows unauthenticated attackers to manipulate the quantity of products in the shopping cart, including setting the quantity to a negative number. Exploiting this flaw enables attackers to reduce the total order cost by subtracting product costs, effectively allowing them to pay less than the intended amount or potentially receive funds if the system processes negative totals. However, the attack is limited to the plugin's Manual Checkout mode, as payment gateways like PayPal and Stripe inherently reject transactions with negative quantities. The vulnerability does not impact confidentiality or availability but compromises the integrity of transaction data. No authentication or user interaction is required, and the attack can be performed remotely over the network. As of the publication date, no known exploits have been observed in the wild, and no official patches have been released. The CVSS v3.1 base score is 5.3, reflecting a medium severity level, with an attack vector of network, low attack complexity, no privileges required, and no user interaction needed. The scope remains unchanged, and the impact is limited to integrity loss without affecting confidentiality or availability.

For European organizations using the WordPress Simple Shopping Cart plugin, this vulnerability poses a financial risk by enabling unauthorized manipulation of order totals. E-commerce sites relying on Manual Checkout mode are particularly vulnerable, as attackers can exploit this flaw to reduce payment amounts or potentially cause accounting discrepancies. Although the impact does not extend to data confidentiality or system availability, the integrity breach can lead to revenue loss, fraud, and undermined customer trust. Organizations with high transaction volumes or those operating in sectors with tight financial controls (e.g., retail, hospitality) may face significant operational and reputational damage. Additionally, the lack of authentication requirement means attackers can exploit the vulnerability at scale, potentially leading to widespread financial impact. The absence of known exploits in the wild suggests limited current threat activity, but the vulnerability remains a latent risk until patched. European businesses must also consider compliance implications, as financial inaccuracies could affect regulatory reporting and audit requirements under frameworks like GDPR and PCI DSS.

1. Immediate mitigation involves disabling Manual Checkout mode in the WordPress Simple Shopping Cart plugin to prevent exploitation, switching to payment gateways like PayPal or Stripe that inherently reject negative quantities. 2. Implement strict input validation and sanitization on all user-controlled parameters, particularly those affecting payment calculations, to prevent negative or otherwise invalid values. 3. Monitor transaction logs for anomalous orders with negative or zero totals and establish alerts for suspicious activity. 4. Restrict access to the 'process_payment_data' endpoint using web application firewalls (WAFs) with custom rules to detect and block requests containing negative quantity parameters. 5. Regularly update the plugin once an official patch is released by the vendor. 6. Conduct code reviews and penetration testing focused on authorization and input validation controls within e-commerce plugins. 7. Educate development and operations teams about the risks of IDOR vulnerabilities and the importance of secure coding practices. 8. Consider implementing additional payment verification steps or manual reviews for orders with unusual totals until the vulnerability is resolved.