Simple Membership Plugin Stored XSS via Stripe Webhook

Compliance Impact

The provided information does not specify how this vulnerability impacts compliance with common standards and regulations such as GDPR or HIPAA.

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Executive Summary

The Simple Membership WordPress plugin before version 4.7.5 has a vulnerability that allows unauthenticated attackers to perform a stored cross-site scripting (XSS) attack via the Stripe webhook API.

This happens because the plugin does not verify the authenticity of Stripe webhook requests when no signing secret is configured, and it fails to escape a value taken from these requests before displaying it in an administrator notice.

An attacker can send a forged Stripe webhook containing malicious JavaScript code in the api_version field, which gets stored and later executed when an administrator views any WordPress admin page, allowing arbitrary scripts to run in the context of a logged-in administrator.

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Impact Analysis

This vulnerability can allow an unauthenticated attacker to inject and execute arbitrary JavaScript code in the browser of a logged-in administrator.

Such an attack can lead to unauthorized actions being performed with administrator privileges, including stealing sensitive information, modifying site settings, or installing malicious software.

Because the attack is stored and triggers whenever an admin views the WordPress admin pages, it can persist and affect multiple administrators.

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Detection Guidance

This vulnerability can be detected by checking if your WordPress installation is running the Simple Membership plugin version before 4.7.5 and if it is configured without a Stripe webhook signing secret.

To confirm exploitation, you can look for suspicious Stripe webhook requests containing malicious payloads in the api_version field that trigger JavaScript execution in administrator notices.

While no specific commands are provided, you can monitor incoming webhook requests to your WordPress site for unusual or forged Stripe webhook payloads, especially those containing script tags or JavaScript code in the api_version field.

Additionally, reviewing the WordPress admin notices for unexpected alerts or script execution when logged in as an administrator can help detect exploitation.

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Mitigation Strategies

The immediate mitigation step is to update the Simple Membership plugin to version 4.7.5 or later, where this vulnerability has been fixed.

If updating immediately is not possible, ensure that a Stripe webhook signing secret is configured to verify the authenticity of incoming webhook requests.

Also, monitor and restrict access to the WordPress admin area to trusted users only, as the vulnerability allows script execution in the context of logged-in administrators.

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