CVE-2026-103899 in EU VAT Assistant for WooCommerce Plugin
Summary
by MITRE • 10/17/2026
The EU VAT Assistant for WooCommerce plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'vat_number' parameter in all versions up to, and including, 2.1.30.260413 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The stored payload executes in the administrator's browser context when reviewing an order via WooCommerce → Orders → Edit Order, enabling privilege escalation up to and including full site takeover via rogue administrator account creation.
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Analysis
by VulDB Data Team • 10/17/2026
The EU VAT Assistant for WooCommerce plugin represents a critical component in e-commerce infrastructure built on WordPress, facilitating compliance with European Union value-added tax regulations by validating customer VAT numbers during checkout processes. The vulnerability identified within versions up to 2.1.30.260413 stems from fundamental flaws in input validation and output encoding mechanisms specifically affecting the vat_number parameter. This architectural weakness allows unauthenticated remote attackers to inject malicious JavaScript payloads directly into the database through standard user-facing interfaces, such as the checkout page or account registration forms. The core technical failure lies in the application's inability to properly sanitize incoming data upon receipt and its subsequent failure to escape this data when rendering it within administrative interface pages. This dual deficiency creates a classic stored cross-site scripting environment where malicious content persists on the server side rather than being transiently passed via URL parameters or headers, thereby increasing the attack surface significantly by targeting any administrator who views specific dynamic content.
From an operational perspective, the impact of this vulnerability is severe due to its persistent nature and high-privilege execution context. Unlike reflected XSS attacks that require social engineering tricks like phishing links, stored vulnerabilities automatically execute when legitimate users access affected pages. In this specific case, the malicious script triggers whenever a site administrator navigates to the WooCommerce orders section to edit or review an order containing the tainted VAT number field. Because administrative interfaces typically possess elevated privileges and often include functionality for managing user accounts, plugin settings, and core configurations, the execution of arbitrary JavaScript in this context effectively grants the attacker control over the browser session of the privileged user. This capability bypasses standard authentication mechanisms since the script runs with the same permissions as the logged-in administrator, allowing for actions such as changing passwords, modifying site options, or creating new administrative accounts without detection by traditional security controls that monitor login attempts rather than client-side behavior.
The exploitation path leads directly to privilege escalation and potential full site takeover through several distinct mechanisms enabled by the stored payload. Attackers can utilize JavaScript Object Notation (JSON) endpoints exposed by WordPress core or WooCommerce plugins to perform state-changing actions on behalf of the administrator, a technique categorized under CWE-79 Improper Neutralization of Input During Web Page Generation in industry standards. Furthermore, the ability to create rogue administrator accounts provides persistent access even if initial credentials are rotated, aligning with ATT&CK techniques related to Account Manipulation and Persistence. The attacker can also exfiltrate sensitive data including customer personally identifiable information (PII), payment details stored within order metadata, or internal site configurations by sending this data to an external server controlled by the adversary. This scenario highlights the critical risk associated with insufficient output escaping in administrative contexts where trust is implicitly placed in the integrity of displayed content.
Mitigation strategies must address both immediate remediation and long-term defensive posture improvements. The primary corrective action involves upgrading the EU VAT Assistant plugin to a version later than 2.1.30.260413, which includes patches for input sanitization and output escaping functions. For organizations unable to patch immediately due to compatibility constraints or deployment cycles, temporary mitigations should include implementing Web Application Firewall rules that detect and block common XSS payload patterns within the vat_number parameter during HTTP POST requests. Additionally, enabling Content Security Policy headers with strict directives can limit the execution of inline scripts, thereby reducing the effectiveness of injected payloads even if they are successfully stored in the database. Developers reviewing similar codebases should enforce rigorous input validation using allowlists for expected data formats and ensure that all dynamic content rendered in HTML contexts is properly escaped according to OWASP guidelines. Regular security audits focusing on admin panel interactions and automated testing pipelines incorporating static analysis tools can further prevent such vulnerabilities from reaching production environments, ensuring the integrity of e-commerce operations and protecting sensitive customer data from unauthorized access or manipulation.