CVE-2026-18547 in Ultimate Member Plugin
Summary
by MITRE • 08/25/2026
The Ultimate Member – User Profile, Registration, Login, Member Directory, Content Restriction & Membership Plugin plugin for WordPress is vulnerable to Stored Cross-Site Scripting via Textarea Profile Field with HTML Support (DOM Gadget via id Attribute) in all versions up to, and including, 2.12.1 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with subscriber-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. This is possible because the wp_kses 'templates' allowlist permits the id attribute on div elements but does not neutralize unescaped quotes within its value; when pickadate.js concatenates the stored id value into an HTML string via jQuery .html() on profile page load, the smuggled onfocus and autofocus attribute syntax breaks out of attribute context and executes as JavaScript.
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Analysis
by VulDB Data Team • 08/25/2026
The Ultimate Member plugin for WordPress is a widely used solution that facilitates user registration, login processes, member directories, content restriction mechanisms, and comprehensive membership management features within the WordPress ecosystem. A critical security vulnerability has been identified in all versions up to and including 2.12.1, specifically classified as a Stored Cross-Site Scripting (XSS) flaw. This vulnerability arises from insufficient input sanitization and output escaping practices embedded within the plugin's architecture, particularly concerning how it handles user-supplied data for profile fields that support HTML content. The severity of this issue is compounded by its stored nature, meaning that malicious payloads are saved to the database rather than being executed immediately upon request submission, allowing them to persist and affect multiple users over time.
The technical root cause of this vulnerability lies in the interaction between WordPress's sanitization functions and specific JavaScript libraries used for date picking functionality. Specifically, the wp_kses function is configured with a templates allowlist that explicitly permits the id attribute on div elements. However, while this allows certain structural attributes to pass through validation, it fails to properly neutralize unescaped quotes within the value of these attributes. When an authenticated attacker with subscriber-level access or higher creates or modifies their profile, they can inject malicious HTML into a textarea field designated for such content. Because the system trusts the id attribute context without rigorous quote escaping, attackers can craft payloads that break out of the intended attribute scope.
The exploitation mechanism relies on the concatenation of stored data by the pickadate.js library via jQuery's .html() method during profile page loads. When a victim user views the attacker's profile or interacts with pages containing this tainted data, the browser parses the injected HTML string. The crafted payload utilizes specific syntax involving onfocus and autofocus attributes to trigger JavaScript execution. By embedding these event handlers within an element that is automatically focused upon rendering, such as through the use of the autofocus attribute, the malicious script executes without requiring any further user interaction beyond viewing the page. This technique effectively bypasses standard browser security models because the code runs in the context of the victim's session and domain.
From a classification perspective, this vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The specific mechanism involving DOM manipulation via attribute injection also relates to CWE-80, although the primary vector is stored XSS rather than reflected or local variants. In terms of offensive security frameworks, this attack pattern corresponds to MITRE ATT&CK technique T1059.007, which covers JavaScript execution within web browsers. The ability for low-privileged users to execute arbitrary scripts represents a significant escalation of privilege in the context of application logic, even if it does not directly compromise server-side infrastructure.
The operational impact of this vulnerability is substantial due to its stored nature and broad applicability across all affected versions. Attackers can deploy persistent malicious code that targets any user who views the compromised profile or interacts with pages containing the tainted data. This can lead to session hijacking, where attackers steal authentication cookies to impersonate legitimate users. It also enables credential theft through fake login forms injected into the page context, phishing attacks against other community members, and defacement of site content. Furthermore, because the execution is triggered automatically via autofocus attributes, social engineering efforts are minimized as victims do not need to click on links or buttons for the attack to succeed.
Mitigation strategies must address both immediate remediation and long-term defensive coding practices. The most effective solution is to upgrade the Ultimate Member plugin to a version later than 2.12.1 where this specific input sanitization flaw has been patched by the developers. Until an update is available, administrators should consider restricting HTML capabilities in profile fields or implementing additional server-side validation that strictly enforces quote escaping for all attribute values before they are stored in the database. Additionally, deploying a Web Application Firewall with rules capable of detecting and blocking payloads containing onfocus events within id attributes can provide a layer of defense against exploitation attempts from external sources.
To prevent similar vulnerabilities in future development cycles, it is essential to adopt secure coding standards that prioritize output encoding over input filtering alone. Developers should ensure that all dynamic content inserted into HTML contexts using JavaScript methods like .html() undergoes proper escaping for the specific context, whether that be attribute values, script blocks, or event handlers. Utilizing Content Security Policy headers can also mitigate the impact of successful XSS attacks by restricting the sources from which scripts are allowed to load and execute, thereby reducing the effectiveness of injected payloads even if they bypass initial sanitization checks.