CVE-2026-112958 in ApostropheCMS
Summary
by MITRE • 10/20/2026
ApostropheCMS is an open-source Node.js content management system. Prior to 4.34.0, a project-defined free-form widget style field can carry an editor-controlled quote into an inline style attribute because the wrapper serializer does not apply HTML-attribute encoding. The quote can terminate the style attribute and inject executable attributes when published content, or an affected draft viewed by a higher-privileged user, is rendered. This issue is fixed in version 4.34.0.
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Analysis
by VulDB Data Team • 10/20/2026
ApostropheCMS, a widely used open-source content management system built on Node.js, contains a critical input validation flaw that allows for stored cross-site scripting attacks through its widget styling interface. The vulnerability exists within the project-defined free-form widget style field mechanism, which is designed to allow users to apply custom CSS styles to specific widgets. In versions prior to 4.34.0, the system fails to properly sanitize user-supplied input before embedding it into HTML attributes. Specifically, when a content editor enters an apostrophe or other quote character as part of their style definition, this value is passed directly into an inline style attribute without undergoing necessary HTML-attribute encoding by the wrapper serializer. This lack of sanitization creates a vector for injection attacks where malicious actors can manipulate the structure of the rendered HTML document.
The technical nature of this flaw constitutes a classic case of improper input validation leading to cross-site scripting, aligning with CWE-79: Improper Neutralization of Input During Web Page Generation Commonly Known as Cross-Site Scripting. Furthermore, because the attack involves injecting executable code into web pages viewed by other users, it falls under the MITRE ATT&CK technique T1059.007: JavaScript via Stored XSS. The vulnerability is particularly dangerous due to its stored nature; once a malicious style definition containing injected attributes or script tags is saved and published, it becomes part of the persistent content on the site. This means that every time a page with this widget is loaded by any user, including administrators or other high-privileged users who view drafts before publication, the embedded code executes within their browser context under the trust relationship established with the CMS domain.
The operational impact of this vulnerability extends beyond simple script execution to potential account takeover and data exfiltration if an administrator views a compromised draft. An attacker can terminate the intended style attribute using a quote character and inject new attributes such as onerror or onclick, which trigger JavaScript execution upon page load or user interaction. This allows for session hijacking, where attackers steal authentication cookies or tokens to impersonate legitimate users. It also enables phishing attacks by modifying the visual presentation of the site to trick users into entering credentials, or it can be used to deface the website's appearance and spread malware through drive-by downloads if combined with other vulnerabilities in client-side libraries. The risk is amplified because content editors may not have administrative privileges but can still introduce payloads that affect privileged users who review their work.
Mitigation strategies primarily involve upgrading to version 4.34.0 or later, where the wrapper serializer has been patched to apply proper HTML-attribute encoding to all user-supplied style data. This ensures that special characters like quotes are escaped and cannot be interpreted as structural delimiters by the browser's parser. For organizations unable to immediately upgrade, temporary mitigations include implementing strict input validation on the server side to reject any widget style definitions containing quote characters or other HTML metacharacters. Additionally, deploying a Web Application Firewall with rules tuned to detect and block XSS payloads in POST requests related to content editing can provide an additional layer of defense. Regular security audits of custom widgets and third-party modules are also recommended to ensure that similar encoding oversights do not exist elsewhere in the application's rendering pipeline.