CVE-2026-31876 is a stored Cross-Site Scripting (XSS) vulnerability identified in the Notesnook note-taking application, specifically in versions prior to 3.3.9. The vulnerability arises from improper neutralization of user input during web page generation, classified under CWE-79. The root cause is in the tweetToEmbed() function within the editor embed component (component.tsx), which processes user-supplied Twitter/X embed URLs. Instead of sanitizing or escaping these URLs, the function directly interpolates them into an HTML string that is assigned to the srcdoc attribute of an iframe element. Since srcdoc allows embedding HTML content directly inside an iframe, this unsanitized input can contain malicious JavaScript code. When the iframe renders this content, the malicious script executes in the context of the Notesnook application, potentially allowing attackers to steal sensitive data, manipulate notes, or perform actions on behalf of the user. Exploitation requires the attacker to have at least limited privileges (PR:L) and user interaction (UI:R), such as convincing a user to open a crafted note containing the malicious embed URL. The vulnerability has a CVSS v3.1 base score of 5.4, reflecting medium severity, with network attack vector (AV:N), low attack complexity (AC:L), partial confidentiality and integrity impact (C:L/I:L), no availability impact (A:N), and scope change (S:C). The flaw is fixed in Notesnook version 3.3.9 by properly escaping or sanitizing the user input before embedding it in the iframe srcdoc attribute. No known exploits have been reported in the wild as of the publication date.

The primary impact of this vulnerability is the potential execution of arbitrary JavaScript code within the Notesnook application context, which can lead to partial compromise of user confidentiality and integrity. Attackers could steal sensitive note contents, session tokens, or other private information stored or accessible within the app. They might also manipulate note data or perform unauthorized actions on behalf of the user. Since Notesnook is a privacy-focused note-taking app, such breaches undermine user trust and privacy guarantees. The vulnerability does not affect availability, so denial-of-service is unlikely. Given the requirement for limited privileges and user interaction, exploitation is somewhat constrained but still poses a significant risk, especially in environments where users share notes or embed external content frequently. Organizations using Notesnook for sensitive or regulated data storage could face compliance and reputational risks if exploited. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as the vulnerability is publicly disclosed.

To mitigate this vulnerability, organizations and users should upgrade Notesnook to version 3.3.9 or later, where the issue is fixed. Until upgrading, users should avoid opening notes containing embedded Twitter/X URLs from untrusted or unknown sources. Developers and security teams should audit any custom or third-party embed components for proper input sanitization and escaping, especially when using iframe srcdoc attributes. Implement Content Security Policy (CSP) headers to restrict script execution and reduce the impact of potential XSS attacks. Employ runtime application self-protection (RASP) or endpoint detection to monitor for suspicious script execution within the app. Regularly review and update dependencies and third-party components to ensure known vulnerabilities are patched promptly. Educate users about the risks of opening notes with embedded external content and encourage cautious behavior. Finally, consider sandboxing or isolating embedded content to limit the scope of script execution.