CVE-2026-26993 is a stored Cross-Site Scripting (XSS) vulnerability classified under CWE-79 affecting FlintSH's Flare platform, versions 1.7.0 and earlier. Flare is a Next.js-based, self-hostable file sharing solution that integrates with screenshot tools. The vulnerability stems from insufficient sanitization and validation of user-uploaded files containing active content formats such as SVG, HTML, or XML. Attackers can embed malicious JavaScript payloads within these files. When a victim with authenticated access views the file in “raw” mode, the embedded script executes in the context of the application’s origin, enabling potential theft of sensitive user data or session tokens. The attack vector requires the attacker to have at least limited privileges to upload files and relies on user interaction to trigger the malicious script. The flaw does not affect availability but impacts confidentiality and integrity. The issue was addressed in Flare version 1.7.1 by implementing proper content validation and sanitization mechanisms to neutralize active content before rendering. No public exploits have been reported to date, but the vulnerability poses a risk to any deployment of affected versions, especially in environments where multiple users share files and trust is implicit.

The primary impact of this vulnerability is the potential compromise of user confidentiality and integrity within affected Flare deployments. Successful exploitation allows attackers to execute arbitrary JavaScript in the victim’s browser under the application’s origin, which can lead to session hijacking, theft of sensitive data, or unauthorized actions performed on behalf of the user. Since Flare is a file sharing platform, the risk extends to any organization using it internally or externally for collaboration, potentially exposing sensitive files or user credentials. Although the vulnerability does not affect system availability, the breach of trust and data leakage can have significant reputational and operational consequences. Organizations with multi-tenant or public-facing Flare instances are at higher risk. The requirement for authenticated upload and user interaction limits the attack surface but does not eliminate it, especially in environments with many users or less stringent access controls.

Organizations should immediately upgrade all FlintSH Flare instances to version 1.7.1 or later, where the vulnerability is patched. In addition to patching, administrators should implement strict access controls to limit file upload permissions to trusted users only. Employ content security policies (CSP) to restrict script execution contexts and reduce the impact of potential XSS attacks. Disable or restrict the use of “raw” mode file viewing if possible, or sanitize files before rendering them in this mode. Conduct regular security audits and penetration testing focusing on file upload functionalities. Educate users about the risks of interacting with untrusted files and encourage reporting of suspicious content. Finally, monitor logs for unusual file uploads or access patterns that may indicate exploitation attempts.