CVE-2026-27119 identifies a cross-site scripting vulnerability in the Svelte JavaScript framework, specifically affecting server-side rendering output of <option> elements. Between versions 5.39.3 and 5.51.4, the framework fails to properly escape content within these elements during SSR, leading to improper neutralization of input (CWE-79). This flaw allows an attacker with sufficient privileges to inject arbitrary HTML into the SSR output, potentially leading to HTML injection attacks. Since client-side rendering is unaffected, the vulnerability is limited to SSR contexts. The vulnerability requires high attack complexity, privileges, and authentication, and does not require user interaction. The CVSS 4.0 vector indicates network attack vector, high complexity, privileged attacker, and low impact on confidentiality, integrity, and availability. The vulnerability was published on February 20, 2026, and fixed in version 5.51.5 of Svelte. No known exploits have been reported, but the flaw could be leveraged in environments where SSR is used to generate dynamic option elements with untrusted input.

The primary impact of this vulnerability is the potential for HTML injection via SSR output, which could lead to cross-site scripting attacks if exploited. This can compromise the integrity of web pages rendered by the server, potentially allowing attackers to execute malicious scripts in the context of the affected web application. While the impact on confidentiality and availability is low, the integrity and trustworthiness of the web content can be undermined. Organizations relying on Svelte SSR for dynamic content generation, especially those rendering user-supplied data within <option> elements, are at risk. Exploitation requires authenticated, privileged access, limiting the threat to insider attackers or compromised accounts. However, successful exploitation could facilitate further attacks such as session hijacking or defacement. The absence of known exploits reduces immediate risk but does not eliminate the potential for future attacks.

1. Upgrade all Svelte instances to version 5.51.5 or later, where this vulnerability is patched. 2. Review server-side rendering code to ensure that all user-supplied input is properly sanitized and escaped before rendering, especially within <option> elements. 3. Implement strict Content Security Policy (CSP) headers to reduce the impact of potential XSS attacks. 4. Limit privileges and access controls to reduce the risk of attackers gaining the necessary authentication and privileges to exploit this vulnerability. 5. Conduct code audits and penetration testing focused on SSR output to detect any residual injection issues. 6. Monitor application logs and user activity for suspicious behavior indicative of attempted exploitation. 7. Educate developers on secure coding practices related to SSR and input handling in Svelte.