CVE-2026-27902 is a cross-site scripting (XSS) vulnerability classified under CWE-79 that affects the Svelte JavaScript framework, specifically versions from 5.53.0 up to but not including 5.53.5. The vulnerability stems from improper neutralization of input during web page generation: errors returned by the transformError function were not correctly escaped before being embedded into the HTML output. This flaw allows an attacker to inject malicious HTML or JavaScript code if they can influence the content returned by transformError. Since Svelte is a performance-oriented framework used for building reactive web applications, this vulnerability can be exploited when an attacker causes an error condition that includes attacker-controlled input, which then gets reflected in the rendered HTML without proper escaping. The CVSS 4.0 score is 5.3 (medium severity), reflecting that the attack vector is network-based but requires user interaction and has high attack complexity. No privileges are required, but the vulnerability scope is limited to client-side confidentiality due to the reflected XSS nature. The vulnerability was publicly disclosed on February 26, 2026, and fixed in version 5.53.5. No known exploits are currently in the wild. This vulnerability primarily impacts developers and organizations that build web applications using the affected Svelte versions, potentially exposing end users to script injection attacks that can lead to session hijacking, data theft, or defacement.

The primary impact of CVE-2026-27902 is on the confidentiality and integrity of client-side data in web applications built with vulnerable Svelte versions. Successful exploitation can allow attackers to execute arbitrary JavaScript in the context of the victim's browser, potentially leading to session hijacking, theft of sensitive information such as cookies or tokens, and unauthorized actions performed on behalf of the user. While the vulnerability does not directly affect server-side systems or availability, the reputational damage and loss of user trust can be significant for organizations. The requirement for user interaction and high attack complexity somewhat limits widespread exploitation, but targeted attacks against high-value users or administrators remain a concern. Organizations relying on Svelte for frontend development may face increased risk if they do not promptly update or implement mitigations, especially those with public-facing web applications handling sensitive user data.

To mitigate CVE-2026-27902, organizations should immediately upgrade all Svelte dependencies to version 5.53.5 or later, where the vulnerability is patched. Developers should audit their code to ensure that any error messages or dynamic content derived from transformError or similar functions are properly escaped or sanitized before being embedded in HTML output. Employing Content Security Policy (CSP) headers can help reduce the impact of potential XSS by restricting the execution of unauthorized scripts. Additionally, implementing strict input validation and output encoding practices throughout the application will reduce the risk of injection flaws. Security teams should monitor for unusual client-side behavior or reports of script injection. Finally, educating developers on secure coding practices and regularly updating third-party dependencies will help prevent similar vulnerabilities.