CVE-2026-27902 in svelte
Summary
by MITRE • 02/26/2026
Svelte performance oriented web framework. Prior to version 5.53.5, errors from `transformError` were not correctly escaped prior to being embedded in the HTML output, causing potential HTML injection and XSS if attacker-controlled content is returned from `transformError`. Version 5.53.5 fixes the issue.
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Analysis
by VulDB Data Team • 03/05/2026
The vulnerability described in CVE-2026-27902 affects the Svelte web framework, specifically impacting versions prior to 5.53.5 where improper handling of error messages during the transformation process creates a significant security risk. This issue stems from the framework's failure to properly escape error content generated by the `transformError` function before incorporating it into HTML output, which directly exposes applications to cross-site scripting attacks. The vulnerability represents a classic HTML injection flaw that can be exploited when attacker-controlled data flows through the error handling pipeline, particularly in scenarios where error messages might contain untrusted input from users or external sources.
The technical flaw manifests in the way Svelte processes error messages within its development and production environments, where the `transformError` function is responsible for formatting error details for display. When this function processes input that contains HTML or script content, the framework fails to sanitize or escape these elements before embedding them directly into the HTML output. This creates a vector for malicious actors to inject arbitrary HTML or JavaScript code that will execute in the context of other users' browsers. The vulnerability is particularly concerning because it operates at the framework level, meaning that any application built with affected Svelte versions could be compromised regardless of the application-specific security measures in place. This aligns with CWE-79, which describes improper neutralization of input during web page generation, and represents a direct violation of secure coding principles that require proper output encoding in web applications.
The operational impact of this vulnerability extends beyond simple XSS exploitation, as it can enable attackers to perform a wide range of malicious activities including session hijacking, data theft, and privilege escalation within affected applications. When attacker-controlled content flows through the `transformError` function, it can be rendered as executable script in the browser, potentially allowing unauthorized access to user sessions or sensitive application data. The vulnerability is particularly dangerous in development environments where error messages might contain sensitive information about the application's internal structure, database schema, or configuration details. Additionally, the issue affects both development and production deployments, making it a comprehensive security concern that requires immediate attention across all Svelte applications. This vulnerability can be mapped to ATT&CK technique T1211 which involves the exploitation of vulnerabilities in web applications to execute malicious code.
The remediation for CVE-2026-27902 requires upgrading to Svelte version 5.53.5 or later, where the framework properly implements HTML escaping for error content generated by `transformError`. This update ensures that any potentially malicious input contained within error messages is properly sanitized before being embedded in HTML output, preventing the execution of unauthorized scripts. Organizations should conduct thorough testing of their applications after applying the patch to ensure that error handling continues to function correctly while maintaining security. The fix demonstrates the importance of proper output encoding in web applications, particularly when dealing with error messages that may contain untrusted data. Security teams should also implement monitoring for error handling patterns in their applications to detect potential exploitation attempts and ensure that all components are updated to secure versions. The vulnerability serves as a reminder of the critical importance of input validation and output encoding in preventing web-based attacks, and highlights the need for comprehensive security testing throughout the application development lifecycle.