CVE-2026-27122 in svelte
Summary
by MITRE • 02/21/2026
svelte performance oriented web framework. Prior to 5.51.5, when using <svelte:element this={tag}> in server-side rendering, the provided tag name is not validated or sanitized before being emitted into the HTML output. If the tag string contains unexpected characters, it can result in HTML injection in the SSR output. Client-side rendering is not affected. This vulnerability is fixed in 5.51.5.
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Analysis
by VulDB Data Team • 02/22/2026
The vulnerability identified as CVE-2026-27122 affects the svelte web framework version 5.51.4 and earlier, specifically within the server-side rendering functionality of the framework. This issue arises when developers utilize the <svelte:element this={tag}> directive in their server-side rendered applications, creating a potential security risk that could be exploited by malicious actors. The vulnerability represents a critical flaw in the framework's handling of dynamic element creation during server-side processing, where the framework fails to properly validate or sanitize user-provided tag names before incorporating them into the HTML output.
The technical flaw stems from the absence of input validation mechanisms within the server-side rendering pipeline when processing dynamic tag names through the svelte:element directive. When a developer passes a tag parameter to this directive, the framework directly incorporates the provided string into the generated HTML without any sanitization or validation checks. This oversight allows for the injection of arbitrary HTML content into the server-side rendered output, potentially enabling attackers to inject malicious scripts or manipulate the structure of rendered pages. The vulnerability specifically impacts the server-side rendering context and does not affect client-side rendering operations, which maintain proper validation mechanisms.
The operational impact of this vulnerability is significant as it could enable attackers to perform HTML injection attacks against server-side rendered svelte applications. An attacker who can influence the tag parameter passed to the svelte:element directive could inject malicious HTML or script content that would be executed in the context of the rendered page. This could lead to various security consequences including cross-site scripting attacks, content manipulation, or potential data exfiltration from server-side rendered applications. The vulnerability is particularly concerning because it operates at the rendering layer where the framework generates the initial HTML output, making it difficult to detect and potentially allowing for widespread impact across applications using affected versions.
The remediation for this vulnerability requires updating to svelte version 5.51.5 or later, which implements proper input validation and sanitization for tag names used in server-side rendering contexts. Organizations should prioritize this update as a critical security measure and conduct thorough testing to ensure that the update does not introduce any regressions in their applications. Security teams should also implement monitoring for any potential exploitation attempts and review their server-side rendering code to identify any other potential injection vectors that might exist within their applications.
This vulnerability aligns with CWE-79 which describes Cross-Site Scripting (XSS) vulnerabilities that occur when untrusted data is included in web pages without proper validation or sanitization. The issue also maps to ATT&CK technique T1203 which involves the exploitation of vulnerabilities to gain access to systems or data, specifically through server-side injection attacks. The vulnerability demonstrates a classic case of insufficient input validation in server-side rendering contexts, where the framework fails to properly sanitize user-provided parameters before incorporating them into generated content. Organizations should consider implementing additional security controls such as content security policies and regular security assessments to detect and prevent similar vulnerabilities in their applications.