svelte vulnerable to Cross-site Scripting
Date: 2026-01
Affected: npm/svelte
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2025-15265
CWE: CWE-79

## Summary An XSS vulnerability exists in Svelte 5.46.0-2 resulting from improper escaping of `hydratable` keys. If these keys incorporate untrusted user input, arbitrary JavaScript can be injected into server-rendered HTML. ## Details When using the [`hydratable`](https://svelte.dev/docs/svelte/hydratable) function, the first argument is used as a key to uniquely identify the data, such that the value is not regenerated in the browser. This key is embedded into a `<script>` block in the server-rendered `<head>` without escaping unsafe characters. A malicious key can break out of the script context and inject arbitrary JavaScript into the HTML response. ## Impact This is a cross-site scripting vulnerability affecting applications that have the `experimental.async` flag enabled and use `hydratable` with keys incorporating untrusted user input. - **Impact**: Arbitrary JS execution in the client’s browser. - **Exploitability**: Remote, single-request if key is attacker-controlled. - **Typical Outcomes**: - Session/token theft - DOM defacement - CSRF bypass via injected JS - Account takeover depending on cookie/session strategy Affected applications should upgrade to a patched version immediately.


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Architectural layers (MAESTRO):