CVE-2025-15265: Svelte 5.46.0 - Hydratable Key Script-Breakout XSS (SSR)
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 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.
Other sources
An SSR XSS exists in async hydration when attacker‑controlled keys are passed to hydratable. The key is embedded inside a <script> block without HTML‑safe escaping, allowing </script> to terminate the script and inject arbitrary JavaScript. This enables remote script execution in users' browsers, with potential for session theft and account compromise. This issue affects Svelte: from 5.46.0 before 5.46.3.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2025-15265?
CVE-2025-15265 is classified as a high severity XSS vulnerability that enables arbitrary JavaScript injection in server-rendered HTML.
How do I fix CVE-2025-15265?
To mitigate CVE-2025-15265, upgrade Svelte to version 5.46.4 or later.
Which versions of Svelte are affected by CVE-2025-15265?
Svelte versions from 5.46.0 to 5.46.3 are affected by CVE-2025-15265.
What type of vulnerability is CVE-2025-15265?
CVE-2025-15265 is a Cross-Site Scripting (XSS) vulnerability caused by improper escaping of hydratable keys.
How does CVE-2025-15265 impact web applications?
CVE-2025-15265 allows attackers to inject and execute arbitrary JavaScript in web applications using the affected Svelte versions.