CVE-2026-42599 Cross-site scripting via spread attributes in Svelte SSR
Attackers can inject malicious event handlers via spread attributes in Svelte SSR, leading to code execution in victims' browsers.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-42599?
CVE-2026-42599 is a medium-severity cross-site scripting vulnerability in sveltejs svelte, published on 9 Jun 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Official description
Svelte is a performance oriented web framework. Prior to version 5.55.7, when using spread syntax to render attributes from untrusted data, event handler properties are included in the rendered HTML output. If an application spreads user-controlled or external data as element attributes, an attacker can inject malicious event handlers that execute in victims' browsers. Note that this vulnerability only triggers if the user's browser has JavaScript enabled but Svelte's hydration mechanism does not reach the vulnerable element before the event fires. This issue has been patched in version 5.55.7.
Weakness: CWE-79
Which versions are affected by CVE-2026-42599?
2 open source packages ship an affected version, including svelte. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
svelte npm | All versions | 5.55.7 |
svelte npm | All versions | 5.55.7 |
Products include Svelte Svelte.
How do I fix CVE-2026-42599?
Upgrade svelte to 5.55.7 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-42599 being exploited?
There is no known exploitation of CVE-2026-42599 in the wild and no public exploit code.
Other vulnerabilities in sveltejs svelte
- CVE-2021-29261 Code injection in n/a
- CVE-2026-42567 Svelte: ReDoS in `<svelte:element>` Tag Validation
- CVE-2024-45047 Potential mXSS vulnerability due to improper HTML escaping in svelte
- CVE-2022-25875 Cross-site Scripting (XSS)
- CVE-2026-42573 Svelte: XSS via DOM Clobbering of Internal Framework State
Sources
Also known as CLSA-2026-1784077486, CLSA-2026-1784769502, CLSA-2026-1784769535, CLSA-2026-1784769620, GHSA-pr6f-5x2q-rwfp.