# 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.

- **Severity:** Medium 5.1 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Jun 2026
- **Updated:** 9 Jun 2026
- **Also known as:** CLSA-2026-1784077486, CLSA-2026-1784769502, CLSA-2026-1784769535, CLSA-2026-1784769620, GHSA-pr6f-5x2q-rwfp

## 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 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-42599?

2 open source packages ship an affected version, including svelte. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `svelte` | npm | All versions | 5.55.7 |
| `svelte` | npm | All versions | 5.55.7 |

Products: Svelte Svelte.

## How do I fix CVE-2026-42599?

Upgrade svelte to 5.55.7 or later.

## 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](https://insights.sbomify.com/CVE-2021-29261/): Code injection in n/a (high 7.8)
- [CVE-2026-42567](https://insights.sbomify.com/CVE-2026-42567/): Svelte: ReDoS in `<svelte:element>` Tag Validation (medium 5.9)
- [CVE-2024-45047](https://insights.sbomify.com/CVE-2024-45047/): Potential mXSS vulnerability due to improper HTML escaping in svelte (medium 5.4)
- [CVE-2022-25875](https://insights.sbomify.com/CVE-2022-25875/): Cross-site Scripting (XSS) (medium 5.4)
- [CVE-2026-42573](https://insights.sbomify.com/CVE-2026-42573/): Svelte: XSS via DOM Clobbering of Internal Framework State (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42599 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-42599 in the wild and no public exploit code.

Affected packages and fixed versions:
- svelte (npm): affected all versions; fixed in 5.55.7
- svelte (npm): affected all versions; fixed in 5.55.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-42599/ (Markdown: https://insights.sbomify.com/CVE-2026-42599/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42599
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42599
- OSV: https://osv.dev/vulnerability/CVE-2026-42599

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42599/
Summaries are generated and may contain mistakes; the official description above is the record.