CVE-2026-55791 Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs
Attackers can exploit Host header poisoning to trigger SSRF and inject arbitrary JavaScript, potentially leading to code execution or data theft.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55791?
CVE-2026-55791 is a medium-severity server-side request forgery vulnerability in craftcms cms, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Craft CMS is a content management system (CMS). Versions 4.0.0-RC1 and above, prior to 4.18.0 and 5.0.0-RC1, and above, prior to 5.10.0, are vulnerable to Server-Side Request Forgery (SSRF) and Arbitrary JavaScript Injection through the /actions/app/resource-js endpoint. By exploiting the default permissive trustedHosts configuration, an attacker can poison the Host or X-Forwarded-Host header to manipulate the application’s $baseUrl. This bypasses the endpoint’s internal URL validation, forcing the backend Guzzle client to fetch a malicious payload from an attacker-controlled server and reflect it to the client with a Content-Type: application/javascript header. The vulnerability manifests when assetManager.cacheSourcePaths is set to false. This issue has been fixed in versions 4.18.0 and 5.10.0.
Which versions are affected by CVE-2026-55791?
4 open source packages ship an affected version, including craftcms/cms.
| Package | Affected | Fixed in |
|---|---|---|
craftcms/cms Packagist | From 4.0.0-RC1 | 4.18 |
craftcms/cms Packagist | From 4.0.0-RC1 | 4.18 |
craftcms/cms Packagist | From 5.0.0-RC1 | 5.10 |
craftcms/cms Packagist | From 5.0.0-RC1 | 5.10 |
How do I fix CVE-2026-55791?
Upgrade craftcms/cms to a fixed version for your release line: 4.18 or 5.10.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-55791 being exploited?
There is no known exploitation of CVE-2026-55791 in the wild and no public exploit code.
Sources
Also known as GHSA-c55v-343g-5xff.