# 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.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 Jul 2026
- **Updated:** 2 Jul 2026
- **Also known as:** GHSA-c55v-343g-5xff

## 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.

Weakness: CWE-918 (Server-Side Request Forgery), CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-644 (Improper Neutralization of HTTP Headers for Scripting Syntax)

## Which versions are affected by CVE-2026-55791?

4 open source packages ship an affected version, including craftcms/cms.


| Package | Ecosystem | 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.

## Is CVE-2026-55791 being exploited?

There is no known exploitation of CVE-2026-55791 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55791 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-55791 in the wild and no public exploit code.

Affected packages and fixed versions:
- craftcms/cms (Packagist): affected from 4.0.0-RC1; fixed in 4.18
- craftcms/cms (Packagist): affected from 4.0.0-RC1; fixed in 4.18
- craftcms/cms (Packagist): affected from 5.0.0-RC1; fixed in 5.10
- craftcms/cms (Packagist): affected from 5.0.0-RC1; fixed in 5.10

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-2026-55791/ (Markdown: https://insights.sbomify.com/CVE-2026-55791/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55791
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55791
- OSV: https://osv.dev/vulnerability/CVE-2026-55791

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55791/
Summaries are generated and may contain mistakes; the official description above is the record.