How severe is GHSA-c55v-343g-5xff?

No CVSS score has been assigned to GHSA-c55v-343g-5xff yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-c55v-343g-5xff?

GHSA-c55v-343g-5xff affects the following packages: craftcms/cms (Packagist), craftcms/cms (Packagist). Ecosystems affected: Packagist.

How do I fix GHSA-c55v-343g-5xff?

Update craftcms/cms to 5.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c55v-343g-5xff is resolved across your whole dependency graph.

How do I detect GHSA-c55v-343g-5xff in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for craftcms/cms. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate GHSA-c55v-343g-5xff if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against GHSA-c55v-343g-5xff?

O3 pinpoints whether GHSA-c55v-343g-5xff is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is GHSA-c55v-343g-5xff actively exploited in the wild?

No public exploit code has been indexed for GHSA-c55v-343g-5xff yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is GHSA-c55v-343g-5xff?

GHSA-c55v-343g-5xff is classified as Cross-site Scripting (XSS) (CWE-79), CWE-644 (CWE-644), Server-Side Request Forgery (SSRF) (CWE-918). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was GHSA-c55v-343g-5xff published, and has it been updated?

GHSA-c55v-343g-5xff was published on June 19, 2026 and was last updated on June 19, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘 Packagist

GHSA-c55v-343g-5xff

Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs

Also known asCVE-2026-55791

Published

Jun 19, 2026

Updated

Jun 19, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Blast Radius

2 pkgs affected

🐘craftcms/cms🐘craftcms/cms

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

1. Overview

Craft CMS is 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.

2. Vulnerability Mechanism (Root Cause) The vulnerability manifests when assetManager.cacheSourcePaths is set to false. The attack chain relies on three structural flaws and insecure defaults:

A. Default Proxy Trust (trustedHosts): Craft’s default GeneralConfig::$trustedHosts is set to ['any']. This allows an attacker to bypass front-end web server (Nginx/Apache) strict Host header validations by simply injecting an X-Forwarded-Host header. Yii2 will parse this and globally set $baseUrl to the attacker's domain.
B. Insecure HTTP Client (actionResourceJs): In AppController::actionResourceJs(), the str_starts_with($url, $baseUrl) validation is bypassed because $baseUrl is already poisoned by the attacker. The core then uses Craft::createGuzzleClient()->get($url). Unlike the GraphQL Asset fetcher, this Guzzle instance defaults to ALLOW_REDIRECTS => true.
C. Forced JS Content-Type: The response fetched from the attacker's server is blindly returned to the user via $this->asRaw() with the header Content-Type: application/javascript.

3. Attack Scenario & Impact (Proof of Exploitability) This endpoint acts as a proxy, taking remote, unverified content and serving it as valid JavaScript. While the direct SSRF allows for internal network probing, the most devastating impact occurs when caching layers are involved.

If the Craft CMS instance is behind a caching layer, this vulnerability leads directly to Web Cache Poisoning:

An unauthenticated attacker sends the poisoned request.
The caching layer caches the malicious JavaScript response for the legitimate /actions/app/resource-js URI.
When an authenticated Administrator logs into the Control Panel, their browser loads the poisoned cached JavaScript (Stored XSS).
The malicious script extracts window.Craft.csrfTokenValue and silently sends a POST request to /admin/actions/plugins/install-plugin, achieving 1-Click Remote Code Execution (RCE) via Session Riding.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`craftcms/cms`	≥ 5.0.0-RC1&&< 5.10	5.10
🐘Packagist	`craftcms/cms`	≥ 4.0.0-RC1&&< 4.18	4.18

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for craftcms/cms. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update craftcms/cms to 5.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-c55v-343g-5xff is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-c55v-343g-5xff is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-c55v-343g-5xff. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

**1. Overview** Craft CMS is 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. **2. Vulnerability Mechanis

O3 Security · Impact-Aware SCA

Is GHSA-c55v-343g-5xff in your dependencies?

O3 detects GHSA-c55v-343g-5xff across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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