CVE-2026-55791: Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs

Should you act?

Normal patch cycle

No exploitation signal; fix it in your next routine update if you are affected. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.5%, percentile 39: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 39% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes craftcms/cms (Composer (PHP)) >= 5.0.0-RC1, < 5.10, fixed in 5.10; craftcms/cms (Composer (PHP)) >= 4.0.0-RC1, < 4.18, fixed in 4.18. Check your lockfile, including transitive dependencies.

What it is

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.

From the GitHub advisory

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 $baseUrl to the attacker's domain. - B. Insecure HTTP Client (actionResourceJs): In AppController::actionResourceJs(), the strstartswith($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 ALLOWREDIRECTS => 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.

Weakness type: CWE-918, CWE-644, CWE-79, CWE-346.

Product: craftcms/cms (Composer (PHP)) · Assigned by: GitHub_M · Published: 2026-07-01

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact partial.
• FIRST EPSS (model of 2026-10-21): 0.46% probability of exploitation activity in the next 30 days; percentile 39.

Timeline

• 2026-06-19 — GitHub-reviewed advisory GHSA-c55v-343g-5xff published (critical)
• 2026-07-01 — CVE record published by GitHub_M
• 2026-07-02 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-c55v-343g-5xff github.com
2. CVE.org record CVE-2026-55791 cve.org
3. NVD entry CVE-2026-55791 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

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