How severe is CVE-2026-55791?

No CVSS score has been assigned to CVE-2026-55791 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-55791?

CVE-2026-55791 affects the following packages: craftcms/cms (Packagist), craftcms/cms (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-55791?

Update craftcms/cms to 5.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55791 is resolved across your whole dependency graph.

How do I detect CVE-2026-55791 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for craftcms/cms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-55791 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2026-55791 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55791 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 is the EPSS score for CVE-2026-55791?

CVE-2026-55791 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 39th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-55791?

CVE-2026-55791 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 CVE-2026-55791 published, and has it been updated?

CVE-2026-55791 was published on July 1, 2026 and was last updated on October 21, 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

Not in CISA KEV

CVE-2026-55791 — cms

Fix: craftcms/cms#18559

CVE-2026-55791 is a Cross-site Scripting (XSS) vulnerability in craftcms/cms. A fix is available for craftcms/cms — see the affected versions and patch details below.

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

Also known asGHSA-c55v-343g-5xff

Published

Jul 1, 2026

Updated

Oct 21, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55791.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs39th percentile — riskier than 39% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

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`composer require craftcms/cms:^5.10`
🐘Packagist	`craftcms/cms`	≥ 4.0.0-RC1&&< 4.18	4.18`composer require craftcms/cms:^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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update craftcms/cms to 5.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55791 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

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

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