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Critical

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

  • Disclosed: 19.06.2026
  • Updated: 11.10.2026 10:22

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:

  1. An unauthenticated attacker sends the poisoned request.
  2. The caching layer caches the malicious JavaScript response for the legitimate /actions/app/resource-js URI.
  3. When an authenticated Administrator logs into the Control Panel, their browser loads the poisoned cached JavaScript (Stored XSS).
  4. 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 software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Composer package craftcms/cms >= 5.0.0-RC1, < 5.10, >= 4.0.0-RC1, < 4.18 5.10, 4.18

References

CVSS score -
CVSS vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:L/SI:N/SA:N
Weakness (CWE)
CWE-346
Platform
Composer
Package
craftcms/cms
Disclosed
19.06.2026
Updated
11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-c55v-343g-5xff - CC BY 4.0

Vulnerability history: craftcms/cms

All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (127) in the archive
Total entries
127
First → latest
07.2021 08.2026
Without a fix
6
Exploited in attacks
4

Severity breakdown

  • Critical 9
  • High 41
  • Medium 65
  • Low 12

Disclosures per year

Severity:
  1. Craft CMS: Passkey login accepts replayed WebAuthn assertions
    Critical CVE-2026-72780 >= 5.0.0-RC1, < 5.10.5 5.10.5
  2. Craft CMS: Arbitrary user password reset leading to administrator account takeover
    High CVE-2026-72786 >= 5.0.0-RC1, < 5.10.8 5.10.8
  3. Craft CMS: Authenticated RCE through Twig sandbox escape
    High CVE-2026-72781 >= 5.0.0-RC1, < 5.10.7, >= 4.0.0-RC1, < 4.18.3 5.10.7, 4.18.3
  4. Craft CMS: Authenticated RCE via `condition.config` JSON cleanse bypass
    High CVE-2026-72778 >= 5.0.0-RC1, < 5.10.6, >= 4.0.0-RC1, < 4.18.2 5.10.6, 4.18.2
  5. Craft CMS: RCE via missing cleanseConfig in FieldsController::actionRenderCardPreview
    High CVE-2026-56382 >= 5.5.0, < 5.9.14 5.9.14
  6. Craft CMS: Potential authenticated Remote Code Execution via referrer redirect
    High CVE-2026-55794 >= 5.9.0, < 5.10.0 5.10.0
  7. Craft CMS: DOM XSS via GitHub issue title in CraftSupport widget
    High CVE-2026-55790 >= 5.0.0-RC1, < 5.9.23, >= 4.0.0-RC1, < 4.17.16 5.9.23, 4.17.16
  8. Craft CMS Vulnerable to Unauthorized Deletion of Destination Folders During Forced Moves
    High CVE-2026-50282 >= 5.0.0-RC1, < 5.9.21, >= 4.0.0-RC1, < 4.17.14 5.9.21, 4.17.14
  9. Craft CMS's mass assignment via id in newAttributes during bulk duplicate overwrites existing elements
    High CVE-2026-50281 >= 5.7.0, < 5.9.21 5.9.21
  10. Craft CMS: Missing peer-permission check in `AssetsController::actionDeleteFolder` allows deletion of other users' assets
    High CVE-2026-50284 >= 5.0.0-RC1, < 5.9.22, >= 4.0.0-RC1, < 4.17.15 5.9.22, 4.17.15
  11. Craft CMS: Authorship spoofing in `entries/save-entry` via pre-check/post-mutation authorization gap
    High CVE-2026-50279 >= 5.0.0-RC1, < 5.9.21 5.9.21
  12. Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs this entry
    Critical CVE-2026-55791 >= 5.0.0-RC1, < 5.10, >= 4.0.0-RC1, < 4.18 5.10, 4.18
  13. Craft CMS's Missing Volume Permission Check in AssetsController::actionShowInFolder Allows Information Disclosure
    High CVE-2026-44012 >= 5.0.0-RC1, < 5.9.18 5.9.18
  14. Craft CMS has Potential Authenticated Remote Code Execution via Malicious Attached Behavior
    High CVE-2026-44011 >= 4.0.0, < 4.17.12, >= 5.0.0, < 5.9.18 4.17.12, 5.9.18
  15. Craft CMS's Missing Authorization in GraphQL Address Resolver Allows Cross-Scope PII Disclosure
    High CVE-2026-44010 >= 5.0.0, < 5.9.18, >= 4.0.0, < 4.17.12 5.9.18, 4.17.12
  16. Craft CMS is Vulnerable to Authenticated Remote Code Execution via Malicious Attached Behavior
    High CVE-2026-33157 >= 5.6.0, < 5.9.13 5.9.13
  17. Craft CMS Vulnerable to Privilege Escalation/Bypass through UsersController->actionImpersonateWithToken()
    High CVE-2026-32267 >= 4.0.0-RC1, < 4.17.6, >= 5.0.0-RC1, < 5.9.12 4.17.6, 5.9.12
  18. Craft CMS vulnerable to behavior injection RCE ElementIndexesController and FieldsController
    High CVE-2026-32264 >= 4.0.0-RC1, < 4.17.5, >= 5.0.0-RC1, < 5.9.11 4.17.5, 5.9.11
  19. Craft CMS vulnerable to behavior injection RCE via EntryTypesController
    High CVE-2026-32263 >= 5.6.0, < 5.9.11 5.9.11
  20. CraftCMS has an RCE vulnerability via relational conditionals in the control panel
    High CVE-2026-31857 >= 5.0.0-RC1, < 5.9.9, >= 4.0.0-beta.1, < 4.17.4 5.9.9, 4.17.4
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