# Craft CMS: Blind SSRF and Arbitrary JavaScript Injection via Host Header Poisoning in actionResourceJs Advisory record · `GHSA-c55v-343g-5xff` - **Identifiers:** `GHSA-c55v-343g-5xff` · `CVE-2026-55791` · `PKSA-5xds-5mf3-ckxn` - **State:** Confirmed - **Severity:** CVSS 6.9 Medium - **Critical:** No - **Weakness (CWE):** [CWE-79](https://cwe.mitre.org/data/definitions/79.html) · [CWE-346](https://cwe.mitre.org/data/definitions/346.html) · [CWE-644](https://cwe.mitre.org/data/definitions/644.html) · [CWE-918](https://cwe.mitre.org/data/definitions/918.html) - **Tags:** [Cross-site scripting (XSS)](https://agentprime.io/advisories/tag/xss.md) · [Server-side request forgery (SSRF)](https://agentprime.io/advisories/tag/ssrf.md) · [CWE-346](https://agentprime.io/advisories/tag/cwe-346.md) · [CWE-644](https://agentprime.io/advisories/tag/cwe-644.md) - **Published:** 19 Jun 2026 - **Updated:** 10 Sept 2026 - **Reviewed by a person:** No Published by the GitHub Advisory Database at 19 Jun 2026, 21:15 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. ## Affected packages - `craftcms/cms` (Packagist): affected `>=5.0.0-RC1, <5.10 || >=4.0.0-RC1, <4.18`; fixed in `5.10`, `4.18` ## Machine-written summary Summary written by Claude Haiku from the source advisory. Not reviewed by a person. > Craft CMS (craftcms/cms) versions from 4.0.0-RC1 to before 4.18, and from 5.0.0-RC1 to before 5.10, are affected through the resource JavaScript action, which can be made to fetch and return attacker-controlled content as JavaScript when a Host header is poisoned and assetManager.cacheSourcePaths is set to false. An unauthenticated attacker can send a crafted request to trigger server-side request forgery and arbitrary JavaScript injection, and the source says this can extend to cache poisoning when a caching layer is present. Upgrade to 4.18, 5.10, or a later release. Written 31 Oct 2026. ## Not analysed yet We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check. ## Severity and exploitation - 6.9 Medium, CVSS 4.0: `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`, from CNA (GitHub_M) - **CISA KEV:** Not listed. - **EPSS:** 0.0046, the 40th percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days. ## Timeline - 31 Oct 2026, 13:57 UTC: First seen by agentprime - 31 Oct 2026, 13:57 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-c55v-343g-5xff) ## Sources - [GitHub Advisory Database: GHSA-c55v-343g-5xff](https://github.com/advisories/GHSA-c55v-343g-5xff) - [OSV: GHSA-c55v-343g-5xff](https://osv.dev/vulnerability/GHSA-c55v-343g-5xff) - - - Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code. ## Check whether this affects you A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet. - With the agentprime MCP server (), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: - In a browser: reads your lockfile in the browser, never uploading it. - Whether it touches your code, with file and line: the Claude Code plugin, - As JSON: , or `POST https://api.agentprime.io/v1/match` with your packages () --- The web page: . Everything for agents: .