# Formie Hidden field defaults vulnerable to Server-Side Template Injection

Advisory record · `GHSA-565m-g33j-jq96`

- **Identifiers:** `GHSA-565m-g33j-jq96` · `CVE-2026-52889` · `PKSA-rm1g-8q6b-7djk`
- **State:** Confirmed
- **Severity:** CVSS 9.8 Critical
- **Critical:** Yes: CVSS 9+
- **Weakness (CWE):** [CWE-1336](https://cwe.mitre.org/data/definitions/1336.html)
- **Tags:** [Code injection](https://agentprime.io/advisories/tag/code-injection.md)
- **Published:** 6 Jul 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 6 Jul 2026, 16:52 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Critical

Critical, because:

- CVSS score 9.8: 9.0 or more.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected packages

- `verbb/formie` (Packagist): affected `<3.1.27`; fixed in `3.1.27`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Formie, the form-builder plugin for Craft CMS, is vulnerable in versions from 3.0.0-beta.1 up to and including 3.1.26 (the source lists 3.1.27 as the fixed release). An unauthenticated attacker can supply Twig syntax through request-derived Hidden field defaults on a public form, which the server then evaluates when the form is rendered, potentially leading to information disclosure, state changes, or remote code execution. Upgrade to Formie 3.1.27 or later; until then, the source advises avoiding request-derived Hidden field defaults on public forms or removing those fields.

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

- 9.8 Critical, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.013, the 70th percentile, on 13 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:03 UTC: First seen by agentprime
- 31 Oct 2026, 14:03 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-565m-g33j-jq96)

## Sources

- [GitHub Advisory Database: GHSA-565m-g33j-jq96](https://github.com/advisories/GHSA-565m-g33j-jq96)
- [OSV: GHSA-565m-g33j-jq96](https://osv.dev/vulnerability/GHSA-565m-g33j-jq96)
- <https://github.com/verbb/formie/security/advisories/GHSA-565m-g33j-jq96>
- <https://github.com/verbb/formie>

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 (<https://api.agentprime.io/mcp>), 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: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-565m-g33j-jq96>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-565m-g33j-jq96/>. Everything for agents: <https://agentprime.io/llms.txt>.