CVE-2026-52889: Formie Hidden field defaults vulnerable to Server-Side Template Injection

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. 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 1.3%, percentile 70: FIRST’s model estimates a 1.3% chance of exploitation activity in the next 30 days, higher than 70% of all scored CVEs (moderate). 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 verbb/formie (Composer (PHP)) < 3.1.27, fixed in 3.1.27. Check your lockfile, including transitive dependencies.

What it is

Formie is a Craft CMS plugin for creating forms. Prior to 3.1.27, Formie can pass request-derived Hidden field defaults such as HTTP User Agent, Referer URL, Current URL, Current URL without Query String, Query Parameter, and Cookie Value to Craft's Twig rendering layer during front-end form rendering. An unauthenticated attacker can place Twig syntax in one of these request-controlled inputs when a public form contains an affected Hidden field. Hidden::getFrontEndInputOptions() then assigns the value to defaultValue and calls renderString, causing server-side template evaluation rather than treating the request data as a plain string. Depending on the Craft site configuration and available Twig capabilities, exploitation can disclose sensitive information, modify application state, or achieve remote code execution. This issue is fixed in version 3.1.27.

From the GitHub advisory

Summary Formie Hidden fields could evaluate request-derived values as Twig during front-end form rendering.

When a Hidden field used a dynamic default value such as HTTP User Agent, Referer URL, Current URL, Query Parameter, or Cookie Value, the value was copied from the incoming request and later passed to Craft’s Twig rendering layer. This allowed an unauthenticated attacker to provide Twig syntax in request-controlled input and have it evaluated server-side when the form was rendered.

Affected Versions verbb/formie for Craft 5: - Affected: >= 3.0.0-beta.1,

Impact An unauthenticated attacker could trigger server-side template evaluation by visiting a public form containing a Hidden field configured with a request-derived default value.

Because Craft’s normal Twig environment exposes application objects, this may lead to disclosure of sensitive information, modification of application state, or remote code execution depending on the site configuration and available Twig capabilities.

Weakness type: CWE-1336.

Product: verbb/formie (Composer (PHP)) · Assigned by: GitHub_M · Published: 2026-08-19

Affected and fixed versions

Severity scores

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

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 total.
• FIRST EPSS (model of 2026-10-21): 1.27% probability of exploitation activity in the next 30 days; percentile 70.

Timeline

• 2026-07-06 — GitHub-reviewed advisory GHSA-565m-g33j-jq96 published (critical)
• 2026-08-19 — CVE record last updated
• 2026-08-19 — CVE record published by GitHub_M

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-565m-g33j-jq96 github.com
2. CVE.org record CVE-2026-52889 cve.org
3. NVD entry CVE-2026-52889 nvd.nist.gov

Other references from the CVE record and advisory

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

Critical Composer (PHP) advisories · Today’s Security Digest