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CVE-2026-63429 – Unauthenticated File Upload – HeyForm prior to 3.0.0-rc.9

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Summary

CVE-2026-63429 is a high-severity unauthenticated arbitrary file upload vulnerability in HeyForm, an open-source self-hosted form builder. Prior to version 3.0.0-rc.9, the POST /api/upload endpoint accepts file uploads from any anonymous internet user with no authentication or session validation, allowing attackers to store arbitrary files on an exposed HeyForm instance and obtain permanent public URLs hosted under the victim’s domain. The vulnerability carries a CVSS v3.1 score of 8.6 (HIGH).

Technical details

  • Root cause: The POST /api/upload controller carries no @UseGuards() decorator, and the application’s global authentication middleware is registered only as a NestJS provider rather than as an APP_GUARD token, leaving the endpoint entirely unprotected. No form-context binding, openToken requirement, or session cookie check is enforced.
  • Trigger condition: Any HTTP request to POST /api/upload on an internet-exposed HeyForm instance — no credentials, session token, or prior interaction required.
  • Attack vector: Network (AV:N), low complexity, no privileges required, no user interaction.
  • Accepted file types: PDF, DOC/DOCX, XLS/XLSX, CSV, TXT, MP4, and image formats; files up to 10 MB per upload.
  • Impact: An attacker receives a permanent, publicly accessible URL hosted under the victim’s HeyForm domain. This can be exploited to host phishing pages, distribute malware, pre-stage payloads on legitimate infrastructure, and cause storage resource exhaustion. CVSS impact dimensions: Confidentiality — Low, Integrity — High, Availability — Low.
  • Weaknesses: CWE-306 (Missing Authentication for Critical Function), CWE-434 (Unrestricted Upload of File with Dangerous Type).

Affected software

  • HeyForm all versions prior to 3.0.0-rc.9 (i.e., ≤ 3.0.0-rc.8)

Severity

CVSS v3.1 Base Score: 8.6 (HIGH)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L

Mitigation and recommended actions

  • Immediate: Upgrade HeyForm to version 3.0.0-rc.9 or later, which contains the vendor-provided patch for this issue.
  • If immediate patching is not possible: Restrict network access to the POST /api/upload endpoint at the reverse proxy or firewall level, and block unauthenticated external access to the HeyForm instance until the upgrade can be applied.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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