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CVE-2026-52111 – Authentication Bypass / Privilege Escalation – fast-note-sync-service ≤2.13.7

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Summary

CVE-2026-52111 is a critical improper access control vulnerability in fast-note-sync-service, a self-hosted note synchronization and REST API backend, affecting versions 2.13.7 and earlier. The flaw allows a remote, unauthenticated attacker to escalate privileges and ultimately impersonate any user, including administrators, by abusing an admin configuration endpoint that exposes JWT signing material. The issue has a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: an administrative configuration endpoint exposes authTokenKey, the secret used to sign user JWT authentication tokens.
  • A related authorization flaw treats a user with admin-uid value of 0 as an administrator, which can allow a self-registered, low-privilege account to reach the admin endpoint.
  • JWT tokens are signed using the exposed authTokenKey combined with a machine identifier; in common Docker deployments this identifier can be empty or predictable, allowing the signing key to be reconstructed.
  • Attack vector: network, no privileges or user interaction required to reach the vulnerable functionality once an attacker can self-register (self-registration is enabled by default).
  • Impact: forgery of valid JWTs for arbitrary users, resulting in administrator account takeover, unauthorized access to private notes/metadata, and modification or deletion of synchronized content.

Affected software

  • fast-note-sync-service, all versions up to and including 2.13.7

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: No confirmed patched release has been publicly identified at the time of writing; monitor the vendor’s GitHub repository for a fix beyond 2.13.7 and upgrade as soon as one is published.
  • If no patch is available:
    • Disable open/public self-registration on the service.
    • Restrict or firewall access to the admin configuration endpoint so it is not reachable from untrusted networks.
    • Rotate the authTokenKey and use a strong, unique, non-predictable value per deployment; avoid relying on default or empty machine identifiers, particularly in containerized/Docker deployments.
    • Correct administrator authorization logic so a admin-uid of 0 is not treated as an administrator.
    • Invalidate existing sessions/tokens after rotating signing keys.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Page title: Fast Note Sync

References

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