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CVE-2026-54047 – Authentication Bypass / Account Takeover – Laci Synchroni server < 1.2.3

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Summary

CVE-2026-54047 is a critical improper authentication vulnerability in the Laci Synchroni server, a self-hosted mod/appearance synchronization backend used with the Dalamud plugin ecosystem. The flaw exists in the server’s OAuth2 login flow, which trusts a client-supplied UID value from a local configuration file instead of verifying it against the authenticated identity, allowing an attacker to fully impersonate any target user. The issue affects all server versions prior to 1.2.3 and carries a CVSS score of 9.2 (Critical).

Technical details

  • Root cause: The server’s AuthorizeOauthAsync method, part of SecretKeyAuthenticatorService, does not properly bind the User ID being requested to the identity actually verified through the OAuth2 authentication flow.
  • Trigger conditions: A user (or attacker with local access to a client’s configuration) edits the local config.json file to inject an arbitrary UID value before initiating the OAuth2 login flow.
  • Attack vector: Network — the server accepts the OAuth2 login request and fails to cross-check the authenticated OAuth2 identity against the requested UID, so the manipulated UID is honored without additional verification.
  • Impact: Full account takeover — an attacker can impersonate any target user and perform actions on their behalf, resulting in high impact to confidentiality, integrity, and availability of the affected account and its data.

Affected software

  • Laci Synchroni server (ghcr.io/LaciSynchroni/server Docker image): all versions prior to 1.2.3

Severity

  • CVSS v4.0 Base Score: 9.2 (Critical)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade the Laci Synchroni server to version 1.2.3 or later, which enforces strict server-side validation that the authenticated OAuth2 identity matches the requested UID before issuing session tokens.
  • If no patch can be applied immediately: No workarounds are available according to the vendor advisory. Self-hosted server operators should restrict access to the login/OAuth2 endpoints where possible and monitor for anomalous authentication activity until the upgrade can be completed.

References

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