LaciSynchroni server up to 1.2.2 OAuth2 Login Flow config.json AuthorizeOauthAsync UID improper authentication

Summaryinfo

A vulnerability classified as very critical has been found in LaciSynchroni server up to 1.2.2. This vulnerability affects the function AuthorizeOauthAsync of the file config.json of the component OAuth2 Login Flow. Performing a manipulation of the argument UID results in improper authentication. This vulnerability is identified as CVE-2026-54047. The attack can be initiated remotely. There is not any exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as very critical has been found in LaciSynchroni server up to 1.2.2. This affects the function AuthorizeOauthAsync of the file config.json of the component OAuth2 Login Flow. The manipulation of the argument UID with an unknown input leads to a improper authentication vulnerability. CWE is classifying the issue as CWE-287. When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

Laci Synchroni is a decentralized mod and appearance sync server and plugin for Dalamud. Versions of the backend prior to 1.2.3 have an improper authentication vulnerability in the application's OAuth2 login flow. The application relies on client-side state by trusting the `UID` field inside the `Authentications` object of a user's local `config.json` file. By manually editing this local file on their PC prior to logging in, a user can supply an arbitrary UID. Because the server fails to validate that the authenticated OAuth2 identity matches the requested UID, an attacker can fully impersonate any target user and perform actions on their behalf. This issue has been resolved in version 1.2.3. The patch modifies `AuthorizeOauthAsync` inside the `SecretKeyAuthenticatorService` to strictly bind the lookup of the requested User ID (`requestedUid`) to the record of the successfully authenticated identity (`primaryUid`). The server will no longer load or return session tokens for a requested UID unless it matches the verified, authenticated database record. No known workarounds are available.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-54047 since 06/11/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 09/11/2026).

By approaching the search of inurl:config.json it is possible to find vulnerable targets with Google Hacking.

Upgrading to version 1.2.3 eliminates this vulnerability.

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Productinfo

Vendor

• LaciSynchroni

Name

• server

Version

• 1.2.0
• 1.2.1
• 1.2.2

Website

• Product: https://github.com/LaciSynchroni/server/

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