SillyTavern has Authentication Bypass via SSO Header Injection

Resolution

SillyTavern 1.18.0 now includes a configuration option to limit which IP addresses can authorize using SSO headers, limiting to just loopback addresses by default. A setting can be customized according to user's needs.

Documentation: https://docs.sillytavern.app/administration/sso/

Summary

SillyTavern accepts Remote-User (Authelia) and X-Authentik-Username (Authentik) HTTP headers to automatically log in users when SSO is configured. There is no validation that these headers originate from a trusted reverse proxy. Any network client that can reach the SillyTavern port directly can inject these headers and authenticate as any user, including administrators, without a password. This vulnerability is exploitable only when sso.autheliaAuth: true or sso.authentikAuth: true is set in config.yaml (both default to false).

Detials

SillyTavern implements header-based SSO for Authelia and Authentik. When enabled, the tryAutoLogin function (called on every request to /login) invokes headerUserLogin, which reads an HTTP header set by the upstream proxy and automatically creates an authenticated session for the matching user:

src/users.js:779-801:

async function headerUserLogin(request, header = 'Remote-User') {
    if (!request.session) { return false; }

    const remoteUser = request.get(header);  // reads any header from any client
    if (!remoteUser) { return false; }

    const userHandles = await getAllUserHandles();
    for (const userHandle of userHandles) {
        if (remoteUser.toLowerCase() === userHandle) {
            const user = await storage.getItem(toKey(userHandle));
            if (user && user.enabled) {
                request.session.handle = userHandle; 
                return true;
            }
        }
    }
    return false;
}

request.get(header) is Express's wrapper for req.headers[name.toLowerCase()]. Express does not distinguish between headers set by a trusted upstream proxy and headers injected by the end client. Without an IP allowlist check, any client can set Remote-User: and receive an authenticated session cookie.

User Enumeration Pre-Condition

The /api/users/list endpoint is registered before requireLoginMiddleware in src/server-main.js:236, making it publicly accessible without authentication:

: