User Impersonation

User impersonation allows authorized users (typically administrators) to temporarily authenticate as another user for debugging, support, or testing purposes. This feature provides a secure way to troubleshoot user-specific issues while maintaining full audit logging.

Features

Opt-in Configuration: Disabled by default, must be explicitly enabled
Short-lived Sessions: Impersonated sessions expire quickly (15 minutes default)
No Token Refresh: Impersonated sessions cannot be refreshed for security
Audit Logging: Full audit trail via Nuxt server hooks
Chain Prevention: Cannot impersonate while already impersonating
Custom Authorization: Implement your own authorization logic via hooks
Original Context Access: Server routes can access both current and original user

Configuration

Enable impersonation in your nuxt.config.ts:

typescript

export default defineNuxtConfig({
  modules: ['@peterbud/nuxt-aegis'],
  
  nuxtAegis: {
    impersonation: {
      enabled: true,
      tokenExpiration: 900, // 15 minutes in seconds (default)
    },
  },
})

Configuration Options

Option	Type	Default	Description
`enabled`	`boolean`	`false`	Enable or disable impersonation feature
`tokenExpiration`	`number`	`900`	Impersonated token expiration in seconds
`originalUserLookupClaim`	`string`	`'sub'`	Token claim used to refetch the original user when ending impersonation

originalUserLookupClaim is useful when your JWT sub is a provider-specific identifier but your impersonation.fetchTarget() handler expects an application-level identifier such as userId.

Required Handlers

To use impersonation, you must implement the impersonation handlers using defineAegisHandler in a server plugin.

Implementation Example

typescript

// server/plugins/aegis.ts
export default defineNitroPlugin((nitroApp) => {
  defineAegisHandler({
    impersonation: {
      // 1. Authorization Check (Optional, defaults to allowing if fetchTarget succeeds)
      canImpersonate: async (requester, targetId, event) => {
        // Check if user is allowed to impersonate
        if (requester.role !== 'admin') {
          return false
        }
        return true
      },

      // 2. Fetch Target User (Required)
      fetchTarget: async (targetUserId, event) => {
        // Fetch the target user from your database
        const targetUser = await getUserById(targetUserId)
        
        if (!targetUser) {
          return null
        }
        
        // Optional: Additional authorization checks
        if (requester.organizationId !== targetUser.organizationId) {
           // You can throw errors for specific messages
           throw createError({
             statusCode: 403,
             message: 'Cannot impersonate users from different organizations',
           })
        }
        
        // Return user data that will become JWT claims
        return {
          sub: targetUser.id,
          email: targetUser.email,
          name: targetUser.name,
          picture: targetUser.picture,
          role: targetUser.role,
          permissions: targetUser.permissions,
          // ... any other custom claims
        }
      }
    }
  })
})

Optional Audit Hooks

Implement these hooks for audit logging:

Start Impersonation Hook

typescript

import { useServerHooks } from 'nuxt/server'

useServerHooks().hook('nuxt-aegis:impersonate:start', async (payload) => {
  // Log impersonation start
  await auditLog.create({
    action: 'IMPERSONATION_START',
    adminId: payload.requester.sub,
    adminEmail: payload.requester.email,
    targetId: payload.targetUser.sub,
    targetEmail: payload.targetUser.email,
    reason: payload.reason,
    ip: payload.ip,
    userAgent: payload.userAgent,
    timestamp: payload.timestamp,
  })
})

End Impersonation Hook

typescript

import { useServerHooks } from 'nuxt/server'

useServerHooks().hook('nuxt-aegis:impersonate:end', async (payload) => {
  // Log impersonation end
  await auditLog.create({
    action: 'IMPERSONATION_END',
    adminId: payload.restoredUser.sub,
    adminEmail: payload.restoredUser.email,
    wasImpersonatingId: payload.impersonatedUser.sub,
    wasImpersonatingEmail: payload.impersonatedUser.email,
    ip: payload.ip,
    userAgent: payload.userAgent,
    timestamp: payload.timestamp,
  })
})

Client-Side Usage

The useAuth() composable provides methods and properties for impersonation:

vue

<script setup lang="ts">
const { 
  user,
  isImpersonating, 
  originalUser, 
  impersonate, 
  stopImpersonation 
} = useAuth()

const targetUserId = ref('')
const reason = ref('')

async function handleImpersonate() {
  try {
    await impersonate(targetUserId.value, reason.value)
    console.log('Now impersonating:', user.value?.email)
  } catch (error) {
    console.error('Impersonation failed:', error)
  }
}

async function handleStopImpersonation() {
  try {
    await stopImpersonation()
    console.log('Restored to:', user.value?.email)
  } catch (error) {
    console.error('Failed to stop impersonation:', error)
  }
}
</script>

<template>
  <div>
    <!-- Impersonation Banner -->
    <div v-if="isImpersonating" class="banner">
      <p>
        You ({{ originalUser?.originalUserEmail }}) are impersonating 
        {{ user?.email }}
      </p>
      <button @click="handleStopImpersonation">
        Stop Impersonation
      </button>
    </div>
    
    <!-- Admin Controls -->
    <div v-if="user?.role === 'admin' && !isImpersonating">
      <h3>Impersonate User</h3>
      <input v-model="targetUserId" placeholder="User ID or Email" />
      <textarea v-model="reason" placeholder="Reason (optional)" />
      <button @click="handleImpersonate">Start Impersonation</button>
    </div>
  </div>
</template>

Composable API

Properties

isImpersonating: ComputedRef<boolean> - True when currently impersonating
originalUser: ComputedRef<{ originalUserSub, originalUserEmail, originalUserName } | null> - Original user subject and profile data when impersonating

Methods

impersonate(targetUserId: string, reason?: string): Promise<void> - Start impersonating another user
stopImpersonation(): Promise<void> - End impersonation and restore original session

Server-Side Usage

Server routes can access both the impersonated user and the original user:

typescript

// server/routes/api/admin/action.post.ts
export default defineEventHandler(async (event) => {
  const user = event.context.user // Current user (impersonated if active)
  const originalUser = event.context.originalUser // Original user (if impersonating)
  
  if (originalUser) {
    // User is impersonating
    console.log(`Admin ${originalUser.email} is impersonating ${user.email}`)
    
    // You can enforce additional restrictions
    if (someSensitiveAction) {
      throw createError({
        statusCode: 403,
        message: 'This action cannot be performed while impersonating',
      })
    }
  }
  
  // Perform action as current user
  return performAction(user)
})

Context Properties

When impersonating, the middleware injects:

event.context.user: Current user (the impersonated user)
event.context.originalUser: Original user who started impersonation (if applicable)

typescript

interface OriginalUserContext {
  sub: string
  email?: string
  name?: string
}

Security Considerations

Token Characteristics

Short Expiration: Impersonated tokens expire in 15 minutes (configurable)
No Refresh: Impersonated sessions cannot be refreshed - user must re-impersonate
Chain Prevention: Cannot impersonate while already impersonating another user
Immediate Validation: Authorization is checked on every impersonation attempt

Best Practices

Restrict to Admins: Implement strict role checks in the canImpersonate handler
Require Reasons: Always prompt for and log the reason for impersonation
Organization Boundaries: Consider preventing impersonation across organization boundaries
Audit Everything: Log all impersonation events with full context
Time Limits: Keep the token expiration short (default 15 minutes)
Sensitive Actions: Block certain actions when impersonating (e.g., password changes, account deletion)
Notification: Consider notifying users when they're impersonated (optional)

Example Authorization Logic

typescript

defineAegisHandler({
  impersonation: {
    canImpersonate: async (requester, targetId) => {
      if (requester.role !== 'admin' && requester.role !== 'support') return false
      if (!requester.permissions?.includes('impersonate')) return false

      const target = await db.users.findById(targetId)
      return !!target && target.organizationId === requester.organizationId
    },
    fetchTarget: targetId => db.users.findById(targetId),
  },
})

JWT Structure

When impersonating, the JWT includes an impersonation object:

json

{
  "sub": "target-user-id",
  "email": "target@example.com",
  "name": "Target User",
  "role": "user",
  "impersonation": {
    "originalUserSub": "auth-provider-admin-sub",
    "originalUserLookupId": "database-admin-id",
    "originalUserEmail": "admin@example.com",
    "originalUserName": "Admin User",
    "impersonatedAt": "2024-01-15T10:30:00.000Z",
    "reason": "Debugging issue #123",
    "originalClaims": {
      "role": "admin",
      "permissions": ["read", "write", "delete"]
    }
  },
  "iat": 1705315800,
  "exp": 1705316700,
  "iss": "nuxt-aegis"
}

The originalClaims field stores all custom claims from the original user's token, ensuring they can be fully restored when impersonation ends.

API Endpoints

Start Impersonation

http

POST /auth/impersonate
Authorization: Bearer {admin-token}
Content-Type: application/json

{
  "targetUserId": "user-id-or-email",
  "reason": "Debugging user issue #123"
}

Response:

json

{
  "accessToken": "eyJhbGc..."
}

Stop Impersonation

http

POST /auth/unimpersonate
Authorization: Bearer {impersonated-token}

Response:

json

{
  "accessToken": "eyJhbGc..."
}

A new refresh token cookie is also set to restore the full session.

Error Handling

Common error scenarios:

Status	Error	Description
`404`	Feature not enabled	Impersonation is disabled in configuration
`403`	Insufficient permissions	User doesn't have permission to impersonate
`403`	Cannot chain impersonation	Already impersonating another user
`404`	Target user not found	User ID doesn't exist
`403`	Organization mismatch	Cannot impersonate users from other organizations
`400`	Not impersonating	Tried to stop impersonation when not impersonating
`403`	Cannot refresh	Tried to refresh an impersonated session

TypeScript Types

typescript

import type { NuxtRequestEvent, RequestEventContext } from 'nuxt/schema'
import type { 
  ImpersonateCheckPayload,
  ImpersonateFetchTargetPayload,
  ImpersonateStartPayload,
  ImpersonateEndPayload,
  ImpersonationContext,
} from 'nuxt-aegis'

// Hook payload types
type AegisHookEvent = Omit<NuxtRequestEvent, 'context'> & { context: RequestEventContext }

interface ImpersonateCheckPayload {
  requester: BaseTokenClaims
  targetUserId: string
  reason?: string
  event: AegisHookEvent
  ip: string
  userAgent: string
}

interface ImpersonateFetchTargetPayload {
  requester: BaseTokenClaims
  targetUserId: string
  event: AegisHookEvent
}

interface ImpersonateStartPayload {
  requester: BaseTokenClaims
  targetUser: BaseTokenClaims
  reason?: string
  event: AegisHookEvent
  ip: string
  userAgent: string
  timestamp: Date
}

interface ImpersonateEndPayload {
  restoredUser: BaseTokenClaims
  impersonatedUser: BaseTokenClaims
  event: AegisHookEvent
  ip: string
  userAgent: string
  timestamp: Date
}

// Impersonation context in JWT
interface ImpersonationContext {
  originalUserSub: string
  originalUserLookupId: string
  originalUserEmail?: string
  originalUserName?: string
  impersonatedAt: string
  reason?: string
  originalClaims?: Record<string, unknown>
}

Complete Example

See the playground implementation for a complete working example with:

Hook implementations in server/plugins/aegis.ts
UI controls in app/pages/index.vue
Demo API route showing dual context in server/routes/api/admin/impersonate-demo.get.ts

User Impersonation ​

Features ​

Configuration ​

Configuration Options ​

Required Handlers ​

Implementation Example ​

Optional Audit Hooks ​

Start Impersonation Hook ​

End Impersonation Hook ​

Client-Side Usage ​

Composable API ​

Properties ​

Methods ​

Server-Side Usage ​

Context Properties ​

Security Considerations ​

Token Characteristics ​

Best Practices ​

Example Authorization Logic ​

JWT Structure ​

API Endpoints ​

Start Impersonation ​

Stop Impersonation ​

Error Handling ​

TypeScript Types ​

Complete Example ​

User Impersonation

Features

Configuration

Configuration Options

Required Handlers

Implementation Example

Optional Audit Hooks

Start Impersonation Hook

End Impersonation Hook

Client-Side Usage

Composable API

Properties

Methods

Server-Side Usage

Context Properties

Security Considerations

Token Characteristics

Best Practices

Example Authorization Logic

JWT Structure

API Endpoints

Start Impersonation

Stop Impersonation

Error Handling

TypeScript Types

Complete Example