Header Injection in Adonisjs with Jwt Tokens

Header Injection in an AdonisJS application that uses JWT tokens can occur when values from HTTP request headers are incorporated into the JWT payload or used to influence token construction without proper validation and sanitization. AdonisJS does not inherently introduce this vulnerability; rather, the risk arises when developer code reads headers such as X-User-ID or X-Role and places them directly into the token claims or uses them to decide token acceptance logic.

For example, an attacker may supply a crafted X-User-ID or X-Impersonate header. If the application uses these headers to build the JWT claims and then signs the token without verifying that the authenticated subject matches the supplied header, the attacker can forge identity claims. This can lead to privilege escalation or unauthorized access when the token is subsequently accepted by downstream services that rely on the embedded claims.

Another scenario involves response headers. If the application dynamically sets headers such as Authorization or X-Access-Token based on token validation outcomes without strict validation, an attacker might inject additional header lines (e.g., via newline characters) that cause header splitting. This can manipulate caching layers, proxies, or client-side logic that processes the response, even though the JWT itself remains cryptographically valid.

Because JWT tokens are often used to carry authorization decisions, injected header values that affect claims can bypass intended access controls. For instance, if role information is derived from a header and embedded into the token, an attacker can assign themselves roles like admin or superuser. This does not break the JWT signature mechanism directly but subverts the trust placed in the claims issued by the server.

It is important to note that the JWT signature ensures integrity of the signed payload, but it does not protect against a server-side logic flaw where untrusted header data is introduced into the payload before signing. Therefore, the combination of AdonisJS routing and JWT handling requires strict validation of any header-derived data before it influences token content or token acceptance decisions.

To mitigate Header Injection risks when working with JWT tokens in AdonisJS, ensure that any data used to construct or interpret tokens is validated, sanitized, and sourced only from trusted authentication state rather than raw request headers.

First, avoid reading identity or role information from incoming headers to build JWT claims. Instead, rely on the authenticated user obtained through AdonisJS authentication mechanisms. Here is an example of a safe login route that issues a JWT based on the authenticated user’s database record:

import { HttpContextContract } from '@ioc:Adonis/Core/HttpContext'
import User from 'App/Models/User'
import { schema, rules } from '@ioc:Adonis/Core/Validator'
import { TokenService } from 'App/Services/TokenService'

export default class AuthController {
  public async login({ request, auth, response }: HttpContextContract) {
    const email = request.input('email')
    const password = request.input('password')

    // Validate input
    const loginSchema = schema.create({
      email: schema.string({ trim: true }, [rules.email(), rules.required()]),
      password: schema.string({}, [rules.confirmed(), rules.required()]),
    })
    const validated = await request.validate({ schema: loginSchema })

    // Attempt authentication using AdonisJS auth
    const user = await auth.authenticate()

    // Build token from trusted user model, not headers
    const token = TokenService.generate({
      subject: user.id.toString(),
      jti: user.id.toString(),
      roles: user.roles?.map((r) => r.name) || [],
    })

    return response.ok({ token })
  }
}

Second, if your application supports impersonation or elevated privileges via trusted administrative workflows, do not allow clients to specify the impersonated identity via headers. If such functionality is required, enforce strict access controls and use server-side session state to determine the target identity, then issue a scoped JWT explicitly marked as a proxy token with appropriate claims and limited lifetime:

import { HttpContextContract } from '@ioc:Adonis/Core/HttpContext'
import TokenService from 'App/Services/TokenService'

export default class AdminController {
  public async impersonate({ params, auth, request, response }: HttpContextContract) {
    const admin = await auth.authenticate()
    if (!admin.isAdmin) {
      throw new Error('Unauthorized')
    }

    // Target identity must come from server-side lookup, not headers
    const targetUserId = Number(params.userId)
    const targetUser = await User.findOrFail(targetUserId)

    // Issue a scoped token referencing the admin and the target
    const token = TokenService.generate({
      subject: targetUser.id.toString(),
      jti: `impersonation-${admin.id}-${targetUser.id}`,
      roles: targetUser.roles?.map((r) => r.name) || [],
      impersonatedBy: admin.id,
    })

    return response.ok({ token })
  }
}

Third, sanitize and validate any headers that may influence token handling, such as Accept or custom routing headers, before using them in middleware that decides token applicability. Use strict allowlists and avoid directly injecting header values into logs or token metadata. For response headers, rely on AdonisJS’s built-in response object methods rather than manually concatenating header strings, which reduces the risk of newline-based injection.