Out Of Bounds Write in Adonisjs with Jwt Tokens

An Out Of Bounds Write occurs when data is written to a memory location outside the intended allocation. In AdonisJS applications that use JWT tokens for stateless authentication, this risk can manifest through unsafe handling of payload size, header content, or signature operations rather than direct memory manipulation. Because JWTs are often decoded, validated, and then processed to build claims objects, an attacker can exploit unchecked input lengths or malformed structures to trigger oversized buffers or unexpected iteration behavior in dependent libraries.

When a token is issued or parsed, the framework typically decodes the base64url-encoded header and payload, validates the signature, and maps claims into a structured payload. If the application does not enforce strict size limits on the decoded payload or on specific claim values (e.g., an array of permissions or scopes), an Out Of Bounds Write can be triggered indirectly. For example, a client might submit a token with an extremely large scope or permissions claim that causes downstream iterators or in-memory structures to exceed expected bounds during processing. This can lead to corrupted runtime state or unpredictable behavior, such as overwriting adjacent data in the process memory space used by the Node.js runtime.

The interaction between AdonisJS middleware and JWT handling packages further influences the attack surface. If the application uses a JWT library that does not enforce strict bounds on array or string lengths during verification, an attacker can supply a token with crafted payload sections. Consider a scenario where the application iterates over scopes to build an access control list without validating the length of the array. An oversized array can cause iteration logic to read or write beyond allocated structures. This becomes especially relevant when integrating third-party libraries that manage token caching or session-like behaviors, even though the token itself is stateless.

Another vector involves the header of the JWT. An attacker might provide a token with an excessively large algorithm field or a deeply nested kid (key ID) value that traverses multiple layers of resolution logic inside key lookup routines. If AdonisJS code resolves the kid by recursively searching through a key store without length or depth limits, this can result in out-of-bounds traversal in the underlying data structures used to store certificates or symmetric keys. Such traversal does not necessarily involve modifying executable code, but it can corrupt adjacent memory regions used by the runtime, leading to crashes or information leakage.

To contextualize this within real-world patterns, consider an API that decodes a JWT and directly assigns the payload to a request-scoped object without normalization. If the payload contains a numeric claim expressed as a string with an extremely large value (e.g., "quota": "9999999999999999999999"), JavaScript number conversion routines may exhibit unexpected behavior. While this does not produce a classic buffer overflow, it can cause logical corruption in quota checks or lead to integer overflow conditions that indirectly enable boundary violations in business logic. This illustrates how improper validation of JWT claims in AdonisJS can facilitate indirect out-of-bounds conditions.

Ultimately, the risk with JWT tokens in AdonisJS is not about traditional memory corruption but about uncontrolled data expansion during decoding, validation, and claim processing. Attackers leverage oversized or malformed tokens to push runtime structures beyond their intended limits. Mitigation requires strict schema validation, bounded iteration, and defensive handling of external input, ensuring that every element derived from a JWT is constrained before being used in security-sensitive operations.

To mitigate Out Of Bounds Write risks when using JWT tokens in AdonisJS, implement strict validation and bounded processing of token payloads and headers. The following code examples demonstrate how to enforce size limits and validate claim structures before use.

First, define a validation schema for the JWT payload using AdonisJS schema builder to ensure that arrays and numeric claims remain within expected bounds:

import { schema } from '@ioc:Adonis/Core/Validator'

const tokenPayloadSchema = schema.create({
  sub: schema.string.optional(),
  scopes: schema.array.optional().ensure(itemList => {
    if (itemList.length > 100) {
      throw new Error('scopes array exceeds maximum allowed length')
    }
  }),
  quota: schema.number.optional([
    (rule) => rule.range(0, 1000000),
  ]),
  permissions: schema.array.optional().ensure(itemList => {
    if (itemList.length > 50) {
      throw new Error('permissions array exceeds maximum allowed length')
    }
  }),
})

Next, integrate this schema into your authentication middleware to validate the decoded token before processing claims:

import { HttpContextContract } from '@ioc:Adonis/Core/HttpContext'
import { authenticator } from 'otplib'
import { Jwt } from '@ioc:Adonis/Addons/Jwt'

export default class AuthMiddleware {
  public async handle({ request, response, auth }: HttpContextContract, next: () => Promise) {
    const token = request.headers().authorization?.replace('Bearer ', '')
    if (!token) {
      return response.unauthorized('Missing token')
    }

    try {
      const decoded = Jwt.verify(token, process.env.JWT_SECRET as string)
      const validated = tokenPayloadSchema.validateSync(decoded, { stripUnknown: true })

      // Ensure no oversized strings in header
      const header = Jwt.getHeader(token)
      if (header.alg && header.alg.length > 10) {
        throw new Error('Algorithm field too long')
      }

      request.authData = validated
      await next()
    } catch (error) {
      return response.badRequest({ error: 'Invalid token structure' })
    }
  }
}

When processing the kid header to locate the appropriate key, enforce depth and length limits to prevent unbounded traversal:

import { logger } from '@poppinss/utils'

export function resolveKey(kid: string) {
  if (typeof kid !== 'string' || kid.length > 64) {
    logger.warn('Invalid kid length')
    throw new Error('Invalid key identifier')
  }

  // Limit recursion depth when resolving keys
  const keys = loadKeyStore()
  if (keys.has(kid)) {
    return keys.get(kid)
  }

  throw new Error('Key not found')
}

Finally, when iterating over claims such as scopes or permissions, use bounded loops and avoid dynamic growth of internal structures:

const scopes = request.authData.scopes || []
if (scopes.length > 100) {
  throw new Error('Too many scopes')
}

for (let i = 0; i < Math.min(scopes.length, 100); i++) {
  const scope = scopes[i]
  // Process each scope within safe bounds
}

These practices ensure that JWT handling remains predictable and that no external input can induce out-of-bounds conditions during token processing in AdonisJS.