Format String in Adonisjs with Basic Auth

A format string vulnerability occurs when user-controlled input is passed directly to a function that performs string formatting without proper sanitization or validation. In AdonisJS, this commonly arises when logging, error reporting, or response construction uses functions like util.format (Node.js) or string interpolation with unchecked input. When Basic Authentication is used, the Authorization header carries credentials in the form Basic base64(username:password). If the application decodes this header and incorporates the username or password into a log message or error response using an unsafe formatter, an attacker can supply format specifiers (e.g., %s, %x, %n) to read stack memory or cause crashes.

Consider a scenario where AdonisJS parses Basic Auth credentials and logs them for debugging:

const authHeader = request.headers().authorization
if (authHeader && authHeader.startsWith('Basic ')) {
  const decoded = Buffer.from(authHeader.split(' ')[1], 'base64').toString('utf-8')
  const [user, pass] = decoded.split(':')
  console.log('Authenticated user:', user) // Safe if using parameterized logging
  console.log(`User: %s`, user)           // Risky if 'user' contains format specifiers
}

If an attacker sends a username like alice%s%s%s, the %s specifiers can cause console.log to read additional arguments from the stack or heap, potentially leaking internal variables or causing denial of service. In more severe cases, format string bugs can lead to information disclosure or memory corruption depending on the underlying C++ bindings or native modules AdonisJS may rely on. Because Basic Auth credentials are transmitted on every request, the attack surface is persistent across sessions.

Another exposure path arises when input validation libraries or route parameter sanitizers treat the decoded credentials as plain strings and pass them into template rendering or response builders that use format-style interpolation. For example, using Lodash’s template with unsanitized user input can allow an attacker to inject format directives that are later interpreted by JavaScript’s String.prototype.replace patterns, leading to unexpected behavior or data leakage.

While AdonisJS itself does not introduce format string flaws, the framework’s flexibility in handling request data means developers must ensure that any user-supplied string—especially from authentication headers—is sanitized before being used in formatting operations. MiddleBrick’s LLM/AI Security checks can detect whether prompts or generated outputs include unsanitized credential handling, but for API endpoints, the focus should remain on strict input validation and avoiding dynamic format strings with external data.

To mitigate format string risks when using Basic Authentication in AdonisJS, ensure that any data derived from the Authorization header is treated as opaque and never used directly in formatting functions. Instead, validate and sanitize credentials before use, and prefer structured logging that avoids string interpolation with raw input.

1. Validate and normalize credentials without string interpolation

Decode the Basic Auth header and split into username and password, but avoid passing these values directly into format-style logging. Use explicit property logging instead:

const authHeader = request.headers().authorization
if (authHeader && authHeader.startsWith('Basic ')) {
  try {
    const decoded = Buffer.from(authHeader.split(' ')[1], 'base64').toString('utf-8')
    const [user, pass] = decoded.split(':')
    // Safe: explicit logging without format specifiers
    console.log({ event: 'auth_attempt', user })
  } catch (error) {
    console.log({ event: 'auth_malformed' })
  }
}

2. Use parameterized logging libraries

AdonisJS applications should use logging libraries that support structured logging (e.g., @adonisjs/logger with JSON output) to avoid format string pitfalls:

const logger = use('Logger')
authHeader = request.headers().authorization
if (authHeader && authHeader.startsWith('Basic ')) {
  const decoded = Buffer.from(authHeader.split(' ')[1], 'base64').toString('utf-8')
  const [user] = decoded.split(':')
  logger.info('auth_event', { username: user })
}

3. Reject suspicious credentials early

Add a pre-validation layer to reject usernames containing format specifiers or non-printable characters before authentication logic executes:

function isValidUsername(username) {
  return /^[\w\-\.]+$/.test(username)
}

const authHeader = request.headers().authorization
if (authHeader && authHeader.startsWith('Basic ')) {
  const decoded = Buffer.from(authHeader.split(' ')[1], 'base64').toString('utf-8')
  const [user] = decoded.split(':')
  if (!isValidUsername(user)) {
    return response.badRequest({ error: 'invalid_credentials' })
  }
  // Proceed with authentication
}

4. Secure route handling with explicit parameters

When using route parameters that might be influenced by authentication context, bind them explicitly rather than interpolating raw strings:

Route.get('/profile/:username', async ({ params, request }) => {
  const authHeader = request.headers().authorization
  // Do not use params.username in format strings with external input
  const safeUsername = params.username.replace(/[^a-zA-Z0-9\-_]/g, '')
  return { user: safeUsername }
})

These practices ensure that Basic Authentication credentials are handled as opaque identifiers, eliminating the risk of format string exploitation while maintaining compatibility with AdonisJS’s request lifecycle.