Format String in Feathersjs with Api Keys

A format string vulnerability in a Feathersjs service becomes high risk when the service authenticates requests using API keys. In Feathers, a typical service file uses app.service('todos').find() or similar methods that accept query parameters supplied directly from the client. If user-controlled input is interpolated into logging, error messages, or debug output using functions like console.log with unsanitized format strings (e.g., console.log(`Processing ${userInput}`) or logger.info(userInput, { meta: extra })), an attacker can supply format specifiers such as %s, %x, or %n to read from the stack or potentially write memory.

When API keys are used for authentication, the API key is usually validated early in the request lifecycle—often via a hook—before the service logic runs. This means the key is present in request state, but if the application later logs or processes user-supplied data using unsafe formatting, the key’s presence in scope can be indirectly exposed. For example, if a hook logs the key for debugging using a vulnerable format string, an attacker can leverage format specifiers to leak the key from memory. This is an example of how insecure logging practices can turn authenticated endpoints into information-disclosure vectors.

Consider a Feathers service where a user-controlled query field is passed into a logging call:

// Before fix: unsafe logging with format string
app.service('todos').hooks({
  before: [
    context => {
      const { query } = context;
      console.log('Processing query:', query.search); // Unsafe if query.search is user-controlled
      return context;
    }
  ]
});

If query.search contains format specifiers (e.g., %s %x %x), the logging call may read arbitrary memory. If the API key is stored in a closure or module-level variable within the same execution context, the attacker might coax the key into the output through repeated probes, demonstrating how authentication data can be implicated even when not directly logged.

Additionally, Feathers allows custom hooks and mixins; if a mixin uses string formatting on request-scoped data (such as IDs or keys) without proper sanitization, it can expose sensitive runtime behavior. The interaction between API-key-based authentication and format string bugs is subtle: the key may not be directly logged, but the surrounding context—including error objects, stack traces, or debug output—can reference the authenticated session, enabling an attacker to infer or reconstruct the key through side channels.

Real-world impact aligns with common patterns in OWASP API Top 10:2023, particularly Improper Logging & Monitoring, and can map to broader classifications such as CWE-134 (Use of Externally-Controlled Format String). While Feathers does not introduce format string risks by itself, the framework’s flexibility in handling hooks and services means developers must ensure that any logging, error handling, or string interpolation involving user input or metadata is strictly controlled.

To mitigate format string risks in Feathersjs when using API keys, focus on safe handling of all user-controlled data and metadata. Never pass unsanitized input directly to logging or formatting functions. Use structured logging with explicit field names, and avoid interpolation that can be interpreted as format specifiers.

1. Replace unsafe logging with structured, non-formatting logs

Instead of using console.log with string interpolation, use a structured logger and pass data as separate arguments or as JSON. This prevents the logging backend from misinterpreting user input as format specifiers.

// Safe: structured logging without format string interpolation
const logger = require('./logger'); // your structured logger

app.service('todos').hooks({
  before: [
    context => {
      const { query } = context;
      logger.info({
        event: 'todos_query',
        search: query.search,
        timestamp: new Date().toISOString()
      });
      return context;
    }
  ]
});

2. Validate and sanitize query and body inputs early

Use a validation layer before the request reaches service logic. This ensures only expected data shapes are processed and reduces the chance that attacker-controlled strings reach logging or business logic.

// Example using express-validator-like checks in a Feathers before hook
app.service('todos').hooks({
  before: [
    context => {
      const search = context.query.search;
      if (typeof search !== 'string' || search.length > 100) {
        throw new Error('invalid_search');
      }
      // Safe: search is now a controlled string
      context.params.safeSearch = search;
      return context;
    }
  ]
});

3. Avoid logging API keys or any sensitive metadata

Ensure that API keys and other secrets are never included in logs, even indirectly. If you must correlate requests, use a request ID that is separate from authentication material.

// Safe: do not log API keys
app.service('todos').hooks({
  before: [
    context => {
      // context.params.apiKey should never be logged
      logger.debug({
        requestId: context.params.requestId,
        path: context.path,
        method: context.method
      });
      return context;
    }
  ]
});

4. Use consistent error handling that avoids leaking context

Custom error handlers should not echo raw user input back in messages that might be processed by logging frameworks that interpret format strings.

// Safe: generic error messages
class HttpError extends Error {
  constructor(statusCode, message) {
    super(message);
    this.statusCode = statusCode;
  }
}

app.use((err, req, res, next) => {
  if (err instanceof HttpError) {
    res.status(err.statusCode).json({ error: err.message });
  } else {
    // Do not include raw input in production errors
    res.status(500).json({ error: 'Internal error' });
  }
});

By combining these practices—structured logging, input validation, and strict separation of authentication material from logs—you reduce the risk that format string bugs can interact with API-key-based authentication in Feathersjs services.