Format String in Feathersjs with Hmac Signatures

In FeathersJS, a Format String vulnerability can emerge when user-controlled input is directly passed to functions that interpret format specifiers, such as util.format or logging utilities, without proper sanitization. When Hmac Signatures are used for request integrity—common in webhook validation or internal service authentication—improper handling of these signatures can amplify the risk. For example, if a developer logs the raw signature or error messages using a format string that incorporates attacker-supplied data, an attacker may supply format specifiers like %s, %x, or %n to read stack contents or inject unintended output.

Consider a FeathersJS hook that validates an Hmac Signature extracted from headers. If the validation failure path uses a format string that embeds the signature value, a malicious payload such as ${signature}%s%s%s could disclose memory contents through the formatted log output. This becomes especially relevant when the signature is generated using a shared secret and included in logs for debugging. An attacker can probe the endpoint with crafted format strings, turning what appears as a routine authentication failure into a data exfiltration vector. Because the signature itself is sensitive, exposing even partial information can aid in offline brute-force or cryptanalysis attempts.

The interaction between format strings and Hmac Signatures is not about breaking the Hmac algorithm itself—hashing and key secrecy remain intact—but about insecure error handling and logging practices. A developer might write code like logger.info('Invalid signature: %s', receivedSignature), which is safe, but mistakenly use logger.info('Invalid signature: ' + receivedSignature) with user input embedded directly into the format string. If receivedSignature originates from an unvalidated source and the logging library uses printf-style formatting internally, the stage is set for a format string bug.

Real-world impact includes information disclosure, denial of service through crashes induced by malformed specifiers, or in rare cases, arbitrary memory writes depending on the runtime and logging backend. This maps to common OWASP API Top 10 categories around Security Misconfiguration and Improper Error Handling, and may intersect with Data Exposure findings in a middleBrick scan. Sensitive values like Hmac Signatures should never be interpolated into log messages or error responses, and developers must ensure that any formatting operation treats user input as data, not as part of the format template.

Remediation centers on strict separation of data and format templates, using safe logging utilities, and validating Hmac Signatures through constant-time comparison. Avoid concatenating or interpolating raw signature values into log messages or error strings. Instead, use parameterized logging where the format string is a static literal and user data is passed as separate arguments.

Example of unsafe code to avoid:

const receivedSignature = req.headers['x-hmac-signature'];
// Unsafe: direct concatenation can trigger format string issues if receivedSignature contains specifiers
logger.info('Invalid signature: ' + receivedSignature);

Safe alternative using parameterized logging:

const receivedSignature = req.headers['x-hmac-signature'];
// Safe: format string is static, signature is a separate argument
logger.info('Invalid signature: %s', receivedSignature);

When validating the Hmac Signature, use a constant-time comparison to prevent timing attacks. Below is a complete FeathersJS hook example that demonstrates secure handling:

const crypto = require('crypto');

function verifyHmac(data, receivedSignature, secret) {
  const hmac = crypto.createHmac('sha256', secret);
  const expected = hmac.update(data).digest('hex');
  // Use timing-safe comparison
  return crypto.timingSafeEqual(
    Buffer.from(expected),
    Buffer.from(receivedSignature)
  );
}

// FeathersJS hook
function hmacValidationHook(options = {}) {
  return async context => {
    const secret = process.env.HMAC_SECRET;
    const receivedSignature = context.headers['x-hmac-signature'];
    const payload = JSON.stringify(context.data);
    if (!receivedSignature || !verifyHmac(payload, receivedSignature, secret)) {
      // Safe logging: static format string
      logger.warn('Hmac signature validation failed');
      throw new Error('Unauthorized');
    }
    return context;
  };
}

// Apply the hook to a service
const app = require('feathers')();
app.service('secure-endpoint').hooks({
  before: {
    all: [hmacValidationHook()]
  }
});

Additionally, ensure that any debugging or error reporting tools do not inadvertently log raw headers. If using a framework that automatically logs requests, configure it to exclude sensitive headers. middleBrick can help identify leftover sensitive values in logs or misconfigured error handlers through its unauthenticated scans, supporting compliance mappings to OWASP API Top 10 and other frameworks.