Header Injection in Fiber with Api Keys

Header Injection in a Fiber application occurs when user-controlled input is directly reflected into HTTP response headers without validation or sanitization. This becomes particularly risky when API keys are involved because keys are often passed via headers (for example, x-api-key), and an attacker can manipulate headers to exfiltrate keys, bypass intended routing, or poison downstream services.

In Go Fiber, developers sometimes read a key from a request header and then set custom headers in the response without sanitization. If the application uses the header value to construct other headers, or logs headers directly into responses, an attacker can inject newline sequences (e.g., \r\n) to append or override headers. This can lead to response splitting, header smuggling, or information disclosure. Because API keys are high-value secrets, an injected header that echoes the key back to the client or to an untrusted origin can expose credentials.

Consider an endpoint that forwards an API key to a backend and then reflects some metadata in a custom header. Without proper validation, an attacker can supply a crafted header such as x-api-key: abc123\r\nSet-Cookie: malicious=1. If Fiber appends user input into another header (e.g., X-Key-Origin), the newline can terminate the current header and inject a new one, causing the client to store a malicious cookie or to trust a different origin. This is especially dangerous when combined with CORS misconfigurations, as injected headers may widen the allowed origins for the attacker’s domain.

Another scenario involves server-side logging or debugging endpoints that include incoming headers in responses or logs. If an API key is passed in Authorization or a custom key header and later echoed in an X-Debug-Info header without escaping, the key may be unintentionally exposed to any party that can force the server to include that header in a response they can observe. This can happen when developers use string concatenation or simple interpolation to build header values instead of using typed structures.

Because Fiber is a fast, expressive web framework, developers may inadvertently trust headers from proxies or load balancers. If the API key is extracted from a header and then forwarded to other internal services without validation, header injection can lead to request smuggling or SSRF-like effects where injected headers alter routing or authentication behavior in downstream components.

To detect these patterns, scans should examine whether response headers are constructed from user input, whether newline characters are filtered out, and whether API keys are ever reflected in headers or logs. The scanner checks for missing input validation on headers that carry authentication material and highlights places where untrusted data reaches sensitive header positions.

Remediation focuses on strict validation, avoiding direct reflection of user-controlled data into headers, and using constants or secure configuration for keys. Never concatenate or interpolate user input into header names or values. Instead, treat API keys as opaque credentials and use Fiber’s context methods safely.

Below are concrete, safe patterns for handling API keys in a Fiber application. The first example shows a secure way to read and validate an API key without reflecting it in response headers.

const allowedKeys = new Set([process.env.VALID_API_KEY]);

app.Get('/resource', (c *fiber.Ctx) => {
  const apiKey = c.get('x-api-key');
  if (!apiKey || !allowedKeys.has(apiKey)) {
    return c.Status(fiber.StatusUnauthorized).JSON({ error: 'invalid key' });
  }
  // Proceed with request handling; do not set any header derived from apiKey
  c.Set('X-Request-Id', uuidv4());
  return c.JSON({ data: 'ok' });
});

The second example demonstrates how to forward a key to a backend safely by using a constant header name and sanitizing any user input that might reach logs. It avoids setting headers based on raw user input and uses structured logging instead of header echoing.

app.Post('/forward', (c *fiber.Ctx) => {
  const userKey := c.Get('x-api-key');
  if !isValidKey(userKey) {
    return c.SendStatus(fiber.StatusBadRequest);
  }
  // Use a constant header name when forwarding; never reuse a user-supplied header name
  req, _ := http.NewRequestWithContext(c.Request().Context(), 'GET', 'https://backend.example.com/data', nil)
  req.Header.Set('Authorization', 'ApiKey '+userKey)
  req.Header.Set('X-Client-Id', c.Get('x-client-id')) // only if already validated
  // Do not copy arbitrary headers from the client to the request
  resp, err := http.DefaultClient.Do(req)
  if err != nil {
    return c.SendStatus(fiber.StatusBadGateway)
  }
  defer resp.Body.Close()
  // Avoid including raw user input in any header or log line
  c.Set('X-Forwarded-By', 'service-proxy')
  return c.Send(resp.Body)
});

Additional measures include rejecting requests that contain newline characters in header values, using strict allowlists for known header names, and ensuring that any logging or debugging output does not concatenate raw header values. If you use middleware for key validation, keep the logic centralized and avoid per-route concatenation.

When you need to expose minimal metadata, use response headers with static values or values derived from server-side configuration, never from user-controlled input. This prevents attackers from manipulating the response chain through injected headers.