Type Confusion in Fiber with Basic Auth

Type confusion in the Fiber web framework occurs when input that should be a specific type (such as a numeric identifier or a structured object) is interpreted as another type due to weak validation. When Basic Authentication is used, credentials are often extracted from the request header, parsed, and bound to request context or middleware variables. If the parsing logic assumes a particular type—such as a numeric user ID or a boolean flag—but receives a different type (for example, a string that looks like a number or a JSON object), the application can behave unexpectedly.

Consider a scenario where a Fiber route expects an integer ID from a decoded Basic Auth payload or a related token, but an attacker supplies a string or a JSON object. Without strict type checks, the route may incorrectly treat the string as an integer, leading to wrong comparisons, bypassed authorization checks, or unsafe operations. This becomes especially dangerous when combined with routes that perform privilege-sensitive actions, such as administrative endpoints or user data exports.

In the context of a security scan, checks such as BOLA/IDOR and Input Validation examine whether type assumptions are enforced. If the scan detects that a numeric comparison or authorization decision is performed on user-controlled input without verifying the type, it can flag a potential confusion issue. This is compounded when Basic Auth credentials are reused across multiple handlers, increasing the attack surface. An attacker who can manipulate the type of data derived from authentication context may be able to trigger logic flaws that lead to unauthorized data access or improper privilege escalation.

Using OpenAPI/Swagger spec analysis, a scanner can cross-reference the expected schema for authentication-related parameters with runtime observations. If the spec defines an integer user identifier but runtime data shows strings or objects, the spec-to-runtime mismatch highlights a potential type confusion path. This is not about parsing errors alone; it is about how the application reacts when the type deviates from expectations in authenticated requests.

Real-world examples in other frameworks and APIs show that type confusion can lead to bypasses in access control, information leakage, or unsafe deserialization. In Fiber, this often surfaces in custom middleware where authentication data is decoded and assigned to context values without strict type assertions. The presence of Basic Auth does not inherently introduce type confusion, but it can amplify the impact when type assumptions are incorrectly applied to authenticated request contexts.

To mitigate type confusion in Fiber when using Basic Authentication, ensure that all data derived from authentication is validated and explicitly typed before use. Do not rely on interface{} or implicit type assertions. Instead, parse credentials into concrete structures and enforce type checks before any authorization or business logic.

Below are concrete, working examples for secure Basic Auth handling in Fiber.

import (\n  "encoding/base64"\n  "net/http"\n  "strings"\n  "strconv"\n  "github.com/gofiber/fiber/v2"\n)

// Safe Basic Auth parsing with explicit types\nfunc parseBasicAuth(auth string) (string, int64, error) {\n  parts := strings.SplitN(auth, ":", 2)\n  if len(parts) != 2 {\n    return "", 0, http.ErrMissingAuthorization\n  }\n  username := parts[0]\n  // Explicitly convert to int64 to avoid type confusion\n  id, err := strconv.ParseInt(parts[1], 10, 64)\n  if err != nil {\n    return "", 0, http.ErrMissingAuthorization\n  }\n  return username, id, nil\n}\n
app.Get("/user/:id", func(c *fiber.Ctx) error {\n  auth := c.Get("Authorization")\n  if auth == "" {\n    return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "missing authorization"})\n  }\n  const bearerPrefix = "Basic "\n  if !strings.HasPrefix(auth, bearerPrefix) {\n    return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization scheme"})\n  }\n  encoded := strings.TrimPrefix(auth, bearerPrefix)\n  decoded, err := base64.StdEncoding.DecodeString(encoded)\n  if err != nil {\n    return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization header"})\n  }\n  username, id, err := parseBasicAuth(string(decoded))\n  if err != nil {\n    return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid credentials"})\n  }\n  // Use explicit types; id is int64, preventing confusion with strings or other types\n  paramID, err := strconv.ParseInt(c.Params("id"), 10, 64)\n  if err != nil || paramID != id {\n    return c.Status(fiber.StatusForbidden).JSON(fiber.Map{"error": "forbidden"})\n  }\n  return c.JSON(fiber.Map{"user": username, "id": id})\n})

Key remediation practices:

• Always decode and parse Basic Auth credentials into concrete types (e.g., string for username, int64 for identifiers) instead of using empty interfaces.
• Validate input types before comparisons or authorization checks; do not assume numeric IDs from strings or objects.
• Use strict error handling for parsing failures and reject requests with malformed credentials.
• Avoid reusing authentication-decoded values in multiple contexts without re-validation; keep the typed contract explicit.

These steps reduce the risk that an attacker can manipulate type expectations in authenticated requests, addressing both the detection signals highlighted by the scanner and the underlying logic flaws.