Unicode Normalization in Fiber with Basic Auth

Unicode normalization inconsistencies can affect HTTP Basic Authentication in Fiber-based services when usernames or passwords contain characters that have multiple binary representations. For example, the character é can be represented as a single code point U+00E9 or as a composed sequence e + combining acute accent U+0301. If a Fiber application normalizes credentials differently than the identity provider or directory service, authentication may fail in ways that can be leveraged in security testing.

In a black-box scan, middleBrick tests authentication surfaces without credentials. When Basic Auth is used, the scanner observes how the service handles malformed or overlong headers, including those that include non-ASCII characters. A server that does not enforce a canonical normalization form may accept Authorization: Basic base64("user\u0301pass") and Authorization: Basic base64("user\u00e1pass") as distinct credentials, potentially bypassing intended access controls. This discrepancy can be surfaced in findings under the Authentication and Input Validation checks, where the scanner flags inconsistent handling of normalized input.

Related findings may also appear in the Property Authorization and BOLA/IDOR checks if access controls are derived from normalized usernames without consistent canonicalization. For instance, a directory that stores usernames in NFC may match one request but not another, leading to authorization mismatches that are reported with remediation guidance to normalize both storage and comparison logic. middleBrick’s cross-referencing of OpenAPI specs with runtime behavior helps highlight such mismatches when doc-defined security schemes omit normalization requirements.

Under LLM/AI Security checks, non-ASCII characters in Basic Auth headers are probed indirectly through prompt injection and output scanning, ensuring that leakage or inconsistent handling does not expose system details. Because Basic Auth credentials are base64-encoded rather than encrypted, any normalization-related inconsistency that leads to acceptance of multiple equivalent forms increases the risk of unintended access paths, which is surfaced as a high-severity finding with remediation guidance to enforce NFC consistently.

To mitigate Unicode normalization issues with Basic Auth in Fiber, normalize usernames and passwords to a canonical form before comparison or storage. For Go services using Fiber, implement a normalization layer in the authentication middleware using the golang.org/x/text/unicode/norm package to convert incoming credentials to NFC.

Example middleware that decodes and normalizes Basic Auth credentials:

package main

import (
    "encoding/base64"
    "strings"

    "github.com/gofiber/fiber/v2"
    "golang.org/x/text/unicode/norm"
)

// normalize returns the NFC form of the input string.
func normalize(s string) string {
    return norm.String(norm.NFC, s)
}

// basicAuthMiddleware validates Basic Auth credentials with Unicode normalization.
func basicAuthMiddleware(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    if len(auth) < 6 || strings.TrimPrefix(auth, "Basic ") == auth {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "missing or invalid authorization header"})
    }

    payload, err := base64.StdEncoding.DecodeString(strings.TrimPrefix(auth, "Basic "))
    if err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid authorization encoding"})
    }

    parts := strings.SplitN(string(payload), ":", 2)
    if len(parts) != 2 {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "malformed credentials"})
    }

    username := normalize(parts[0])
    password := normalize(parts[1])

    // Replace with your identity verification logic, ensuring stored values are also normalized.
    if username != "admin" || password != "p@ssw0rd" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid credentials"})
    }

    return c.Next()
}

func main() {
    app := fiber.New()
    app.Use(basicAuthMiddleware)
    app.Get("/secure", func(c *fiber.Ctx) error {
        return c.JSON(fiber.Map{"message": "authorized"})
    })
    app.Listen(":3000")
}

Ensure that any backend directory or database stores usernames and passwords in the same normalized form (NFC). When integrating with external identity providers, verify that their normalization behavior matches or apply normalization on both sides. middleBrick scans can validate that your API consistently rejects equivalent but differently encoded credentials, and findings include prioritized remediation steps and references to relevant standards.

For teams using the CLI, you can run middlebrick scan <url> to validate the behavior of your endpoints. Pro plan users receive continuous monitoring and GitHub Action integration to fail builds if deviations in header handling are detected. The Dashboard can track how remediation changes affect your security scores over time.