Out Of Bounds Read in Fiber with Basic Auth

An Out Of Bounds Read occurs when a program reads memory beyond the intended buffer, potentially exposing stack or heap contents. In Fiber, this can arise when request handling logic does not properly validate the length or source of data used to construct responses or intermediate buffers. When Basic Authentication is used, the Authorization header is parsed by the application or by middleware to extract credentials. If the header value is used in an unchecked way—such as slicing into it with an attacker-controlled index or length—an Out Of Bounds Read can occur.

For example, suppose a Fiber route extracts the credentials substring without validating the header length before accessing bytes at specific positions. An attacker can send a very short or malformed Authorization header, causing the application to read beyond the allocated memory for that string. This may leak process memory contents in the response or logs, depending on how the extracted data is used. Because the scan tests unauthenticated attack surfaces, middleBrick will probe endpoints using Basic Auth headers and check whether responses reveal signs of memory disclosure or inconsistent behavior when header sizes vary.

In the context of the 12 parallel security checks, the BFLA/Privilege Escalation and Input Validation checks look for patterns where authentication data influences memory-sensitive operations. The scan also cross-references OpenAPI specifications—if the spec defines security schemes using Basic Auth but runtime behavior mishandles malformed or missing credentials, middleBrick correlates the mismatch as a finding. Because this check is part of the black-box scan, no credentials are required; the scanner evaluates how the endpoint behaves when the Authorization header is missing, malformed, or abnormally sized.

Real-world attack patterns such as CVE-2021-22505 (a Netfilter-related memory disclosure) illustrate how out-of-bounds reads can expose sensitive data. While not a Fiber-specific vulnerability, it demonstrates the class of risk: unchecked memory access triggered by input. middleBrick’s LLM/AI Security checks do not apply here because this is a memory safety issue, not an LLM endpoint issue. However, the scanner’s Inventory Management and Data Exposure checks help identify whether the leak appears in responses, logs, or error messages.

When you run a scan using the CLI with middlebrick scan <url>, the tool reports any indications of out-of-bounds behavior under the relevant category with severity and remediation guidance. If you use the GitHub Action, you can fail builds when such findings appear, and the Pro plan’s continuous monitoring can alert you on recurring patterns. The Dashboard then tracks how this class of finding evolves over time.

To prevent Out Of Bounds Read when using Basic Authentication in Fiber, always validate the presence, format, and length of the Authorization header before parsing it. Do not assume the header conforms to the expected Basic base64(credentials) format. Use bounded string operations or, better, rely on standard library functions that handle length checks internally.

Below is a secure example in Go using Fiber that decodes Basic Auth safely:

package main

import (
    "encoding/base64"
    "strings"

    "github.com/gofiber/fiber/v2"
)

func basicAuthMiddleware(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    if auth == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header missing"})
    }
    const prefix = "Basic "
    if !strings.HasPrefix(auth, prefix) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization type"})
    }
    encoded := auth[len(prefix):]
    if len(encoded) == 0 {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "credentials missing"})
    }
    decoded, err := base64.StdEncoding.DecodeString(encoded)
    if err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid authorization encoding"})
    }
    // Optionally split decoded into username:password and validate length
    parts := strings.SplitN(string(decoded), ":", 2)
    if len(parts) != 2 || parts[0] == "" || parts[1] == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid credentials format"})
    }
    // Here you would verify parts[0] and parts[1] against your backend logic
    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")
}

Key remediation steps reflected in the code:

• Check that the header exists and uses the Basic scheme before slicing.
• Avoid indexing into the encoded string with hardcoded offsets; use length checks and strings.HasPrefix.
• Validate decoded output length and structure before using credentials.
• Return early with clear error responses instead of proceeding with potentially malformed data.

By applying these practices, the application reduces the risk of reading beyond intended buffers when the Authorization header is malformed or unusually sized. middleBrick will note the implementation of such controls under the Input Validation and Authentication checks, and findings will include severity and remediation guidance. If you automate scans with the GitHub Action, you can enforce that endpoints using Basic Auth adhere to these patterns before deployment.