Command Injection in Fiber with Basic Auth

Command Injection occurs when an attacker can cause a program to execute arbitrary operating system commands. In Go Fiber applications that use Basic Authentication, risk arises when credentials or user-controlled data are passed into shell commands without proper sanitization or isolation. Even though Basic Auth itself is a standard HTTP authentication mechanism, developers sometimes mistakenly treat the decoded username or password as safe input and concatenate it into command invocations, for example via os/exec without validation.

Consider a handler that authenticates a user via Basic Auth and then uses a username to construct a shell command. If the username contains shell metacharacters such as ;, &, or backticks, and the application directly interpolates that value into a command string, an authenticated or unauthenticated attacker can inject additional commands. This can occur in scenarios where the application runs privileged operations or logs user activity by invoking shell utilities. Because the endpoint is protected by Basic Auth, developers might assume the input is trusted, but the authentication boundary does not sanitize or validate the content of credentials for shell safety.

When combined with the broader scanning approach of middleBrick, which runs 12 security checks in parallel including Authentication, Input Validation, and Unsafe Consumption, such patterns are detectable. middleBrick tests unauthenticated attack surfaces and, where applicable, can exercise authentication mechanisms to observe how inputs flow through the application. If a scan includes endpoints that use Basic Auth and subsequently invoke external commands, findings may highlight Command Injection with severity tied to the context and potential impact, such as unauthorized command execution or information disclosure. Remediation focuses on avoiding shell invocation for trusted data, using parameterized commands, and strictly validating inputs even when they originate from authentication headers.

Real-world examples align with known attack patterns outlined in the OWASP API Top 10 and common vulnerability databases. For instance, if a username is passed to a command like ping without escaping, an attacker could attempt username; id to run additional code. Because middleBrick also checks for Data Exposure and Encryption, it can highlight whether sensitive information is inadvertently revealed through such command behavior. The scanner does not fix or block these issues but provides prioritized findings with remediation guidance to help developers address the root cause in their code.

Secure handling of Basic Authentication in Fiber requires keeping credentials out of shell contexts and validating all inputs rigorously. The safest approach is to avoid invoking operating system commands with any data derived from authentication, including usernames or passwords. Instead, use internal logic or safe libraries to perform the necessary operations. If you must execute external commands, ensure that the inputs are not derived from authentication headers or user-controlled data.

Below are concrete, working code examples in Go using Fiber that demonstrate both vulnerable and remediated approaches.

Vulnerable pattern: using Basic Auth data in a shell command

// This example illustrates a risky pattern and should not be used.
package main

import (
    "os/exec"
    "strings"

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

func main() {
    app := fiber.New()

    cfg := basicauth.Config{
        Users: map[string]string{
            "admin": "secret123",
        },
    }
    app.Use(basicauth.New(cfg))

    app.Get("/run", func(c *fiber.Ctx) error {
        // Extract username from Basic Auth context (hypothetical; Fiber does not expose it directly like this)
        username := c.Get("X-Username") // This is illustrative; in practice you would parse the header safely
        cmd := exec.Command("ping", username) // Unsafe if username contains shell metacharacters
        out, err := cmd.Output()
        if err != nil {
            return c.Status(fiber.StatusInternalServerError).SendString(err.Error())
        }
        return c.Send(out)
    })

    app.Listen(":3000")
}

In the above, if the username could contain shell metacharacters, the command becomes vulnerable. Note that Fiber does not directly expose Basic Auth credentials in a parsed form in the context; this example assumes a hypothetical extraction to illustrate the risk pattern.

Remediated pattern: avoid shell usage and validate strictly

// Safe approach: avoid shell commands with authentication-derived data.
package main

import (
    "net"

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

func main() {
    app := fiber.New()

    cfg := basicauth.Config{
        Users: map[string]string{
            "admin": "securepassword",
        },
    }
    app.Use(basicauth.New(cfg))

    // Use internal logic instead of shell commands.
    app.Get("/health", func(c *fiber.Ctx) error {
        // Perform safe operations, e.g., network checks without shell.
        conn, err := net.Dial("tcp", "example.com:80")
        if err != nil {
            return c.Status(fiber.StatusServiceUnavailable).SendString("dependency check failed")
        }
        conn.Close()
        return c.SendString("healthy")
    })

    app.Listen(":3000")
}

If external commands are unavoidable, ensure arguments are statically defined and validated against a strict allowlist, and never concatenate authentication-derived strings. middleBrick can help identify such risky integrations by analyzing input flows and authentication usage across endpoints.