Command Injection in Fiber with Api Keys

Command Injection occurs when an attacker can cause a program to execute arbitrary system commands. In the Go Fiber framework, this risk can arise when API keys are handled in ways that pass untrusted input to shell-like operations or external command construction, even if the keys themselves are not directly used for command execution.

Consider an endpoint that receives an API key as a query parameter or header and uses it to build a command string for logging, external tool invocation, or system interaction. If the API key value is concatenated directly into a command without validation or escaping, an attacker can supply additional shell metacharacters (such as &, |, ;, or $()) to alter command behavior. For example, an API key like abc123; cat /etc/passwd could cause the server to execute arbitrary commands if the input is used unsafely.

Even when API keys are validated against a known set, the surrounding code may still construct commands dynamically using string interpolation. In Fiber, route handlers often read headers with context.Get() and pass values to helper functions. If those functions forward the value to a command builder, the handler may unintentionally expose the application to injection despite the key being treated as an authentication credential. This becomes especially dangerous when combined with logging or monitoring tools that execute external processes, where the API key is included in command arguments or environment variables.

The interaction between authentication mechanisms and system command execution is subtle: the API key is not inherently dangerous, but the way it is incorporated into command construction can turn it into an injection vector. Real-world patterns include using the key to invoke scripts, trigger binaries, or interact with system utilities. Without strict input validation, output encoding, and avoidance of shell metacharacters in command assembly, an API key can act as the entry point for command injection.

An attacker may also probe endpoints that echo or log API keys in verbose error messages or debugging output. If those messages are processed by external tools or scripts that invoke shell commands, the key can be used to inject and escalate within the execution context. This highlights the importance of ensuring that any use of API keys in command-related workflows follows secure coding practices, such as using parameterized commands or avoiding shell invocation entirely.

To mitigate Command Injection risks when handling API keys in Fiber, ensure that API key values are never directly interpolated into commands. Use structured, parameterized approaches and strict validation.

Example of vulnerable code

package main

import (
	"github.com/gofiber/fiber/v2"
	"os/exec"
)

func main() {
	app := fiber.New()
	app.Get("/log", func(c *fiber.Ctx) error {
		apiKey := c.Get("X-API-Key")
		// Vulnerable: direct concatenation with shell command
		cmd := "echo API_KEY=" + apiKey
		exec.Command("sh", "-c", cmd).Run()
		return c.SendString("logged")
	})
	app.Listen(":3000")
}

Secure remediation using exec.Command with arguments

package main

import (
	"github.com/gofiber/fiber/v2"
	"os/exec"
)

func main() {
	app := fiber.New()
	app.Get("/log", func(c *fiber.Ctx) error {
		apiKey := c.Get("X-API-Key")
		// Secure: pass arguments directly without shell
		cmd := exec.Command("echo", "API_KEY="+apiKey)
		cmd.Stdout = nil // or capture output appropriately
		cmd.Stderr = nil
		cmd.Run()
		return c.SendString("logged")
	})
	app.Listen(":3000")
}

Validation and allowlisting

Validate API keys against a strict pattern before use. For example, if keys are expected to be alphanumeric strings of a fixed length:

package main

import (
	"github.com/gofiber/fiber/v2"
	"regexp"
)

func isValidAPIKey(key string) bool {
	// Allow only uppercase letters and digits, length 32
	pattern := regexp.MustCompile(`^[A-Z0-9]{32}$`)
	return pattern.MatchString(key)
}

func main() {
	app := fiber.New()
	app.Get("/validate", func(c *fiber.Ctx) error {
		apiKey := c.Get("X-API-Key")
		if !isValidAPIKey(apiKey) {
			return c.Status(fiber.StatusBadRequest).SendString("invalid key")
		}
		// Proceed with safe usage
		return c.SendString("valid")
	})
	app.Listen(":3000")
}

Avoiding shell invocation

Prefer standard library functions that do not invoke a shell. If external tooling is required, use explicit paths and pass arguments as a slice rather than a single command string.

package main

import (
	"github.com/gofiber/fiber/v2"
	"os/exec"
)

func safeExternal(apiKey string) error {
	// Secure: explicit binary with arguments
	cmd := exec.Command("/usr/local/bin/my-tool", "--key", apiKey)
	out, err := cmd.Output()
	if err != nil {
		return err
	}
	_ = out
	return nil
}

Environment and logging precautions

Do not log or expose raw API keys in structured logs that may be processed by external scripts. If logging is necessary, redact or hash the key. Avoid setting API keys as environment variables that could be read by child processes constructed via shell commands.