Command Injection in Fiber with Jwt Tokens

Command injection occurs when untrusted input is passed to a system shell or command interpreter without proper sanitization. In a Fiber application, this risk can be exposed through endpoints that accept user-controlled data and use it to construct shell commands, even when requests include Jwt Tokens for authentication. Jwt Tokens typically carry claims such as user ID or role, and developers sometimes use these claims directly in command construction, mistakenly assuming that authentication alone prevents abuse.

Consider a scenario where an API endpoint decodes a Jwt Token to extract a username claim and then uses that username as part of a shell command, for example to invoke a user-specific script or to generate a report. If the token payload is not validated for format and the resulting value is concatenated into a command string, an attacker who can influence the token’s payload (for instance, via account creation or token refresh in a weak implementation) or a downstream service that reflects attacker-controlled data into the token may be able to inject shell metacharacters. A malicious username like admin; cat /etc/passwd would cause the constructed command to execute additional instructions, leading to unauthorized file reads or further compromise.

The combination of Jwt Tokens and Fiber routing can inadvertently broaden the attack surface when authorization checks are incomplete. Authentication verifies identity, but it does not automatically enforce least-privilege command execution. If middleware extracts a claim and passes it to an external binary or script without input validation or output sanitization, the unauthenticated attack surface includes the token’s content as a potential injection vector. Moreover, if endpoints rely on optional or nested claims that are not strictly constrained, attackers might exploit missing validation to smuggle shell metacharacters through seemingly benign authenticated requests.

Real-world patterns include using environment variables derived from token claims to assemble commands, or logging user identity into system utilities that invoke subprocesses. Even when Jwt Tokens are verified using a robust middleware, the extracted claims must be treated as untrusted input. Attack patterns such as PATH injection or argument injection can manifest if the command relies on implicit search paths or unescaped arguments. For instance, a command built as report --user " + username + " --output /tmp/report becomes vulnerable if username contains characters like backticks or $(), enabling command substitution. Therefore, treating Jwt Token claims as safe because they originate from a trusted issuer is a common misconception that can lead to severe command injection flaws.

To detect such issues, scanning tools evaluate whether endpoints that require Jwt Tokens still pass untrusted data into shell-like contexts. They examine claim usage in command construction, validate input against strict allowlists, and check for missing escaping or sandboxing. This ensures that authenticated endpoints do not inadvertently expose dangerous functionality. Remember that authentication and authorization are necessary but insufficient defenses; command construction must always follow secure coding practices regardless of token validity.

Remediation focuses on strict input validation, avoiding shell construction altogether, and ensuring Jwt Token claims are never directly concatenated into commands. Use structured, non-shell interfaces such as library calls or parameterized APIs instead of spawning processes with user-influenced data. If command execution is unavoidable, apply rigorous allowlisting and use language features that prevent injection.

Example of vulnerable code in Fiber that uses a Jwt Token claim unsafely:

import (github.com/gofiber/fiber/v2)
import (github.com/golang-jwt/jwt/v5)
import (os/exec)

func unsafeHandler(c *fiber.Ctx) error {
    token := c.Locals("user")
    if claims, ok := token.(*jwt.Token).Claims.(jwt.MapClaims); ok {
        username := claims["username"]
        cmd := exec.Command("sh", "-c", "echo Hello " + username)
        out, _ := cmd.Output()
        return c.SendString(string(out))
    }
    return c.SendStatus(fiber.StatusUnauthorized)
}

In this example, the username claim is concatenated into a shell command, creating a command injection path. An attacker who can influence the token’s payload can execute arbitrary commands.

Secure alternative using argument separation and strict allowlisting:

import (github.com/gofiber/fiber/v2)
import (github.com/golang-jwt/jwt/v5)
import (os/exec)
import (regexp)

var usernameRegex = regexp.MustCompile(`^[a-zA-Z0-9_]{1,64}$`)

func safeHandler(c *fiber.Ctx) error {
    token := c.Locals("user")
    if claims, ok := token.(*jwt.Token).Claims.(jwt.MapClaims); ok {
        username, ok := claims["username"].(string)
        if !ok || !usernameRegex.MatchString(username) {
            return c.SendStatus(fiber.StatusBadRequest)
        }
        // Use exec.Command with separate arguments; no shell involved
        cmd := exec.Command("echo", "Hello", username)
        out, err := cmd.Output()
        if err != nil {
            return c.SendStatus(fiber.StatusInternalServerError)
        }
        return c.SendString(string(out))
    }
    return c.SendStatus(fiber.StatusUnauthorized)
}

This approach avoids the shell entirely, passes arguments directly to the executable, and validates the username against a strict pattern. For operations that require more complex processing, prefer using dedicated libraries instead of invoking external commands. MiddleBrick scans can verify that endpoints using Jwt Tokens do not pass raw claims into shell contexts and flag any missing allowlists or unsafe command construction.

Additional recommendations include using environment variables for configuration rather than dynamic claims, applying principle of least privilege to runtime execution contexts, and logging command attempts without including raw user input. If your workflow depends on external tooling, ensure that invocation paths are fixed and not derived from token data. The Pro plan supports continuous monitoring to detect regressions in command usage patterns, and the GitHub Action can enforce security gates in CI/CD pipelines to prevent insecure deployments.