Command Injection in Buffalo with Jwt Tokens

Command injection occurs when untrusted input is concatenated into system commands, allowing an attacker to execute arbitrary shell commands. In Buffalo applications, this risk can intersect with JWT token handling when token payloads or headers are used to influence command construction. For example, if a JWT claim such as sub or a custom field is passed without validation into a shell command—perhaps to build a filename, a log tag, or a tool argument—an attacker who can influence the token (via a compromised client, a weak signing key, or token injection) can inject shell metacharacters like ;, &, or backticks.

Consider a scenario where a Buffalo handler decodes a JWT and uses a claim to specify a system utility to run. Because Buffalo does not inherently sanitize external inputs, the developer must explicitly validate and escape any data derived from the token before it reaches a command executor. If the token contains a value like admin; rm -rf / and that value is interpolated into a command string, the shell will execute both the intended operation and the injected destructive command. This becomes especially dangerous when the JWT is accepted from unauthenticated or public endpoints, or when the token is parsed without verifying the issuer or signature, effectively turning the JWT into a vector for privilege escalation or data destruction.

The vulnerability is not inherent to JWTs themselves—JWTs are a standard for representing claims—but emerges when developers treat token contents as safe for direct shell interpolation. In Buffalo, which encourages rapid development with Go, the risk is heightened if command execution is performed via os/exec with unsanitized strings derived from token claims. Attack patterns include classic command injection via shell operators, and abuse of environment variables or PATH manipulation if the command relies on external binaries. The OWASP API Top 10 category of Injection and Broken Object Level Authorization (BOLA) applies here, as improperly scoped token usage can allow an attacker to act across authorization boundaries through injected commands.

To mitigate command injection when working with JWT tokens in Buffalo, you must validate, restrict, and escape any token-derived data before it reaches system commands. Below are concrete code examples demonstrating secure practices.

1. Validate and constrain token claims

Only accept expected claims and enforce strict patterns. For example, if a claim is expected to be a username, restrict it to alphanumeric characters.

import (
    "github.com/golang-jwt/jwt/v5"
    "regexp"
    "strings"
)

func safeUsernameFromToken(tokenString string) (string, error) {
    token, _, err := new(jwt.Parser).ParseUnverified(tokenString, jwt.MapClaims{})
    if err != nil {
        return "", err
    }
    claims, ok := token.Claims.(jwt.MapClaims)
    if !ok || !token.Valid {
        return "", errors.New("invalid token")
    }
    username, ok := claims["username"].(string)
    if !ok {
        return "", errors.New("missing username claim")
    }
    // Allow only letters, digits, underscores, and dashes
    match, _ := regexp.MatchString(`^[a-zA-Z0-9_-]+$`, username)
    if !match {
        return "", errors.New("invalid username format")
    }
    return username, nil
}

2. Avoid shell interpolation; use exec.Command with explicit arguments

Never build a command string via concatenation. Use exec.Command with separate arguments so the shell is not involved.

import (
    "os/exec"
)

func runTool(username string) ([]byte, error) {
    // Safe: arguments are passed directly, not interpreted by a shell
    cmd := exec.Command("/usr/bin/logger", "-t", "app-user-"+username)
    return cmd.CombinedOutput()
}

3. Use allowlists for any external command or binary selection

If a JWT claim must select which binary or operation to run, map claims to a predefined set of safe commands rather than using raw token values as paths or command names.

func commandFromClaim(claim string) (*exec.Cmd, error) {
    switch claim {
    case "report":
        return exec.Command("/usr/local/bin/gen-report"), nil
    case "export":
        return exec.Command("/usr/local/bin/export-data"), nil
    default:
        return nil, errors.New("unsupported operation")
    }
}

4. Reject tokens with suspicious content early

Inspect token payloads for shell metacharacters or encoded commands before any processing, and reject such tokens.

func containsShellMetachars(s string) bool {
    metachars := []string{`;`, `&`, `|`, `$`, `(`, `)`, `` ` ``, `\n`}
    for _, c := range metachars {
        if strings.Contains(s, c) {
            return true
        }
    }
    return false
}

5. Enforce token integrity and scope

Always verify the token signature, issuer, and intended audience. Do not accept unsigned or malformed tokens, and scope token usage to the minimum required claims.

func verifyToken(tokenString string) (*jwt.Token, error) {
    return jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method")
        }
        return []byte("your-secure-secret"), nil
    }, jwt.WithValidMethods([]string{"HS256"}), jwt.WithIssuer("myapp"))
}