Shellshock in Chi with Dynamodb

Shellshock is a family of command injection vulnerabilities in the Bash shell that allow attackers to execute arbitrary commands via specially crafted environment variables. When an API built with Chi (a lightweight HTTP router for Go) interacts with DynamoDB, the risk arises if user-controlled data is passed into shell commands or into Lambda function environments that use Bash. DynamoDB streams or Lambda event sources can include item attributes that, if improperly sanitized, become part of shell invocations. For example, a DynamoDB attribute such as command or script could be used to construct a Bash command string in a Lambda handler written in a language that invokes a shell (e.g., using os/exec with sh -c). The Chi router may expose endpoints that trigger these Lambda functions or background workers, and if input validation is weak, an attacker can inject shell metacharacters to alter command behavior.

In practice, consider a Chi endpoint that receives a DynamoDB item via an event payload and runs a system command based on an attribute. If the attribute is not validated, characters like backticks, $(), or ; can lead to command injection, which is the practical manifestation of Shellshock-style behavior in this context. Even when not using Bash directly, DynamoDB data that reaches shell-like interpreters (e.g., via environment variables in containerized Lambda runtimes) can reproduce injection paths. The combination of Chi routing, DynamoDB as the data source, and insecure command construction creates a clear attack surface: unvalidated DynamoDB fields can propagate into execution contexts where Shellshock-style injection is feasible.

Additionally, if the API exposes an endpoint that triggers a Lambda function reading from DynamoDB and that function exports item attributes as environment variables, those attributes may be interpreted by Bash in child processes. This mirrors classic Shellshock exploitation where environment variables are passed into vulnerable shell functions. MiddleBrick’s LLM/AI Security checks and runtime scanning help detect unsafe patterns where DynamoDB-derived data reaches execution contexts, providing findings aligned with OWASP API Top 10 and injection-related risks.

Remediation focuses on strict input validation, avoiding shell invocation, and ensuring DynamoDB data never reaches executable contexts. Prefer using the AWS SDK for DynamoDB operations instead of constructing shell commands. When you must invoke external processes, use command arguments directly rather than a shell, and explicitly whitelist allowed values.

Example 1: Unsafe pattern in Chi with DynamoDB item leading to shell injection

import (
    "os/exec"
    "net/http"
    "github.com/go-chi/chi/v5"
)

// Unsafe: using shell to run a command built from DynamoDB data
func handler(w http.ResponseWriter, r *http.Request) {
    // Assume itemAttr comes from a DynamoDB stream or event
    itemAttr := chi.URLParam(r, "value") // user-controlled
    cmd := exec.Command("sh", "-c", "echo "+itemAttr)
    cmd.Stdout = os.Stdout
    cmd.Run()
}

Example 2: Safe pattern — avoid shell, use exec.Command directly

import (
    "os/exec"
    "net/http"
    "github.com/go-chi/chi/v5"
    "regexp"
)

// Safe: no shell, direct arguments, strict validation
var safeValue = regexp.MustCompile(`^[a-zA-Z0-9_\-]+$`)

func handler(w http.ResponseWriter, r *http.Request) {
    itemAttr := chi.URLParam(r, "value")
    if !safeValue.MatchString(itemAttr) {
        http.Error(w, "invalid input", http.StatusBadRequest)
        return
    }
    // No shell; command and arguments are separate
    cmd := exec.Command("echo", itemAttr)
    cmd.Stdout = os.Stdout
    cmd.Run()
}

Example 3: Safe DynamoDB integration in Chi without shell usage

import (
    "context"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/config"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/go-chi/chi/v5"
    "net/http"
)

func getDynamoItem(w http.ResponseWriter, r *http.Request) {
    cfg, err := config.LoadDefaultConfig(context.TODO())
    if err != nil {
        http.Error(w, "config error", http.StatusInternalServerError)
        return
    }
    client := dynamodb.NewFromConfig(cfg)

    id := chi.URLParam(r, "id")
    // Validate ID format strictly to prevent injection in downstream logic
    if id == "" {
        http.Error(w, "missing id", http.StatusBadRequest)
        return
    }

    req := &dynamodb.GetItemInput{
        TableName: aws.String("MyTable"),
        Key: map[string]aws.AttributeValue{
            "ID": &dynamodb.AttributeValueMemberS{Value: id},
        },
    }

    out, err := client.GetItem(r.Context(), req)
    if err != nil {
        http.Error(w, "dynamodb error", http.StatusInternalServerError)
        return
    }

    // Process out.Item safely; do not pass to shell or template without escaping
    w.Write([]byte("OK"))
}

Key remediation practices:

• Never construct shell commands using DynamoDB data; use the SDK directly.
• If external commands are required, avoid sh -c or bash; pass arguments explicitly.
• Validate and sanitize all input derived from DynamoDB using strict allowlists.
• Review Lambda environment variables and ensure no DynamoDB attributes are exported to Bash context.

MiddleBrick scans can surface these patterns by analyzing endpoint behavior and spec-derived runtime findings, helping you identify insecure command construction and data flows involving DynamoDB.