Command Injection in Echo Go with Hmac Signatures

Command injection occurs when an attacker can control part of a system command executed by the application. In Echo Go, combining external command execution (such as calling exec.Command) with Hmac Signatures for request authentication can still lead to injection if the signature or related parameters are mishandled. Hmac Signatures are typically used to verify request integrity and authenticity, but if the application uses signature values or derived data in constructing shell commands, an attacker may be able to inject shell metacharacters when the input is not properly validated or escaped.

Consider an API endpoint that verifies an Hmac Signature and then uses a payload field to build a shell command for diagnostics or backend processing. If the signature verification passes but the payload is concatenated directly into the command string, an attacker who can influence the payload might append shell operators (e.g., &&, ||, ;) to run arbitrary commands. Even when Hmac Signatures ensure the request comes from a trusted source, trust boundaries can be misaligned: the signature may protect integrity but does not sanitize or restrict the content of user-controlled data used in shell execution.

In Echo Go, this can manifest in handlers that parse JSON requests containing a signature and an action string. If the action is used in exec.Command without proper argument isolation (e.g., using exec.CommandContext with separate arguments), special shell characters in the action can lead to unintended command execution. For example, an action like list && exfiltrate_data could be executed if the application splits the string naively or passes it to a shell. The presence of Hmac Signatures may give a false sense of security, because the vulnerability lies in how the application uses the data after signature validation, not in the cryptographic verification itself.

Additionally, if the application derives subprocess arguments or environment variables from signed claims, and those values are later interpolated into shell commands, an attacker who can influence the claims (e.g., via a compromised client or a replayed request) may be able to control command behavior. This is especially risky when the server uses a shell to interpret commands (explicitly or implicitly via functions like sh -c), as metacharacters become meaningful. The combination of Echo Go routing, Hmac Signatures for client authentication, and improper command construction creates a scenario where trusted-sourced input can still lead to arbitrary command execution if input validation and argument handling are insufficient.

To remediate command injection in Echo Go when using Hmac Signatures, ensure that signed data is never directly interpolated into shell commands. Instead, validate and sanitize all inputs, use fixed command paths with explicit arguments, and avoid shell interpretation entirely. Below are concrete code examples demonstrating secure handling.

Example 1: Safe command execution with explicit arguments

Instead of concatenating user-influenced data into a shell command, use exec.Command with separate arguments. This prevents the shell from interpreting metacharacters.

import (
    "os/exec"
    "net/http"
    "github.com/labstack/echo/v4"
)

func handleAction(c echo.Context) error {
    var req struct {
        Signature string `json:"signature"`
        Action    string `json:"action"`
    }
    if err := c.Bind(&req); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid request")
    }
    // Verify Hmac Signature (pseudocode)
    if !verifyHmac(req.Signature, req.Action) {
        return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
    }
    // Map allowed actions to fixed commands and arguments
    allowed := map[string][]string{
        "list": {"/usr/bin/list", "--safe"},
        "info": {"/usr/bin/info", "--json"},
    }
    cmdArgs, ok := allowed[req.Action]
    if !ok {
        return echo.NewHTTPError(http.StatusBadRequest, "action not allowed")
    }
    cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
    out, err := cmd.CombinedOutput()
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "command failed")
    }
    return c.JSON(http.StatusOK, echo.Map{"output": string(out)})
}

Example 2: Using context timeouts and avoiding shell interpolation

Wrap command execution with context timeouts and ensure no shell is invoked. Never use sh -c or similar with concatenated strings.

import (
    "context"
    "time"
    "os/exec"
)

func runVerified(action string) (string, error) {
    allowedBinaries := map[string]string{
        "list": "/usr/bin/list",
        "info": "/usr/bin/info",
    }
    binary, ok := allowedBinaries[action]
    if !ok {
        return "", fmt.Errorf("action not allowed")
    }
    ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
    defer cancel()
    cmd := exec.CommandContext(ctx, binary) // no shell, explicit args
    output, err := cmd.CombinedOutput()
    if err != nil {
        return "", err
    }
    return string(output), nil
}

Input validation and canonicalization

Treat Hmac Signature verification as a trust signal for authenticity, not for input safety. Validate the action against a strict allowlist and reject any that do not match exactly. Do not attempt to sanitize by removing characters; use allowlisting instead.

Audit and logging

Log verification failures and rejected actions for monitoring. Ensure that error messages do not leak sensitive information that could aid an attacker in crafting malicious payloads.