Command Injection in Strapi with Hmac Signatures

Remediation focuses on ensuring that user-controlled data never reaches the shell as part of command arguments. The preferred approach is to avoid shell invocation entirely by using language-native operations or safe, parameterized APIs. When shell commands are unavoidable, the input must be strictly validated and passed as arguments rather than interpolated into the command string.

First, validate the filename against a strict allowlist or pattern. For image processing, allow only alphanumeric characters, dashes, and underscores, and enforce a known extension list. Reject any input containing shell metacharacters or path sequences. Second, use child_process.execFile or equivalent APIs that accept arguments as an array, preventing the shell from parsing metacharacters.

The following example demonstrates a secure implementation in Strapi using execFile:

const { execFile } = require('child_process');

module.exports = {
  customAction: async (ctx) => {
    const { filename, signature } = ctx.request.body;
    if (!verifyHmac(filename, signature)) {
      ctx.throw(401, 'Invalid signature');
    }
    // Validate filename format
    if (!/^[
a-zA-Z0-9_.-]+\.(jpg|png|gif)$/.test(filename)) {
      ctx.throw(400, 'Invalid filename');
    }
    // Safe: arguments passed as array, no shell injection
    execFile('convert', [filename, 'output.png'], (err, stdout, stderr) => {
      if (err) { ctx.throw(500, err); }
      ctx.body = 'OK';
    });
  },
};

In this fix, the Hmac signature still provides authenticity, but the filename is first validated against a regex that excludes dangerous characters. Using execFile with an array of arguments ensures that the filename is treated as a single argument, not as part of the shell command syntax. This effectively neutralizes command injection while preserving the intended functionality.

For scenarios where shell features are required, such as pipes or redirects, explicitly invoke a shell with controlled input and avoid concatenating user data. If you must use exec, escape or sanitize the input rigorously. However, the safest path is to rely on native modules or libraries that implement the desired behavior without invoking a shell.

Additionally, apply principle of least privilege to the Strapi process. Run the underlying service with a dedicated account that has minimal filesystem permissions. This reduces the impact of any residual vulnerability and aligns with defense-in-depth. MiddleBrick scans can help identify such insecure command patterns in your API surface, and the Pro plan’s continuous monitoring can alert you if new endpoints introduce similar risks.

Finally, document the security expectations for plugins and custom controllers. Ensure that developers understand the distinction between authentication (Hmac Signatures) and input validation. Use the CLI tool to test your endpoints locally with malformed inputs and verify that no shell commands are executed. The GitHub Action can enforce these checks in CI/CD, failing builds when insecure patterns are detected.