Second order command injection

Some shell commands, like git ls-remote, can execute arbitrary commands if a user provides a malicious URL that starts with --upload-pack. This can be used to execute arbitrary code on the server.

Recommendation

Sanitize user input before passing it to the shell command. For example, ensure that URLs are valid and do not contain malicious commands.

Example

The following example shows code that executes git ls-remote on a URL that can be controlled by a malicious user.

const express = require("express");
const app = express();

const cp = require("child_process");

app.get("/ls-remote", (req, res) => {
  const remote = req.query.remote;
  cp.execFile("git", ["ls-remote", remote]); // NOT OK
});

The problem has been fixed in the snippet below, where the URL is validated before being passed to the shell command.

const express = require("express");
const app = express();

const cp = require("child_process");

app.get("/ls-remote", (req, res) => {
  const remote = req.query.remote;
  if (!(remote.startsWith("git@") || remote.startsWith("https://"))) {
    throw new Error("Invalid remote: " + remote);
  }
  cp.execFile("git", ["ls-remote", remote]); // OK
});

References

• Max Justicz: Hacking 3,000,000 apps at once through CocoaPods.
• Git: Git - git-ls-remote Documentation.
• OWASP: Command Injection.
• Common Weakness Enumeration: CWE-78.
• Common Weakness Enumeration: CWE-88.