How do I escape a string for a shell command in node?

user_50505 0
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In nodejs, the only way to execute external commands is via sys.exec(cmd). I'd like to call an external command and give it data via stdin. In nodejs there does yet not appear to be a way to open a command and then push data to it (only to exec and receive its standard+error outputs), so it appears the only way I've got to do this right now is via a single string command such as:

var dangerStr = "bad stuff here";
sys.exec("echo '" + dangerStr + "' | somecommand");

Most answers to questions like this have focused on either regex which doesn't work for me in nodejs (which uses Google's V8 Javascript engine) or native features from other languages like Python.

I'd like to escape dangerStr so that it's safe to compose an exec string like the one above. If it helps, dangerStr will contain JSON data.

v8 node.js shell escaping javascript
7 Answers

You should never rely on escaping unknown input going to a shell parameter - there will almost always be some edge-case that you haven't thought of that allows the user to execute arbitrary code on your server.

Node has support for calling a command and passing each argument separately, with no escaping required. This is the safest way to do it:

const { spawn } = require('child_process');
// Note that the arguments are in an array, not using string interpolation
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

The documentation is here

user_6924100

I second the opinion of Will, whenever possible you should avoid escaping by hand and prefer spawn.

However, in the case that escaping is unavoidable, for example if you need to use exec or you are executing a command through ssh. Then you can use base64 to pass safe characters to bash and rely on bash to escape the unknown.

const dangerStr = 'bad stuff here'
// base64 has safe characters [A-Za-z=0-9+/]
const dangerBase64 = btoa(dangerStr)

sys.exec(`echo "$(echo ${dangerBase64} | base64 -d)" | somecommand`)

The explanation is the following:

dangerBase64 is unknown but it does not contain unsafe characters in bash. Hence echo ${dangerBase64} will output what we want.

Finally the double quote around $(echo ${dangerBase64} | base64 -d) escape the actual value passed by the user inside bash, which is safe and has the same value that the user wanted.

user_15361330

If you are building you own software, you can encode the command to base64 or hex format then decode the arguments from the program.

For my Nodejs applications I use.

var base64_encode = exports.base64_encode = function(non_base64_string){
    return Buffer.from(non_base64_string).toString('base64');
}


var base64_decode = exports.base64_decode = function(base64_string){
    return Buffer.from(base64_string, 'base64').toString('ascii')
}

So when I run a base64 encoded command like this

webman grep --search "aW5jbHVkZV9vbmNlICRfU0VSVkVSWyJET0NVTUVOVF9ST09UIl0uIi9zZXR0aW5ncy5waHAiOw==" --replacement "JGRvY3VtZW50X3Jvb3QgPSBfX0RJUl9fO3doaWxlKHRydWUpe2lmIChmaWxlX2V4aXN0cygkZG9jdW1lbnRfcm9vdC4iL3NldHRpbmdzLmpzb24iKSl7YnJlYWs7fWVsc2V7JGRvY3VtZW50X3Jvb3Q9ZGlybmFtZSgkZG9jdW1lbnRfcm9vdCk7fX08bmV3bGluZT5pbmNsdWRlX29uY2UgJGRvY3VtZW50X3Jvb3QuIi9zZXR0aW5ncy5waHAiOw=="

I can get the arguments search and replacement arguments without stress using base64_decode

user_128692730
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