http-eval

Runs a simple http server on a Unix domain socket, which evals POSTed content inside NodeJS.

This is intended for use as a subprocess "sidecar" to execute JavaScript from non-JavaScript programs.

Table of Contents

• Example
• License
• Security Stance
• Usage instructions
• More examples

Example

$ npx http-eval --udsPath /tmp/foo.sock

and then:

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "return 6*7;"}' \
	| jq .result
42

License

http-eval is distributed under the terms of the MIT license.

Security stance

A server which evals arbitrary JavaScript obviously raises security concerns! If constructed incorrectly, this would essentially be a vector for Remote Code Execution (RCE) attack.

http-eval attempts to guarantee that only the current local Unix user, and (depending on the umask) folks in its Unix group, can send JavaScript to the server.

Note that, as stated in the LICENSE file, http-eval has no warranty of any kind guaranteeing the above security stance. Use at your own risk. Please report any discovered faults using Github issues!

To do this, http-eval only listens on a Unix domain socket. As constructed, it refuses to bind to a TCP/IP interface at all.

Unix domain sockets are ordinarily located in the Unix filesystem, and access is controlled by ordinary filesystem permissions. Thus, only programs which can write to local files can send JavaScript code to the server.

Now, there may be different trust boundaries within a machine and its filesystem! Usually, we don't want one local (non-root) Unix user to have access to run arbitrary commands as another local Unix user (typically we'd call that a privilege escalation vulnerability). To mitigate the risk of such a misconfiguration, by default http-eval validates the file permissions, ensuring that the UDS does not have world-write access. (However, group-write is still allowed.) http-eval effectively mandates a umask of at least 0002.

This file permission check is simply a best-effort attempt to detect a configuration footgun. This does not provide any security guarantee against misconfiguration.

To intentionally disable the file permission check, set the option --ignoreInsecureSocketPermission. This could plausibly be useful in situations where you are using other guarantees (e.g., directory permissions, or chroot) to protect write access to the UDS, and/or you actually intend to enable local privilege escalation via http-eval. Obviously, disable this check at your own risk.

Usage instructions

• Requests must:
  • contain a JSON body (Content-Type: application/json)
    • ... with an object containing the key code which contains the code to execute.
  • accept JSON in UTF-8 (Accept-Encoding: application/json, Accept-Charset: UTF-8).
• Responses contain UTF-8 JSON with:
  • the result in the object key result, or
  • any exception in the object key error.
• Code is evaluated as a function body within an ECMAScript module with a consistent this context
  • ... and thus must return anything it wants to send back to the client.
    • ... and any returned values must be JSON.stringifyable.
  • ... and thus can use dynamic await import(...) but not require (and import is generally best used in async mode; see below).
  • ... and thus may store values on this between calls.
• Code can be run within an async function (and thus use await) using the async=true query parameter
  • ... and as noted above, this is required for use of await import(...).

More examples

Basic sync call

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "return 6*7;"}' \
	| jq .
{
  "result": 42
}

Basic error

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "bad code;"}' \
	| jq .
{
  "error": "HttpEvalError: Error in eval\n [...]",
  "cause": {
    "error": "SyntaxError: Unexpected identifier 'code'\n    at Function (<anonymous>)\n    at [...]"
  }
}

Storing and retrieving values on this

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "this.foo = 6*7;"}' \
	| jq .
{}

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "return this.foo;"}' \
	| jq .
{
  "result": 42
}

Basic async call

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run?async=true' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "await new Promise(resolve => setTimeout(resolve, 2000));"}' \
	| jq .
{}

Async call with a dynamic import

$ curl \
	--silent \
	--unix-socket /tmp/foo.sock \
	'http://bogus/run?async=true' \
	-X POST \
	-H 'Content-Type: application/json' \
	-d '{ "code": "let os = await import(\"os\"); return os.cpus();"}' \
	| jq .
{
  "result": [
    {
      "model": "DMC(R) DeLorean(R) DMC-12 @ 1.21 GW",
      [...]
    },
    [...]
  ]
}