Shescape vulnerable to insufficient escaping of whitespace

Impact

This only impacts users that use the escape or escapeAll functions with the interpolation option set to true. Example:

import cp from "node:child_process";
import * as shescape from "shescape";

// 1. Prerequisites
const options = {
  shell: "bash",
  // Or
  shell: "dash",
  // Or
  shell: "powershell.exe",
  // Or
  shell: "zsh",
  // Or
  shell: undefined, // Only if the default shell is one of the affected shells.
};

// 2. Attack (one of multiple)
const payload = "foo #bar";

// 3. Usage
let escapedPayload;
shescape.escape(payload, { interpolation: true });
// Or
shescape.escapeAll(payload, { interpolation: true });

cp.execSync(`echo Hello ${escapedPayload}!`, options);
// _Output depends on the shell being used_

The result is that if an attacker is able to include whitespace in their input they can:

1. Invoke shell-specific behaviour through shell-specific special characters inserted directly after whitespace.
   • Affected shells: Bash, Dash, Zsh, PowerShell
2. Invoke shell-specific behaviour through shell-specific special characters inserted or appearing after line terminating characters.
   • Affected shells: Bash
3. Invoke arbitrary commands by inserting a line feed character.
   • Affected Shells: Bash, Dash, Zsh, PowerShell
4. Invoke arbitrary commands by inserting a carriage return character.
   • Affected Shells: PowerShell

Patches

Behaviour number 1 has been patched in v1.5.7 which you can upgrade to now. No further changes are required.

Behaviour number 2, 3, and 4 have been patched in v1.5.8 which you can upgrade to now. No further changes are required.

Workarounds

The best workaround is to avoid having to use the interpolation: true option - in most cases using an alternative is possible, see the recipes for recommendations.

Alternatively, you can strip all whitespace from user input. Note that this is error prone, for example: for PowerShell this requires stripping '\u0085' which is not included in JavaScript’s definition of \s for Regular Expressions.

References

• https://github.com/ericcornelissen/shescape/pull/322
• https://github.com/ericcornelissen/shescape/pull/324
• https://github.com/ericcornelissen/shescape/pull/332
• https://github.com/ericcornelissen/shescape/releases/tag/v1.5.7
• https://github.com/ericcornelissen/shescape/releases/tag/v1.5.8

For more information

• Comment on:
  • For behaviour 1 (PowerShell): https://github.com/ericcornelissen/shescape/pull/322
  • For behaviour 1 (Bash, Dash, Zsh): https://github.com/ericcornelissen/shescape/pull/324
  • For behaviour 2, 3, 4 (any shell): https://github.com/ericcornelissen/shescape/pull/332
• Open an issue at https://github.com/ericcornelissen/shescape/issues (New issue > Question > Get started)
• If you’re missing CMD from this advisory, see https://github.com/ericcornelissen/shescape/security/advisories/GHSA-jjc5-fp7p-6f8w