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Introduction
Mobile apps often rely on third-party libraries to handle common tasks like file compression. However, when developers fail to update these dependencies, what was once a convenience becomes a critical security hole. This article dissects a real-world vulnerability in Samsung Notes (CVE-worthy behavior) where an outdated version of Zip4j (1.3.2) allowed arbitrary path traversal via crafted `.memo` files, enabling attackers to write files anywhere on the filesystem – outside the app’s sandbox.
Learning Objectives
- Understand how ZIP path traversal (Zip Slip) works and why old libraries are vulnerable.
- Learn to manually craft malicious ZIP entries to test for path traversal flaws.
- Master mitigation techniques including canonical path validation and dependency updates.
- Apply Linux/Windows commands to detect outdated libraries and harden file extraction routines.
You Should Know
- Anatomy of the Exploit: Breaking Out of the Sandbox with a Single `.memo` File
Samsung Notes processes `.memo` files as ZIP archives. When a user opens such a file, the app extracts its contents using Zip4j version 1.3.2 – a library released in 2013 that performs no path validation. An attacker can create a ZIP entry with a name like ../../../../../../sdcard/saconfig.ini. Upon extraction, the app blindly concatenates the destination directory with the malicious entry name, resulting in a full path outside the intended temp folder.
What this does: It allows arbitrary file write in locations the app has permissions to access – typically external storage (/sdcard/), shared preferences, or even system configuration files if the app runs with high privileges.
Step‑by‑step guide to recreate (for authorized testing only):
- Create a legitimate `.memo` file (or any ZIP file).
- Use a ZIP tool or Python to modify entry names:
import zipfile with zipfile.ZipFile('evil.memo', 'w') as zf: zf.writestr('../../../../../../sdcard/pwned.txt', 'Hacked via path traversal')
3. On Linux, verify the crafted entry:
unzip -l evil.memo Output should show the traversal path
4. On Windows (PowerShell), create a similar archive:
Compress-Archive -Path .\malicious.txt -DestinationPath evil.memo -Update Then hex-edit the ZIP to change entry name to ......\sdcard\test.ini
5. Open the file in the vulnerable Samsung Notes version – observe file written to /sdcard/pwned.txt.
Mitigation: Upgrade to Zip4j 1.3.3 or later, which adds `isValidFileName()` using canonical path validation.
- Zip Slip Deep Dive: Why Path Validation Is Non‑Negotiable
Zip Slip is a broad class of vulnerability affecting any archive extraction routine that trusts entry names. The core issue: `new File(destDir, entryName).getCanonicalPath()` must start with destDir.getCanonicalPath(). Without this check, `../` sequences escape the intended directory.
Step‑by‑step guide to test any app:
1. Decompile the APK (using `apktool` or `jadx`):
apktool d target_app.apk cd target_app grep -r "Zip4j|unzip|extract" .
2. Check library version in `AndroidManifest.xml` or `build.gradle` (if source available).
3. Create a test ZIP with nested traversal entries:
echo "malicious" > payload.txt zip evil.zip payload.txt Use a hex editor to change payload.txt to ../../data/data/com.example.app/shared_prefs/hack.xml
4. Use `zipinfo` or `7z l` to inspect:
7z l evil.zip
5. Run the app in an emulator and trigger extraction – monitor file creation:
adb shell ls -la /sdcard/ adb shell run-as com.example.app ls -la ../shared_prefs/
Secure extraction code (Java/Kotlin):
public void safeExtract(ZipFile zipFile, File destDir) throws IOException {
Enumeration<? extends ZipEntry> entries = zipFile.entries();
while (entries.hasMoreElements()) {
ZipEntry entry = entries.nextElement();
File target = new File(destDir, entry.getName());
if (!target.getCanonicalPath().startsWith(destDir.getCanonicalPath())) {
throw new SecurityException("Path traversal detected: " + entry.getName());
}
// Proceed with extraction
}
}
- Hunting Outdated Dependencies in Your APK – Linux & Windows Commands
The Samsung case is a textbook example of a “dependency zombie” – a library version years behind with known patches. Here’s how to find them before attackers do.
Step‑by‑step guide – Linux:
1. Extract APK:
unzip app.apk -d app_unpacked/
2. Search for library signatures (e.g., Zip4j):
find app_unpacked/ -name ".jar" -exec grep -l "Zip4j" {} \;
find app_unpacked/ -name ".dex" | xargs -I {} strings {} | grep -i "zip4j"
3. Use MobSF (Mobile Security Framework) for automated scanning:
docker run -it -p 8000:8000 opensecurity/mobile-security-framework-mobsf Upload APK – check "Binary Analysis" for library versions
4. Check `AndroidManifest.xml` for known vulnerable libraries:
aapt dump badging app.apk | grep "package:"
Step‑by‑step guide – Windows (PowerShell):
1. Rename `.apk` to `.zip` and extract:
Rename-Item app.apk app.zip Expand-Archive app.zip -DestinationPath .\app_unpacked
2. Search for library strings in `.dex` files (requires `strings.exe` from Sysinternals):
Get-ChildItem -Recurse -Filter .dex | ForEach-Object { strings.exe $_ | Select-String "zip4j" }
3. Use OWASP Dependency-Check:
dependency-check.bat --scan app.apk --format HTML --out report.html
- API Security Parallel: Path Traversal in REST Endpoints
The same path traversal logic applies to cloud APIs. If an endpoint accepts a filename parameter and uses it unsafely in file system operations, attackers can escape directories. This is common in file upload/download features.
Step‑by‑step guide to test API endpoints:
1. Intercept a file download request (Burp Suite):
GET /api/download?file=../../../../etc/passwd
2. Try URL-encoded variants:
GET /api/download?file=..%2F..%2F..%2Fetc%2Fpasswd
3. On Linux backend, check for response containing sensitive files.
4. If the API uses `path.join()` or `os.path.join()` without sanitization, it’s vulnerable.
5. Fix:
import os
base_dir = '/safe/path'
requested = os.path.realpath(os.path.join(base_dir, user_input))
if not requested.startswith(base_dir):
raise Exception("Path traversal detected")
Windows equivalent: Always use `Path.GetFullPath()` and compare with safe root in .NET or Java.
- Cloud Hardening: Preventing Archive Bombs and Traversal in Serverless Functions
AWS Lambda, Azure Functions, and Google Cloud Run often unzip user‑supplied archives. Without proper validation, they expose the same Zip Slip vulnerability – plus denial‑of‑service via ZIP bombs.
Step‑by‑step guide for cloud hardening:
1. Before extraction – validate entry names:
import zipfile, os
def safe_extract(zip_path, target):
with zipfile.ZipFile(zip_path, 'r') as zf:
for member in zf.namelist():
abs_path = os.path.join(target, member)
if not os.path.realpath(abs_path).startswith(os.path.realpath(target)):
raise ValueError("Bad zip entry: " + member)
zf.extractall(target)
2. Limit archive size and entry count:
Use `unzip` with resource limits on Linux unzip -t -q large.zip 2>&1 | head -n 10 unzip -l large.zip | wc -l Check entry count
3. Run extraction in a sandbox (e.g., AWS Lambda in a temporary `/tmp` directory that is cleared per invocation).
4. Use managed services – AWS S3 with Lambda triggers? Validate before processing.
5. Monitor CloudTrail for unusual file writes outside expected prefixes.
6. Vulnerability Exploitation & Mitigation in CI/CD Pipelines
Attackers can slip malicious `.memo` files into app stores as legitimate updates. Defenders must integrate dependency scanning into DevOps.
Step‑by‑step guide to embed security in CI/CD:
1. Add OWASP Dependency-Check to GitHub Actions:
- name: Dependency Check run: | wget https://github.com/jeremylong/DependencyCheck/releases/download/v9.0.0/dependency-check-9.0.0-release.zip unzip dependency-check-.zip ./dependency-check/bin/dependency-check.sh --scan app/build/outputs/apk/ --format HTML
2. Use Snyk or Sonatype Nexus to monitor open‑source libraries – block builds if vulnerable versions are found.
3. Automated patch testing: After updating Zip4j to 1.3.3+, run a regression test that attempts to extract a malicious `.memo` and confirms it fails.
4. Sign your APK and enforce signature verification on the client side – though this won’t fix path traversal, it prevents tampered updates.
5. Create a Software Bill of Materials (SBOM) – use `cyclonedx` for Android:
gradle cyclonedxBom
- Code-Level Fix: Implementing Canonical Path Validation in Any Language
The Samsung vulnerability vanished with a simple library update. But if you’re maintaining legacy code, here’s the manual fix across different environments.
Step‑by‑step guide with code snippets:
- Java (Zip4j or native
java.util.zip):Path targetPath = destDir.toPath().resolve(entryName).normalize(); if (!targetPath.startsWith(destDir.toPath())) { throw new IOException("Bad entry: " + entryName); } - Python (using
zipfile):import zipfile, os with zipfile.ZipFile('evil.zip', 'r') as zf: for info in zf.infolist(): extracted = os.path.realpath(os.path.join('/safe/dir', info.filename)) if not extracted.startswith(os.path.realpath('/safe/dir')): raise RuntimeError('Path traversal') zf.extract(info, '/safe/dir') - Node.js (with
adm-zip):const AdmZip = require('adm-zip'); const path = require('path'); const zip = new AdmZip('file.zip'); zip.getEntries().forEach(entry => { const resolved = path.resolve('/safe/dir', entry.entryName); if (!resolved.startsWith(path.resolve('/safe/dir'))) { throw new Error('Invalid entry'); } }); zip.extractAllTo('/safe/dir', true); - Windows PowerShell (using
System.IO.Compression.ZipFile):Add-Type -AssemblyName System.IO.Compression.FileSystem $zip = [System.IO.Compression.ZipFile]::OpenRead("evil.zip") foreach ($entry in $zip.Entries) { $target = [System.IO.Path]::GetFullPath([System.IO.Path]::Combine($destDir, $entry.FullName)) if (!$target.StartsWith([System.IO.Path]::GetFullPath($destDir), [bash]::OrdinalIgnoreCase)) { throw "Path traversal" } }
What Undercode Say
- Outdated dependencies are silent backdoors. Samsung’s use of Zip4j 1.3.2 – a version with a known missing validation – shows how a single overlooked library can compromise millions of devices. Always monitor your supply chain.
- Path traversal is timeless. Whether in ZIP extraction, API file parameters, or cloud storage mounts, trusting user‑supplied path strings without canonicalization leads to the same flaw. A simple `getCanonicalPath()` check blocks nearly all variants.
- The fix is often free. Zip4j 1.3.3 already contained the patch – Samsung only needed to increment a version number. This reinforces that vulnerability management is not about writing new code but about updating what you already have.
Analysis: The Android ecosystem suffers from slow dependency adoption due to binary size concerns, backward compatibility fears, and lack of automated update tools. Attackers actively scan APKs for library versions like Log4j, OkHttp, and Zip4j. In 2026, we predict that runtime dependency verification (similar to npm audit but for APKs) will become mandatory for app store submission. Until then, manual audits and CI/CD scanning remain your best defense.
Prediction
Within the next 18 months, we will see a wave of mobile supply chain attacks specifically targeting ZIP extraction libraries across note‑taking, backup, and file manager apps. The Samsung Notes case will be cited as a precursor. Regulators (e.g., EU Cyber Resilience Act) will likely mandate SBOMs with version‑specific vulnerability attestations for all apps distributed in official stores. Simultaneously, attackers will shift to exploiting outdated XML parsers and image decoders – the next “Zip4j” waiting to be discovered.
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Reported By: Pallis Okay – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
