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SecurityTrails Blog · Nov 28 · SecurityTrails team

Flan Scan - The New Vulnerability Scanner from Cloudflare

Reading time: 6 minutes

Port scanning is one of the most frequent activities performed by security researchers working as part of a red team. And in other scenarios, blue teams use it along with online vulnerability scanning tools to prevent a security breach in their apps and servers.

Knowing that port and vulnerability scanners are pillars of the cybersecurity world, we’ll analyze one of the newest today, Flan Scan. How new is it? This vulnerability scanner was released only days ago.

What is Flan Scan?

Flan Scan is a lightweight vulnerability scanner created by Cloudflare. It’s based on the famous Nmap, one of the best port scanners of all time (which, if you’re following this blog, you’ll notice we love and evangelize every time we can).

Flan Scan uses Nmap scripts to boost its detection capabilities. In particular, it uses the vulners script to detect CVEs from all services found on the host (as covered previously in our Nmap Vulnerability Scanner article).

Main differences between the classic Nmap and Flan-Scan:

  1. Different deployment and configuration: While Nmap can be installed by compiling the source code, or using a pre-compiled binary package, the only way to install Flan Scan is by using a Docker container or in Kubernetes.

  2. Pushes results into the cloud: This feature allows you to push the results into a centralized cloud location such as Amazon S3 or GCS buckets.

  3. Actionable Reports: By using Python, Flan Scan converts the default Nmap output into actionable LaTeX reports.

Flan Scan was developed after Cloudflare tried using several commercial scanners to audit its internal networks. With an eye on license costs and overall results, they eventually turned to classic Nmap to build their Flan Scan wrapper.

How does Flan Scan work?

As mentioned, Flan Scan works with Nmap as its core scanner. Therefore, the first thing Flan Scan does is perform a network exploration with an Nmap ICMP scan.

Then, it performs a SYN scan against the top 1,000 most popular ports on those addresses marked as live by the previous ICMP scan.

Finally, it performs a service scan to detect which services are listening on the open ports.

Installation

We tested this installation on a Debian system and it worked really well.

The first step will be to clone the source code from https://github.com/cloudflare/flan

git clone https://github.com/cloudflare/flan.git

Output example:

[research@securitytrails.com]# git clone https://github.com/cloudflare/flan.git
Cloning into 'flan'...
remote: Enumerating objects: 171, done.
remote: Total 171 (delta 0), reused 0 (delta 0), pack-reused 171
Receiving objects: 100% (171/171), 33.98 KiB | 164.00 KiB/s, done.
Resolving deltas: 100% (99/99), done.
[research@securitytrails.com]#

Check that you have Docker installed and running:

[research@securitytrails.com ~]# docker --version
Docker version 1.13.1, build 47e2230/1.13.1

Start the service by typing:

[research@securitytrails.com flan]# systemctl start docker

Add the IPs you want to scan:

cd flan
nano -w shared/ips.txt

By default it contains these lines:

1.1.1.1
8.8.8.8

Delete those and replace them with the IPs you wish to scan, then save the changes.

Now, let’s build Flan Scan by typing:

make build

At the end, if the build has finished properly you should get this output:

Removing intermediate container 39365bd6c7c1
Successfully built 54cdaa539811
[research@securitytrails.com flan]#

Flan Scan is now installed. Let’s test it!

Testing Flan Scan

Let the fun begin! To start scanning your target hosts, just type:

make start

If Flan Scan starts successfully, you should see something like this:

research@securitytrails.com:~/flan# make start
docker run --name flan_1574527571 -v /root/flan/shared:/shared flan_scan
# Nmap 7.70 scan initiated Sat Nov 23 16:46:12 2019 as: nmap -sV -oX
/shared/xml_files/2019.11.23-16.46/1.1.1.1.xml -oN - -v1 --script=vulners/vulners.nse 1.1.1.1

When finished, Flan Scan will show you this:

Read data files from: /usr/bin/../share/nmap
Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Sat Nov 23 16:47:57 2019 — 1 IP address (1 host up) scanned in 74.58 seconds
research@securitytrails.com:~/flan#

If you see that, it means the scan was completed successfully.

The default scan consists in the following command:

nmap -sV -oX /shared/xml_files -oN - -v1 \$@ --script=vulners/vulners.nse 11.22.33.44

Here are explanations for each one of the parameters:

  • -sV helps to perform a service detection scan.
  • -oX parameter enables you to save the results in XML format inside the /shared/xml_files directory.
  • -oN parameter helps to print the results into the terminal, and the -v1 parameter boosts the verbosity level.
  • --script=vulners/vulners.nse is the Nmap script used to detect all CVEs from the host.
  • 11.22.33.44 is the target IP address.

If you also want to run UDP scans, or scan IPv6-based addresses, you can do it by adding the Nmap flags manually. Run the container and input the desired parameters like this:

docker run -v \$(shell pwd)/shared:/shared flan_scan <Nmap-flags>

Analyzing the results

Something we like about Flan Scan is its ability to generate the LaTeX summary report, along with the XML format. The LaTeX summary report is located in shared/reports, as well as the XML output in shared/xml_files.

In this case, we analyzed the .tex report, and found it was presented in an elegant way, featuring the most relevant information about the scans performed.

The first part of the scan includes a full description of each test, with the exact Nmap command that was executed. The vulnerability report starts with the most critical ones, as shown below:

Flan scan report

These results include critical, medium and low importance CVEs. In these tests we ran several scans against http://scanme.nmap.org/ to see if Flan Scan was able to detect vulnerable applications and services.

Flan Scan vulnerable applications and services detection

Once the CVE report finishes, it will also add the services with no known vulnerabilities. In this case, apart from Nmap ScanMe page host, we also scanned the popular 1.1.1.1 and 8.8.8.8 servers, as you see in the following screenshot:

Scanned the popular 1.1.1.1 and 8.8.8.8 servers

Conclusion

At the time of Flan Scan’s release, Cloudflare stated in their blog:

“Flan Scan is a thin wrapper around Nmap that converts this popular open source tool into a vulnerability scanner with the added benefit of easy deployment.”

After analyzing this product, we strongly believe that it can be used to discover open ports on your network, explore system services, versions and detect CVEs… in the same way as the original Nmap does.

The truth is there is no need to “convert” Nmap into a vulnerability scanner—it has been a network, port and CVE scanner for a long time.

Unlike Nmap, which works out-of-the-box and comes pre-installed on most modern Linux distros, Flan Scan requires running inside a Docker container to make it work. This may be the result of Cloudflare’s need to meet their compliance requirements, rather than a multi-platform generic solution which ultimately might not be the best way to deploy it for many other IT companies out there.

Pushing results to the cloud seems to be useful for data collection and central processing. However, the coolest thing is the XML and LaTeX reporting feature, which generates a really useful summary of all vulnerabilities found on the remote server.

While it’s great to see new additions and features added to classic Nmap, just as other colleagues in the infosec community found, we expected a little more from Cloudflare’s brilliant engineers. Flan Scan does offer a few cool features, but it’s ultimately not quite as outstanding a product as we thought it would be.

If you need to push results into the cloud, or need a better reporting system, it might fit your needs; otherwise, stick to good, old-fashioned Nmap. And if you’re willing to go big with vulnerability scanning, switch to a truly robust product like Nessus.


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