Following my talk at SSTIC, I’ve released a new tool called opensvp. Its aim is to cover the attacks described in this talk. It has been published to be able to determine if the firewall policy related to Application Layer Gateways is correctly implemented.
Opensvp implements two type of attacks:
- Abusive usage of protocol commands: an protocol message can be forged to open pinhole into firewall. Opensvp currently implements message sending for IRC and FTP ALGs.
- Spoofing attack: if anti-spooofing is not correctly setup, an attacker can send command which result in arbitrary pinhole being opened to a server.
It has been developed in Python and uses scapy to implement the spoofing attack on ALGs.
For usage and download, you can have a look at opensvp page. The attack against helpers is described in the README of opensvp.
The document “Secure use of iptables and connection tracking helpers” was published some months ago and it describes how to protect a Netfilter firewall when using connection tracking helpers (the Netfilter name for ALGs).
Les transparents de ma présentation du SSTIC sont disponibles : Utilisation malveillante des suivis de connexions. Merci aux organisateurs du SSTIC d’avoir accepté mon papier!
Des vidéos de démonstration sont disponibles sur ce post: Playing with Network Layers to Bypass Firewalls’ Filtering Policy
L’outil de test openvsp est disponible sur cette page.
Scapy BPF filtering is not working when some exotic interface are used. This includes Virtualbox interface such as vboxnet.
For example, the following code will not work if the interface is a virtualbox interface:
build_filter = "src host %s and src port 21"
sniff(iface=iface, prn=callback, filter=build_filter)
To fix this, you can use the lfilter option. The filtering is now done inside Scapy. This is powerful but less efficient.
The code can be modified like this:
build_lfilter = lambda (r): TCP in r and r[TCP].sport == 21 and r[IP].src == ip
sniff(iface=iface, prn=callback, lfilter=build_lfilter)
Tanks a lot to Guillaume Valadon for the tips!