Trying to fool ASA stateful FTP inspection

3 minute read

Hi there!

I was doing some CCNA Security study, playing with ASAs, and the ability of the firewall to inspect FTP traffic in order to open ports for FTP passive mode connections.

For example, when I tell my FTP server that I want to connect in passive mode:

PASV
227 Entering passive mode (10,1,2,3,253,232).

If this FTP session goes through the ASA, and FTP traffic is inspected by the ASA (as it is by default in the global_policy policy-map, at least in ASA 9.1(5)21, see below), the ASA will open up the access to the passive port on the FTP server (65000 (253 * 256 + 232) in the above):

policy-map global_policy
 class inspection_default
 inspect dns migrated_dns_map_1
 inspect ftp
 inspect h323 h225
 inspect h323 ras
 inspect ip-options
 inspect netbios
 inspect rsh
 inspect rtsp
 inspect skinny
 inspect esmtp
 inspect sqlnet
 inspect sunrpc
 inspect tftp
 inspect sip
 inspect xdmcp

So, what if I, a malicious user on the inside network of a company with a restrictive outbound firewall policy, wants to SSH to a server that is blocked in the firewall policy, but the firewall allows FTP out. Could I craft an FTP server to tell the firewall that the FTP client needs to connect to port 22 for passive mode?

I thought I’d try!

Initially I thought I’d do this using scapy, which I hadn’t played with before, so I found this great, easy to follow basics guide.

Unfortunately, I got sidetracked while learning that, and thought I might just be able to find an open source FTP server that I could fiddle with instead, which I did: pyftpdlib by giampaolo.

I modified the section of pyftpdlib where it sends back the 227 Entering Passive Mode message, so that it sent back a port number of my choosing.

I then spun up my GNS3 ASA lab. I used a router on the inside of the ASA as the client, my Mac as the FTP server running my modified pyftpdlib server, and the access-list on the inside of the ASA was this:

access-list inside_in extended permit tcp any any eq 21

Everything else will get dropped, which is what I want to test.

Now, my server has been told to say 227 Entering Passive Mode (10,1,2,3,0,22), 10.1.2.3 being my Mac’s IP address, and 0,22 meaning 0*256+22, or just port 22.

Alright, let’s try it then! From R1:

R1#telnet 10.1.2.3 21
Trying 10.1.2.3, 21 ... Open
220 pyftpdlib 1.6.0 ready.
user user
331 Username ok, send password.
pass 12345
230 Login successful.
pasv

[Connection to 10.1.2.3 closed by foreign host]

Hmmm, I got disconnected when running PASV? Let’s see what the ASA had to say:

%ASA-4-406001: FTP port command low port: 10.1.2.3/22 to 10.1.0.1 on interface outside
%ASA-4-507003: tcp flow from inside:10.1.0.1/58593 to outside:10.1.2.3/21 terminated by inspection engine, reason - inspector drop reset.
%ASA-6-302014: Teardown TCP connection 11 for outside:10.1.2.3/21 to inside:10.1.0.1/58593 duration 0:00:18 bytes 141 Flow closed by inspection

Damn! It’s on to me!

So I tried a couple of other ports in the passive mode response, it turns out that the ASA will terminate the connection if it sees a passive mode response with a port of 1023 or below.

Okay, what about tricking the ASA into letting me connect to another host, by getting the server to respond with 227 Entering Passive Mode (1,2,3,4,4,0) (1.2.3.4 on port 1024).

R1#telnet 10.1.2.3 21
Trying 10.1.2.3, 21 ... Open
220 pyftpdlib 1.6.0 ready.
user user
331 Username ok, send password.
pass 12345
230 Login successful.
pasv

[Connection to 10.1.2.3 closed by foreign host]

Bugger! Foiled again.

Alright ASA, what now?

%ASA-4-406002: FTP port command different address: 10.1.2.3(1.2.3.4) to 10.1.0.1 on interface outside
%ASA-4-507003: tcp flow from inside:10.1.0.1/28040 to outside:10.1.2.3/21 terminated by inspection engine, reason - inspector drop reset.
%ASA-6-302014: Teardown TCP connection 13 for outside:10.1.2.3/21 to inside:10.1.0.1/28040 duration 0:00:12 bytes 196 Flow closed by inspection

So, it turns out that if the FTP server responds with a different IP address, or a port less than 1024 for the passive mode connection, the firewall will destroy the FTP connection altogether.

I did, however notice that if the passive connection was opened (the port was above 1023, and the host was the same), I could do whatever I want with that port, and the ASA would just let it happen.

I’m glad to know the ASA is doing it’s job (mostly) properly!

If you have any other ideas on things to try, or any questions about what I’ve done, I can be found on twitter @XORcat.

Thanks for reading!