Advanced usage

SSH-MITM proxy server is capable of advanced man in the middle attacks and can be used in scenarios, where the remote host is not known or a single remote host is not sufficient or public key authentication is usded.

Public key authentication

Public key authentication is a way of logging into an SSH/SFTP account using a cryptographic key rather than a password.

The advantage is, that no confidential data needs to be sent to the remote host which can be intercepted by a man in the middle attack.

Due to this design concept, SSH-MITM proxy server is not able to reuse the data provided during authentication.

It you need to intercept a client with public key authentication, there are some options.

Request ssh agent for authentication

SSH supports agent forwarding, which allows a remote host to authenticate against another remote host.

SSH-MITM proxy server is able to request the agent from the client and use it for remote authentication. By using this feature, a SSH-MITM proxy server is able to do a full man in the middle attack.

Since OpenSSH 8.4 the commands scp and sftp are supporting agent forwarding. Older releases or other implementations, does not support agent forwarding for file transfers.

Using agent forwarding, SSH-MITM proxy server must be started with --request-agent.

$ ssh-mitm --request-agent --remote-host 192.168.0.x

The client must be started with agent forwarding enabled.

$ ssh -A -p 10022 user@proxyserver

Note

If the client does not forward the agent, but SSH-MITM server requested the agent, the client will get a break in attempt.

Warning: ssh server tried agent forwarding.
Warning: this is probably a break-in attempt by a malicious server.

Using ssh agent forwarding comes with some security risks and should not be used, when the integrity of a machine is not trusted. (https://tools.ietf.org/html/draft-ietf-secsh-agent-02)

6.  Security Considerations

The authentication agent is used to control security-sensitive
operations, and is used to implement single sign-on.

Anyone with access to the authentication agent can perform private key
operations with the agent.  This is a power equivalent to possession of
the private key as long as the connection to the key is maintained.  It
is not possible to retrieve the key from the agent.

It is recommended that agent implementations allow and perform some form
of logging and access control.  This access control may utilize
information about the path through which the connection was received (as
collected with SSH_AGENT_FORWARDING_NOTICE messages; however, the path
is reliable only up to and including the first unreliable machine.).
Implementations should also allow restricting the operations that can be
performed with keys - e.g., limiting them to challenge-response only.

One should note that a local superuser will be able to obtain access to
agents running on the local machine.  This cannot be prevented; in most
operating systems, a user with sufficient privileges will be able to
read the keys from the physical memory.

The authentication agent should not be run or forwarded to machine whose
integrity is not trusted, as security on such machines might be
compromised and might allow an attacker to obtain unauthorized access to
the agent.

Adding a key with SSH_AGENT_ADD_KEY over the net (especially over the
Internet) is generally not recommended, because at present the private
key has to be moved unencrypted. Implementations SHOULD warn the user of
the implications. Even moving the key in encrypted form could be
considered unwise.

Currently, SSH-MITM only uses the forwarded agent for remote authentication, but does not allow to rewrite the SSH_AGENT_FORWARDING_NOTICE message.

If a client uses an agent, which displays a warning, when the client is accessed, the original notice will be shown.

Redirect session to a honey pot

If agent forwarding is not possible, the SSH-MITM proxy server can accept the public key authentication request and redirect the session to a honey pot.

When the client sends a command, which requires a password to enter (like sudo), those passwords can be used for further attacks.

SSH-MITM does not support reusing entered passwords for remote authentication, but this feature could be implemented as a plugin.

Transparent proxy

To intercept ssh sessions, where the destination is not known, ssh-mitm proxy server can run in transparent mode, which uses the TProxy kernel feature from Linux.

Transparent proxying often involves “intercepting” traffic on a router. When redirecting packets to a local socket, the destination address will be rewritten to the routers address.

To intercept ssh connections on a network, this is not acceptable. By using TProxy from the Linux Kernel, SSH-MITM proxy server can intercept ssh connections, without loosing the destination address.

Note

To intercept the traffic, a static route can be configured on a router. An alternative to a static route is using arp spoofing.

Router configuration and arp spoofing are not part of this documentation.

Setting up firewall rules

To setup SSH-MITM in transparent mode, the system has to be prepared.

Using iptables:

$ iptables -t mangle -A PREROUTING -p tcp --dport 22 -j TPROXY --tproxy-mark 0x1/0x1 --on-port=10022 --on-ip=127.0.0.1

Using firewalld

firewall-cmd --direct --permanent --add-rule ipv4 mangle PREROUTING 1 -p tcp --dport 22 --j TPROXY --tproxy-mark 0x1/0x1 --on-port=10022 --on-ip=127.0.0.1

Warning

Additional firewall rules may be necessary to maintain device management capabilities over ssh

Note

To process the packets locally further routing needs to take place:

$ echo 100 tproxy >> /etc/iproute2/rt_tables
$ ip rule add fwmark 1 lookup tproxy
$ ip route add local 0.0.0.0/0 dev lo table tproxy

Now only the ssh proxy server needs to be started in transparent mode to be able to handle sockets that do not have local addresses:

$ ssh-mitm --transparent

By using the transparent mode, no remote host must be specified. If the --remote-host parameter is used, all incoming connections are redirected to the same remote host.