INTEGRITY

(mis)Trusting and (ab)Using SSHTips and Tricks for Pentesters and Sysadmins

Herman Duarte <hd@integrity.pt>Bruno Morisson <bm@integrity.pt>

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About usBruno Morisson<bm@integrity.pt>

Consultant and Partner @ INTEGRITY Leading Consulting and Penetration Testing engagementsBreaking things, and finding how to fix themCISSP-ISSMP, CISA, ISO27001LACurrently doing the MSc in Information Security @ Royal Holloway, University of London.

Herman Duarte<hd@integrity.pt>

Consultant @ INTEGRITYPenetration testing engagementsBSc in Information Systems and Computer EngineeringCISSP Associate / ISO27001LA / CCNA

@morissonhttp://www.linkedin.com/in/morisson

@hdontwithttp://www.linkedin.com/in/hcoduarte

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Security Competition Winner

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Show common pitfalls when using SSH

Understand how you can use SSH more securely

Understand what you cannot assure when using SSH

Objectives

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Key agreement (DH)

Encrypted communications (C&I from CIA)

Multiple authentication options (password, public keys, kerberos, etc...)

Channel Multiplexing

Port Forwarding

VPN

...and so much more!

SSH* features

* for this talk SSH==SSHv2INTEGRITY

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SSH 101- The Basics

Key Agreement (DH), Host auth, Integrity, Encryption, Re-Keying

User Authentication (password, Pubkey, etc)

Session Multiplexing, TCP forwarding, socket forwarding, sftp subsystem, etcConnection

User Auth

Transport

TCP

IP

SSH

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SSH 101- The Basics

Client Server

Encrypted Channel Setup

User Authentication

Connection

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SSH 101- The BasicsUser authentication methods:

GSSAPI

Host-Based

Public Key

Challenge-Response

Password

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Password Authentication

Encrypted Channel Setup

username, use password

OK

Password

Auth Ok / NOk

Client

ssh

Server

sshd passwd file

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If the server is compromised...

sshd binary is changed with one that logs passwords

keylogger is installed on the server

..the password is compromised!

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PublicKey Authentication

Encrypted Channel Setup

username, use publickey

OK

Signature

Auth Ok / NOk

Client

id_dsa ssh

Server

sshd authorized_keys

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DEMO

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What if I have a lot of servers, or login a lot ??

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Control Master

Connection multiplexing allows for multiple sessions on one connection

It’s fast

No need for extra authentication

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DEMO

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SSH AgentEncrypted Channel Setup

username, use publickey

OK

Signature

Auth Ok / NOk

Server

sshd

authorized_keys

Client

Agent

id_dsa

ssh

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DEMO

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What if I manage a lot of servers ?

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Typical setup

SSH

SSH

SSH

Options:Use passwords ?Copy private key to host ?

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Multiplexing / Forwarding

Client Server

Transport

Connection

Interactive Shell

Interactive Shell #2

127.0.0.1:1234 www.google.com:80

TCP Port Forward

www.google.com:80 127.0.0.1:80ssh -L 1234:www.google.com:80

ssh -R 80:www.google.com:80

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SSH Agent Forwarding

No need to copy private key to other servers

Key is kept on the original source host

Agent is forwarded, using a tunnel

Passwordless!

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Client

Agent

id_dsa

Server #1

sshd

authorized_keys

Server #2

sshd

authorized_keys

SSH Agent ForwardingTransport

Connection

Interactive Shell

Agent Forwarding

Transport

Connection

Interactive Shell

ssh ssh

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DEMO

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Caveat Emptor(s)

You must trust the server(s)

What if the server was compromised ?

Can SSH Agent be abused ?

Can Control Master be abused ?

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DEMO

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Help us Obi Wan

You’re our only hope!

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Freak on a Leash

When adding keys to ssh-agent use ssh-add with:

-t <secs> to set a maximum lifetime on the identities being added to the agent

-c to indicate that identities being added should be subject to confirmation before being used for auth

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ssh-agent queries /usr/libexec/ssh-askpass for confirmation

“ssh-add -c -t 3600 < /dev/null” makes ssh-add use env var SSH_ASKPASS to query for passphrase

Freak on a Leash

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DEMO

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But we still need passwords!

If you su / sudo, you still type your password...

What if we could use the SSH Agent for sudo ?

Yes we can! :)

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DEMO

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Paranoia is reality on a finer scale

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What if...

we could SSH without using SSH ?

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Using SSH Gangnam Style

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Using SSH w/o using SSH(but still using SSH)

ssh -W trusted:22 untrusted

Open socket to trusted Server...

...through an untrusted Server

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ssh -o “ProxyCommand ssh -a -W trusted:22 untrusted” trusted

Open Socket to trusted via untrusted

Disable Agent Forwarding

Connect to the socket created

Just for user and key validation

Using SSH w/o using SSH(but still using SSH)

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Untrusted

sshd

authorized_keys

Owned

sshd

authorized_keys

Trusted

sshd

authorized_keys

Client

Agent

id_dsa

Transport

Connection

-W (Open Socket to Server #2)Transport

ssh

Connection

Interactive Shell

Using SSH w/o using SSH(but still using SSH)

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DEMO

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Control your SSH

.ssh/config

Host trusted1 trusted2 trusted3ForwardAgent yesProxyCommand ssh -a -W %h:22 untrusted.server.com

Host *ControlMaster noForwardAgent noPasswordAuthentication no

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Wrapping up

Don’t use passwords.When using publickey auth, control your private key.Be careful with multiplexing options (tunnels, CM, etc).Read the Fu^Hine Manual, before using exotic features.Create ~/.ssh/config file with a secure default configuration, and configure exceptions when needed.Don’t use passwords.

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Live long and prosper

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ReferencesRTFM :)

RFCs 4251-4256,4335,4344,4345,4419,4432,4462,4716,5656

http://www.linuxjournal.com/article/9566

http://pamsshagentauth.sourceforge.net/

http://www.jedi.be/blog/2010/08/27/ssh-tricks-the-usual-and-beyond/

http://www.openbsd.org/papers/auug2002-ssh.pdf

http://pentestmonkey.net/cheat-sheet/ssh-cheat-sheet

http://www.hsc.fr/ressources/breves/passe-partout.html.en