Gateway Cryptography
Hacking Impossible Tunnels Through
Improbable Networks with OpenSSH
et al.
Originally By Dan Kaminsky, CISSP
http://www.doxpara.com
Shamelessly mangled, retargeted, and reissued by Titus
Winters
Summary
• This is not how to crack SSH. This is
SSH on crack.
• 1) How to get there from here
• 2) What to do once you get there
• 3) Making getting there easier
The Basics
• Bringing people up to speed
– This is not another talk about the wonders of a simple
local port forward
• What OpenSSH does
– Forwards a shell (w/ transparent X support)
– Forwards a single command (with full stdio)
– Forwards a single TCP port
SSH under Windows
• 1) Install Cygwin from www.cygwin.com
• 2) Create a shortcut to rxvt
– C:\cygwin\bin\rxvt.exe -rv -sl 20000 -fn “Courier-12" -e
/usr/bin/bash
• bash doesn’t work under whistler yet, so use zsh if you want to retain
your tab-completion sanity
• 3) Finally enjoy a usable Unix environment under Win32
• Everything in this talk is cross platform, as long as you’ve
made Windows cross to another platform
• (Still some other things you can do, more on this later.)
Forwarding Shells
• ssh user@host
• Encryption: 3DES/Blowfish/AES
• Authentication: Password, RSA/DSA
– Key Generation
• ssh2: ssh-keygen –t dsa
– Key Authorization
• ssh2: cat ~/.ssh/id_dsa.pub | ssh user@host
‘umask 0600; mkdir .ssh; cat >>
authorized_keys2’
– SSH 1 / 2: Separate auth – so remember ssh -1 and ssh 2
Forwarding Commands
• ssh user@host ls
ssh –t user@host top
• Fully 8 bit clean for most commands,
supports (unclean) TTYs for anything that
wants to redraw screen (like top) using –t
• Full STDIO(stdin/stdout/stderr) support
– Allows pipelines across multiple systems
Command Forwarding:
CD Burning Over SSH
• mkisofs reads in files and spits out a burnable
image
• cdrecord burns the image.
– Normal CD Burning
mkisofs –JR files | cdrecord dev=#,#,# speed=# -
– Remote CD Burning
mkisofs –JR files | ssh user@host cdrecord dev=#,#,# speed=# -
– Remote CD Burning From Windows
mkisofs.exe –JR files | ssh.exe user@host cdrecord dev=#,#,#
speed=# -
Command Forwarding:
File Transfer w/o SCP
•
# GET
alicehost$ ssh alice@bobhost “cat file” > file
# PUT
falicehost$ cat file > ssh alice@bobhost “cat > file”
# LIST
alicehost$ ssh alice@bobhost “ls”
# MGET
alicehost$ ssh alice@bobhost “tar -cf - /etc” | tar -xf –
# RESUME GET
alicehost$ ssh alice@bobhost “tail –c 231244 file” >> file
Forwarding Ports
• ssh user@host -L8000:127.0.0.1:80
ssh user@host -R80:127.0.0.1:8000
• Separates into “listener” vs. “location”
– If local listens, the destination is relative to the
remote location
– If remote listens, the destination is relative to
the local location
Limitations on Port Forwards
• By default, only the systems directly
hosting the listener can connect to it
– Local forwards can be made public using the –g
option, but remote “gateway ports” must be
enabled using GatewayPorts Yes
• Destination locations are unrestricted
Accessing a Port Forward
• Application Layer
– Connect Directly to 127.0.0.1 or “localhost”
• Operating System Layer (“systemspace”)
– Pre-empt DNS lookup in hosts file
• Unix: /etc/hosts
• Win95: \windows\hosts
• WinNT: \WINNT\system32\drivers\etc\hosts
• All forwards must be preannounced, and share the
same IP (127.0.0.1)
Problem:
Static Forwards Are Inflexible
• Work decently only when:
– Each port is only used once
• Passes:
– Mail(smtp, pop3, imap)
– Simple Web(HTTP)
• Fails:
– Web Surfing Multiple Sites (HTTP)
– P2P File Transfer(Napster, Gnutella),
– Ports are predictable in advance
• Fails miserably
– FTP, both Active and Passive
Solution:
Dynamic Forwarding w/ SOCKS
• ssh user@host -D1080
• SOCKS4/5: An in-band protocol header,
nothing more, that allows the client to very
quickly tell a proxy server where its actual
destination was
• SOCKS4 is extraordinarily simple
– ~9 bytes from Client, 8 byte response, and the client has
informed the “proxy” where it actually wants to go!
– “Library Preloads” are excessive
• The idea: Run a trivial SOCKS daemon in the ssh
client; use it to redirect the destination of each
channel.
Dynamic Forwarding:
Application Support
• Most major Windows applications support
SOCKS proxies directly
– Internet Explorer, CuteFTP, IM Clients, P2P
Clients(Napster, Gnutella)
– Dialpad (Voice over IP to a telephone for free over
SSH!)
• SocksCap32 can be used to “Socksify” remaining
apps on Windows
– Outlook Express, LeechFTP, Media Player, etc.
• Unix applications can be reasonably socksified too
ProxyCommand:
Blind Proxying w/ SSH
• ssh -o 'ProxyCommand arbitrary_tool proxy %u %h
%p' user@10.1.0.1
• A ProxyCommand is an arbitrary tool that,
after it finishes executing, leads to an 8 bit
clean path to an SSH daemon
– OpenSSH's excuse for SOCKS support :-)
• Allows end-to-end crypto through any 8bit
clean link
– Like SSL over HTTP Connect
No Internet Accessible Bastion
Proxy: Now What?
• proxy$ ssh user@client -R2022:127.0.0.1:22
client$
ssh user@127.0.0.1 -o "HostKeyAlias
proxy" -L8000:www-internal:80
• Turns inability to trust into irrelevancy of trust
– Negative: “You can’t trust the addresses of x, y, or z!”
– Positive: “It doesn’t matter if you think you’re talking
to the addresses of x, y, or z.”
• MUST CHECK HOSTKEY – it’ll work even if
you don’t
Cross-Connecting Mutually
Firewalled Hosts
• server$
ssh proxyuser@proxy
-R10022:127.0.0.1:22
client$
ssh -o 'ProxyCommand ssh
proxyuser2@proxy ‘nc 127.0.0.1 10022'
user@server
or in my syntax
client$ ssh proxyuser2@proxy/10022 user@server
• Again, as long as IP addresses cannot be
trusted, it doesn’t matter that you’re talking
to the proxy and connecting through one of
its ports.
Expanding Escape Syntax
•
noname# ~?
Supported escape sequences:
~. - terminate connection
~R - Request rekey (SSH protocol 2 only)
~^Z - suspend ssh
~# - list forwarded connections
~& - background ssh (when waiting for connections to
terminate)
~? - this message
~~ - send the escape character by typing it twice
(Note that escapes are only recognized immediately after
newline.)
• Eventual goal: Port both ssh_config syntax and
ssh command line syntax to the escape character
mode
– Allow on-demand things like activation of X
forwarding
((Secure SU:
The Battle Against Direct Root
• Most “security gurus” will decry direct root login
– Holdover from the battle against admins doing
everything as root
– SU is a painless enough context switch
• If it hurts to switch, people will just do it all as root
– Advantages to being forced to switch accounts
• Inertia
• Emotion – significance of the action is emphasized
• Accounting – logs show who used root
– Even though it essentially reduces the security of the
root account to the security of the Alice account, even
OpenBSD (2.7, at least) still exhorts users not to ssh
directly to root, and instead to use SU.
Secure SU:
The Near-Perfect Compromise
•
alicehost$
ssh alice@bobhost -t “su –l root”
SSHD creates a secure execution environment
when commands are explicitly specified
– Shell configuration files not loaded
– su, as a setuid app, can’t generally be traced by ordinary
users
• User logs in as normal, is safely prompted for the
root password, gets a root shell without having to
“slum” in through insecure space
Secure SU:
Developing: Individuated Root
• Individual Public Keys For Root Access
– Nobody learns root password
• authorized_keys contains list of identities allowed to connect as root
to the system
– SSHD modified to log who connected to root
– Scales to multiple security-critical accounts
• Root can modify its own authorized_keys, but other accounts could
have root owned, root readable authorized_keys files.
• Individual Root Accounts
– Multiple accounts all set to same UID, but with different
passwords
• Alice_Root, Bob_Root, etc.
– Really only works for root
Stranded?
• mindterm – Java applet with full port
forwarding / proxying capabilities.
• Google for “mindterm”, find an available
installation of the applet.
• If you’ve got web and outgoing SSH,
you’ve got access to the world.
Conclusion
•
•
•
•
ssh is powerful
ssh is flexible
ssh is fun.
any questions? any requests?