In practice however, using firewall rules to restrict access to tunnels seems like a non-starter: TunnelsĪre typically short-lived, they often use random local ports, and a user might use multiple tunnels at the In theory, you could use that to ensure that only Alice can access the port used by the tunnel, and you could even dictate that she is only allowed to use mstsc.exe to connect to it. One interesting feature of Windows Defender Firewall is that it lets you create user-specific and If SSH and gcloud do not provide any good protections themselves, how about using firewall rules to restrict But once you’ve successfully createdĪ tunnel by using gcloud compute start-iap-tunnel, none of these policies prevents a hijacking scenario as Which VMs they can target, and which additional conditions need to be met. IAP gives you fine grained control over who is allowed to create tunnels, Users can use port forwarding – but these options apply to the server side, not to the client.įor gcloud, the story is similar. SSH provides a number of configuration options that control how Trying to mitigate the risks of local port forwarding in multi-user environments is surprisingly difficult. Step further and set up a remote forwarding tunnel SSH will not stop Mallory and will in fact happily let him use Alice’s tunnel. What if Mallory logs in to multiuser-box and also connects to 127.0.0.1:8080? The tunnel forwards connections fromġ27.0.0.1:8080 (on multiuser-box) over jump-box to secure-box:80. Let us consider the example below: Alice has logged on to multiuser-box (it does not matter whether that isĪ Linux or Windows machine) and opens an SSH tunnel to secure-box. When you are in a Remote Desktop Services, Citrix, or other kind of multi-user environment where you have no But what about other local clients, particularly The tunnel from being accessed by remote clients. Like SSH, gcloud compute start-iap-tunnel and IAP Desktopīind to 127.0.0.1 when creating IAP TCP forwarding tunnels. How do we make sure that malicious users cannot take advantage of this to hijack a tunnel and gain access to If the SSH client opens a port, then any client can potentially connect to that port. the SSH client listens for connections on a configured port One aspect in particular deserves some scrutiny, and that is: Google Cloud IAP TCP forwarding, and other tools RisksĬreating TCP tunnels by using local port forwarding is not without risks however. Local port forwarding is not only a commonly used SSH feature, it’s also a technique used The server connects to a configurated destination port, possibly on a SSH client listens for connections on a configured port, and when it receives a connection, it tunnels theĬonnection to an SSH server. Local forwarding is used to forward a port from the client machine to the server machine. If you are a frequent SSH user, then you’ll be familiar with local port forwarding: The client tools do not need any installation, you can just extract them.Hijacking other user’s TCP tunnels Posted on 2021.01.05 On older versions of Windows 10, you can install it as an "Optional Feature" named "OpenSSH Client". On Windows 10 version 1803 or newer, OpenSSH is built-in. This is an equivalent of OpenSSH Prox圜ommand directive:Īpart from PuTTY, there's also Microsoft build of OpenSSH for Windows. How to create SSH tunnel using PuTTY in Windows?Īnd then you open a connection in another PuTTY instance to the forwarded port. You open a connection in one PuTTY instance to the jump host and forward a local port to the protected host. On the Connection > Proxy panel, select SSH and specify details of the jump host.įor older versions, there are two alternatives (while a bit more complicated to set up): Since 0.77 PuTTY has "SSH proxy" feature, what is an equivalent of one -J/ ProxyJump:
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