客户端服务器SSH登录非可信服务器安全性及会话反向攻击风险问询

Great question—let’s unpack this clearly, since SSH’s stateful, bidirectional model is very different from HTTP’s stateless request-response pattern.

Is SSH a bidirectional channel? #

Yes, absolutely. Unlike HTTP (where a client sends a request, the server responds, and the connection closes or idles), SSH establishes a persistent, encrypted bidirectional session once initial authentication is complete.

• When Server A connects to Server B via SSH (ssh user@serverB), both ends can send data at any time during the session. For example:
  • Server A sends command requests like ls or sudo apt update
  • Server B sends command output back to A
  • If you run interactive tools (like a text editor) over SSH, B will send real-time keystroke updates back to A

But crucially: this bidirectionality doesn’t mean both sides have equal access rights. The session’s permissions are tied to the identity A used to authenticate to B (e.g., a regular user, root, etc.). B doesn’t automatically get any access to A just because the session exists.

What happens if Server B is compromised? #

Let’s break down the risks to Server A based on your authentication method:

1. Password-based authentication #

If you logged into B using a password, there’s a small risk an attacker could capture that password (if B has keyloggers or malicious memory scrapers running). But SSH itself doesn’t send passwords in plaintext—it uses secure hashing during the auth process, so the password isn’t exposed in network traffic.

That said, if the attacker steals your password, they could try to log into A only if A accepts password logins and you reused the password. This isn’t a flaw in the SSH connection itself, though—it’s a credential hygiene issue.

2. SSH key-based authentication #

This is the safer option, and here’s why:

• Your private key stays on Server A (never sent over the network). Server B only has your public key, which can’t be used to reverse-engineer the private key or log into A.
• Even if B is compromised, the attacker can’t get your private key from the existing SSH session. The session uses the private key to sign messages, but the key itself never leaves A.

Can an attacker reverse-login to Server A or execute commands via the existing SSH connection? #

By default, no. Here’s why:

• The SSH connection is initiated by A (client) to B (server). B’s SSH daemon is designed to respond to A’s requests, not initiate new connections to A. Unless A has its own SSH server exposed to the network and the attacker has valid credentials for A, they can’t connect back.
• The encrypted SSH session means the attacker on B can’t read or modify the traffic between A and B without breaking strong encryption (AES, ChaCha20, etc.), which is not feasible for most threat actors.

Exceptions (configuration mistakes to avoid) #

The only way an attacker could leverage the SSH connection to access A is if you’ve misconfigured SSH port forwarding:

• If you set up remote port forwarding (e.g., ssh -R 2222:localhost:22 user@serverB), this forwards traffic from B’s port 2222 to A’s SSH port (22). An attacker on B could then use ssh localhost:2222 to log into A—but only if they have valid credentials for A.
• Always restrict port forwarding to trusted use cases, and never forward your SSH port to an untrusted server.

Key Takeaway #

Switching from HTTP to SSH doesn’t introduce inherent reverse-access risks if you follow secure SSH practices:

• Use key-based authentication (disable password logins entirely on both servers if possible)
• Avoid unnecessary port forwarding
• Keep SSH daemon configurations tight (e.g., disable root login, use modern encryption algorithms)

HTTP’s stateless model is great for untrusted servers because each request is isolated, but SSH’s bidirectional session is secure as long as you control the authentication and don’t grant unnecessary permissions.

内容的提问来源于stack exchange，提问作者nibb11