SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Threats SSH Can Counter

Like any security tool, SSH has particular threats against which it is effective and others that it doesn’t address. We’ll discuss the former first.

3.9.1 Eavesdropping

An eavesdropper is a network snooper who reads network traffic without affecting it in any way. SSH’s encryption prevents eavesdropping . The contents of an SSH session, even if intercepted, can’t be decrypted by a snooper.

3.9.2 Name Service and IP Spoofing

If an attacker subverts your naming service (DNS, NIS, etc.), network-related programs may be coerced to connect to the wrong machine. Similarly, an attacker can impersonate a host by stealing use of its IP address(es). In either case, you’re in trouble: your client program can connect to a false server that steals your password when you supply it. SSH guards against this attack by cryptographically verifying the server host identity. When setting up a session, the SSH client validates the server’s host key against a local list associating server names and addresses with their keys. If the supplied host key doesn’t match the one on the list, SSH complains. This feature may be disabled in less security-conscious settings if the warning messages get annoying. [7.4.3.1]

The SSH-2 protocol allows for including PKI certificates along with keys. In the future, we hope that implementation of this feature in SSH products along with more common deployment of PKI will ease the burden of key management and reduce the need for this particular ...