However, the explosion of Internet of Things devices and other systems that have SSH enabled by default means that this practice is not always followed. Credential stuffers may also try to fly under the radar by distributing their efforts over many accounts so as to never hit the failed login threshold.īest practice is to block all SSH traffic at the network firewall or to implement whitelists to limit remote access to authenticated users. Look in Wireshark for unusual numbers of SSH login attempts, whether or not they are successful. They can be differentiated based off of the bytes being sent from the server (B) to the client (A), where a successful authentication attempt results in much more data being sent than an unsuccessful one.Ī credential-stuffing attack could try to breach a single user or many at once. Looking at the two screenshots above, the top one shows a successful SSH session, while the lower one shows an SSH brute-force guessing attack. Using this information, comparing traffic capture statistics can help to detect successful versus unsuccessful SSH sessions.

Packet timing: Packets that require user interaction will take longer than automated ones, making them easier to detect.

In Zeek, a server is assumed to send a 5 kB response to a login request if the authentication was successful Observing the length of the SSH packets can show whether authentication succeeded or failed.

Packet size: SSH servers have set responses for successful and failed authentications.

Flow length: A successful authentication attempt will result in a longer session than a failed.

However, some features of the traffic can help to reveal whether or not an attempted authentication is successful:

Since SSH traffic is encrypted, it is not easy to differentiate successful versus failed login attempts in Wireshark. While SSH can be configured to lock a user’s account after a certain number of incorrect login attempts, the fact that many people use the same credentials for multiple accounts can mean that only one or two tries are enough. This means that if an attacker has access to a computer running an SSH server, they can perform a credential-stuffing attack against it.Ĭredential stuffing involves trying a list of breached or guessed user credentials to try to gain access to a user’s account. As a result, it can be used for a variety of different purposes, including credential-stuffing attacks, scanning for machines running vulnerable SSH servers and establishing reverse shells. It enables remote, encrypted access to any system running an SSH server. SSH protocol analysis for incident response While this dates the capture (MD5 is deprecated), it shows how SSH works and looks in Wireshark. This SSH session will be using AES-128 in Counter mode for encryption with an HMAC based upon MD5 to ensure message integrity. The use of asymmetric cryptography is shown in the screenshot above as Diffie-Hellman is a protocol for secure key generation.ĭiving into packet 21, as shown above, we see the use of symmetric cryptography as well. SSH uses asymmetric cryptography to establish a shared secret key and then symmetric cryptography for bulk encryption with that key. Asymmetric encryption, on the other hand, doesn’t require a shared secret key but is less efficient. Symmetric cryptography, like the Advanced Encryption Standard (AES), is faster and more efficient for bulk encryption, but it requires a shared secret key.

To accomplish its goals, SSH uses two different types of cryptography. As shown, packets associated with the session are filtered using the built-in ssh filter. The screenshot above shows a sample SSH session in Wireshark. The way that SSH accomplishes this is very similar to SSL/TLS, which is used for encryption of web traffic (HTTPS) and other protocols without built-in encryption. The main difference between SSH and Telnet is that SSH provides a fully encrypted and authenticated session.

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