With this information, an attacker can intercept the communications and decrypt the traffic. Furthermore, having the public and private keys, the attacker can also perform a MITM attack that cannot be detected by the victim (Not detected by looking at the SSL/SSH layer).
LittleBlackBox is a collection of thousands of private SSL and SSH keys extracted from various embedded devices. These private keys are stored in a database where they are correlated with their public certificates as well as the hardware/firmware that are known to use those private keys.
A command line utility is included to aid in the identification of devices or network traffic that use these known private keys. Given a public certificate, the utility will search the database to see if it has a corresponding private key; if so, the private key is displayed and can be used for traffic decryption or MITM attacks. Alternatively, it will also display a table of hardware and firmware that is known to use that private key.