The popular OpenSSH is affected by a user enumeration bug that could be exploited by a remote attacker to check a list of hacked credentials.
A bug (CVE-2016-6210) in the popular OpenSSH crypto library could be exploited by a remote attacker to enumerate users on systems running SSHD. An attacker can exploit the bug to check whether user names are valid on a server exposed online, the issue exists in most systems where the Blowfish algorithm runs faster than SHA256/SHA512.
“By sending large passwords, a remote user can enumerate users on systems that run SSHD. This problem exists in most modern configuration due to the fact that it takes much longer to calculate SHA256/SHA512 hash than BLOWFISH hash.” states the abstract for the flaw in the OpenSSH.
Unfortunately, the bug is still present in the OpenSSH, but according to Verint expert Eddie Harari who published the advisory on Full Disclosure, the OpenSSH developer Darren Tucker is already working to solve the problem.
If the attacker tries to authenticate a user to an OpenSSH server with a user ID and a long, but incorrect password, then the server will respond quickly for fake users, but slower for real users.
“When SSHD tries to authenticate a non-existing user, it will pick up a fake password structure hardcoded in the SSHD source code. On this hard coded password structure the password hash is based on BLOWFISH ($2) algorithm. If real users passwords are hashed using SHA256/SHA512, then sending large passwords (10KB) will result in shorter response time from the server for non-existing users.”
It is easy to imagine possible attack scenarios, a hacker could verify a list of hacked credentials against a server running SSHD.
The exploitation of the bug was tested on the opensshd-7.2p2, but experts speculate it could affect most earlier versions as well.
Pierluigi Paganini is member of the ENISA (European Union Agency for Network and Information Security) Threat Landscape Stakeholder Group and Cyber G7 Group, he is also a Security Evangelist, Security Analyst and Freelance Writer.
Editor-in-Chief at "Cyber Defense Magazine", Pierluigi is a cyber security expert with over 20 years experience in the field, he is Certified Ethical Hacker at EC Council in London. The passion for writing and a strong belief that security is founded on sharing and awareness led Pierluigi to find the security blog "Security Affairs" recently named a Top National Security Resource for US.
Pierluigi is a member of the "The Hacker News" team and he is a writer for some major publications in the field such as Cyber War Zone, ICTTF, Infosec Island, Infosec Institute, The Hacker News Magazine and for many other Security magazines.
Author of the Books "The Deep Dark Web" and “Digital Virtual Currency and Bitcoin”.