Expert found a flaw that affects all OpenSSH versions since 1999

Verschluesselte Kommunikation mit OpenSSH

Security expert discovered a username enumeration vulnerability in the OpenSSH client that affects all versions of the software that was released since 1999.

Security expert Darek Tytko from securitum.pl has discovered a username enumeration vulnerability in the OpenSSH client. The flaw tracked as CVE-2018-15473 affects all versions of the software that was released since 1999. The vulnerability could be exploited by a remote attacker to guess the usernames registered on an OpenSSH server.

OpenSSH maintainers have now released a security fix, but since the OpenSSH client is included in a broad range of software applications many of them could remain vulnerable for a long time.

Researchers from Qualys have published a detailed analysis of the vulnerability once discovered that the bug was fixed.

The flaw could potentially impact billion of devices using the vulnerable software.

Let’s see in detail how attackers can trigger the flaw.

The attacker tries to authenticate on an OpenSSH endpoint using a malformed authentication request (i.e. a truncated packet).

A vulnerable OpenSSH server, in turn, would respond in two different ways.

If the username included in the malformed authentication request does not exist, the server responds with authentication failure reply, otherwise, the server closes the connection without a reply.

“The attacker can try to authenticate a user with a malformed packet (for example, a truncated packet), and:

if the user is invalid (it does not exist), then userauth_pubkey() returns immediately, and the server sends an SSH2_MSG_USERAUTH_FAILURE to the attacker;
if the user is valid (it exists), then sshpkt_get_u8() fails, and the server calls fatal() and closes its connection to the attacker.” states the advisory.

“We believe that this issue warrants a CVE; it affects all operating systems, all OpenSSH versions (we went back as far as OpenSSH 2.3.0, released in November 2000), and is easier to exploit than previous OpenSSH username enumerations (which were all timing attacks):”

The flaw could allow an attacker to guess valid usernames registered on an SSH server, then to launch brute-force attacks to guess the password.

Open SSH versions 1:6.7p1-1 and 1:7.7p1-1— and the 1:7.7p1-4 unstable branch have addressed the flaw.

Proof-of-concept codes for the vulnerability are already available online:

Pierluigi Paganini

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”.