#VU255 User enumeration via covert timing channel in OpenSSH - CVE-2016-6210

Vulnerability identifier: #VU255

Vulnerability risk: Medium

CVSSv4.0: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:A/U:Green]

CVE-ID: CVE-2016-6210

CWE-ID: CWE-200

Exploitation vector: Network

Exploit availability: Yes

Vulnerable software:
OpenSSH
Server applications / Remote management servers, RDP, SSH

Vendor: OpenSSH

Description

The vulnerability allows a remote attacker to enumerate users on system.

The vulnerability exists in most systems where the Blowfish algorithm runs faster than SHA256/SHA512. A remote unauthenticated attacker can determine valid usernames by sending a specially crafted request with a large password (approximately 10,000 characters) to the target ssh daemon. On systems where a valid user's password has been hashed with SHA256/SHA512, the response time will be shorter for a non-existent username than for a valid username.

Successful exploitation of this vulnerability may result in disclosure or user logins.

Mitigation
Install the latest version of OpenSSH 7.3.

Vulnerable software versions

OpenSSH: 6.0p1 - 7.2p2

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External links
https://www.openssh.com/txt/release-7.3

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability. However, a fully functional exploit for this vulnerability is available.