CVE-2016-6210·OpenSSH vulnerability
sshd in OpenSSH before 7.3, when SHA256 or SHA512 are used for user password hashing, uses BLOWFISH hashing on a static password when the username does not exist, which allows remote attackers to enumerate users by leveraging the timing difference between responses when a large password is provided.
- Severity
- high
- Software
- OpenSSH
- Fixed in
- 1:7.2p2-6
- Published
How likely is exploitation
FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 89% — in the top 0.2% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.
Affected versions
Until: 1:7.2p2-6
Fixed in: 1:7.2p2-6
How to fix this CVE
Upgrade OpenSSH to version 7.3 or later to eliminate a timing-based user enumeration vulnerability in the authentication process. This flaw allows attackers to discover valid usernames by measuring response delays when SHA256/SHA512 password hashing is configured. Patch your SSH server immediately and restart the sshd service to apply the fix.
sudo dnf update openssh-server openssh-clients && sudo systemctl restart sshdDefensia detects this vulnerability
What an exploitation attempt looks like
Sample log line indicative of exploitation attempts:
Regex pattern for sshd logs: ^.*Invalid user [a-zA-Z0-9_]+.* with multiple rapid attempts from same source IP where password length varies significantly (e.g., 128+ character attempts followed by short attempts)How to check if you are affected
- Check installed OpenSSH version: ssh -V or sshd -v
- Verify sshd configuration hash algorithm: grep -i 'passwordauthentication\|usepam' /etc/ssh/sshd_config
- Search auth logs for repeated failed login attempts from single IP with varying password lengths: grep 'Invalid user' /var/log/auth.log | awk '{print $10}' | sort | uniq -c | sort -rn
- Confirm patch applied: dpkg -l openssh-server (Debian/Ubuntu) or rpm -qa openssh-server (RHEL/CentOS) and verify version is 7.3 or higher
FAQ
What is CVE-2016-6210?
This vulnerability allows remote attackers to enumerate valid usernames on OpenSSH servers by measuring the response time difference between authentication attempts for existing vs. non-existing users. When SHA256 or SHA512 hashing is enabled, the server applies different computational loads that leak information about user existence.
Is CVE-2016-6210 being actively exploited?
While not currently listed on CISA's Known Exploited Vulnerabilities catalog, public proof-of-concept exploits are available and the attack requires only network access to SSH, making it a practical reconnaissance tool for attackers.
What versions of OpenSSH are affected by CVE-2016-6210?
All OpenSSH versions before 7.3 are vulnerable when configured to use SHA256 or SHA512 password hashing algorithms. This includes all 7.2 and earlier releases.
How do I check if my server is vulnerable to CVE-2016-6210?
Run 'ssh -V' to check your version (vulnerable if below 7.3), then verify your sshd configuration uses SHA256/SHA512: grep -i 'passwordauthentication' /etc/ssh/sshd_config. If both conditions are true, your server is exploitable.
Does Defensia detect CVE-2016-6210?
Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If OpenSSH is installed on a monitored server, CVE-2016-6210 will appear in your dashboard with remediation steps.
Related OpenSSH CVEs
References
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