CVE-2016-6210: OpenSSH sshd user enumeration via password hashing timing leak

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 18 September 2026. Confidence: high.

medium priorityThe flaw only leaks username validity and requires a timing-sensitive attack, but the very high EPSS and public exploits make it a practical precursor to credential attacks.

What it is

OpenSSH before 7.3, when SHA256 or SHA512 password hashing is configured, falls back to BLOWFISH hashing of a static password for nonexistent usernames. Because that path is faster than hashing a large supplied password for a real account, response timing differs and remote attackers can distinguish valid from invalid usernames. The result is reliable account enumeration that feeds later credential attacks.

Impact

An attacker learns which usernames exist on the target host, enabling targeted password guessing and reducing the search space for brute force. No data beyond username validity is exposed.

Attack surface

Reachable over the network through the SSH authentication service; no authentication or user interaction is required, though the timing signal is only observable when SHA256 or SHA512 hashing is in use and a large password is submitted.

Exploitation

Not listed in CISA KEV, but EPSS is very high (0.88944, 99.77th percentile) and public proof-of-concept code exists in Exploit-DB (40113, 40136), indicating active interest and easy reproduction.

What to do

• Upgrade OpenSSH to 7.3 or later, or apply the vendor errata for your distribution (Debian DSA-3626, Red Hat RHSA-2017:2029 and RHSA-2017:2563, Gentoo GLSA 201612-18).
• If immediate upgrade is not possible, restrict SSH exposure to trusted networks and enforce key-based authentication with password authentication disabled.
• Rate-limit and monitor SSH authentication attempts to blunt enumeration and follow-on brute force.
• Review vendor advisories for embedded or third-party products (Siemens SSA-412672, NetApp ntap-20190206-0001) that bundle affected OpenSSH.
• Verify the deployed sshd version and password hashing configuration after patching.

Detection

• Baseline SSH authentication response times per source and alert on repeated timing patterns consistent with username probing.
• Monitor for high volumes of failed SSH authentication attempts from single sources, especially with large password payloads.
• Correlate SSH auth failures across hosts to identify enumeration sweeps followed by targeted guessing.
• Audit sshd version and configuration to confirm SHA256/SHA512 hashing and patched builds.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.