CVE-2016-6210 in OpenSSH
Summary
by MITRE
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.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 05/30/2026
The vulnerability identified as CVE-2016-6210 represents a critical timing-based side-channel attack against OpenSSH's sshd daemon implementation. This flaw exists in versions prior to 7.3 and specifically affects systems where SHA256 or SHA512 password hashing algorithms are configured for user authentication. The vulnerability stems from an inconsistent response time behavior during the authentication process, creating a measurable timing difference that can be exploited by remote attackers to determine the existence of specific user accounts within the system.
The technical implementation of this vulnerability occurs at the password verification stage within the sshd daemon when processing authentication requests. When a client attempts to authenticate using a non-existent username, the system employs the BLOWFISH hashing algorithm with a static password value instead of properly handling the invalid username case. This inconsistent processing creates a timing differential that becomes apparent when attackers provide large password values during authentication attempts. The timing variations occur because the system must perform additional cryptographic operations when dealing with non-existent users compared to when it processes valid usernames with incorrect passwords, allowing attackers to distinguish between these two scenarios through careful timing analysis.
The operational impact of CVE-2016-6210 extends beyond simple user enumeration, potentially enabling more sophisticated attacks including account takeover attempts and credential stuffing operations. Attackers can leverage this vulnerability to systematically test multiple username combinations against a target system, using the timing variations to confirm which accounts exist within the system. This information can then be used to focus subsequent attacks on valid accounts, significantly reducing the search space for brute force or dictionary attacks. The vulnerability particularly affects systems where OpenSSH is configured to use modern hashing algorithms such as SHA256 or SHA512, which are commonly implemented in enterprise environments for enhanced security requirements.
This vulnerability maps directly to CWE-203, which describes "Information Exposure Through Timing Discrepancy," and aligns with ATT&CK technique T1078.004 for Valid Accounts and T1110.001 for Brute Force. The timing-based information leakage creates a significant security risk because it provides attackers with actionable intelligence about system user accounts without requiring direct access or sophisticated exploitation techniques. Organizations implementing this type of vulnerability in their authentication infrastructure face increased risk of credential compromise and unauthorized access attempts, particularly in environments where automated attack tools can rapidly iterate through potential username combinations.
The recommended mitigation strategy involves upgrading OpenSSH to version 7.3 or later, which addresses the timing inconsistency by ensuring consistent response handling regardless of whether the username exists. Additionally, organizations should implement account lockout mechanisms, rate limiting, and monitoring for unusual authentication patterns to detect and prevent exploitation attempts. Network-level protections such as firewall rules and intrusion detection systems can also help identify and block repeated authentication attempts that may indicate exploitation of this vulnerability. Security teams should also consider implementing multi-factor authentication to provide additional protection layers beyond password-based authentication.