OpenSSH sshd Denial of Service via GSSAPI Authentication

Executive Summary

The vulnerability exists in sshd of OpenSSH versions before 10.4. It allows remote attackers to cause a denial of service by consuming resources through excessive authentication attempts. This happens because the MaxAuthTries setting is mishandled when GSSAPIAuthentication is enabled.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Detection Guidance

This vulnerability involves sshd mishandling MaxAuthTries for GSSAPIAuthentication, leading to denial of service from excessive authentication attempts.

Detection can focus on monitoring sshd logs for unusually high numbers of authentication attempts, especially related to GSSAPI authentication failures.

Commands to help detect this might include checking sshd logs with commands like:

• sudo grep 'authentication failure' /var/log/auth.log
• sudo journalctl -u sshd | grep 'GSSAPIAuthentication'
• Monitoring for excessive connection attempts using tools like netstat or ss:
• sudo netstat -anp | grep ':22'
• sudo ss -tnp | grep ':22'

However, no specific detection commands or tools are detailed in the provided resources.

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Mitigation Strategies

The vulnerability is fixed in OpenSSH version 10.4, released on July 6, 2026.

Immediate mitigation steps include upgrading sshd to OpenSSH 10.4 or later, which addresses the mishandling of MaxAuthTries for GSSAPIAuthentication and other related security issues.

Additionally, reviewing and possibly tightening sshd configuration related to authentication attempts, such as lowering MaxAuthTries or disabling GSSAPIAuthentication if not needed, can help reduce risk.

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Impact Analysis

This vulnerability can impact you by causing a denial of service condition on the affected system. Remote attackers can exploit it to consume system resources through repeated authentication attempts, potentially making the sshd service unavailable or degraded.

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