libssh Client Denial of Service via Certificate Authentication Loop

Executive Summary

CVE-2026-59849 is a flaw in libssh where logic errors in automatic certificate-based public key authentication cause clients to loop indefinitely. This happens when configured certificates are missing or rejected by a server, leading to a denial of service.

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Detection Guidance

To detect this vulnerability, monitor for processes using libssh that hang indefinitely during certificate-based authentication attempts. Check for repeated failed authentication loops in logs or system monitoring tools. No specific commands are provided in the context, but inspecting libssh client logs for repeated authentication failures may indicate exploitation.

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Impact Analysis

This vulnerability can cause a denial of service by making the libssh client hang indefinitely during authentication attempts. It only affects non-default certificate configurations and does not impact default setups.

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Compliance Impact

This vulnerability causes a denial of service via an infinite loop during certificate-based authentication, which could disrupt system availability. While it does not directly impact data confidentiality or integrity, prolonged service disruption may violate availability requirements in standards like GDPR (Article 32) or HIPAA (Security Rule). However, the issue is low severity and only affects non-default certificate configurations.

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Mitigation Strategies

Avoid using automatic certificate-based public key authentication in libssh clients. If certificates are required, ensure proper configuration and server acceptance to prevent infinite loops. Monitor for unusual client behavior indicating repeated authentication attempts.

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