The operating system must implement replay-resistant authentication mechanisms for network access to privileged and non-privileged accounts.

From Apple OS X 10.11 Security Technical Implementation Guide

Associated with: CCI-001941 CCI-001942

SV-82089r1_rule The operating system must implement replay-resistant authentication mechanisms for network access to privileged and non-privileged accounts.

Vulnerability discussion

The SSH Version should be explicitly set to Version 2. Version 2 supports strong crypto and was rewritten from scratch to resolve several weaknesses in Version 1 that make it extremely vulnerable to attackers. The weaker crypto in Version 1 is potentially susceptible to certain forms of replay attacks. The system default is to only enable Version 2.A replay attack may enable an unauthorized user to gain access to the operating system. Authentication sessions between the authenticator and the operating system validating the user credentials must not be vulnerable to a replay attack. An authentication process resists replay attacks if it is impractical to achieve a successful authentication by recording and replaying a previous authentication message.Satisfies: SRG-OS-000112-GPOS-00057, SRG-OS-000113-GPOS-00058

Check content

To check which protocol is configured for sshd, run the following: /usr/bin/sudo /usr/bin/grep ^Protocol /etc/ssh/sshd_config If there is no result or the result is not "2", this is a finding.

Fix text

In order to make sure that "Protocol 2" is used by sshd, run the following command: /usr/bin/sudo /usr/bin/sed -i.bak 's/.*Protocol.*/Protocol 2/' /etc/ssh/sshd_config

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