cyberstrikeus CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 5.3.2
Description
An SSH private key is one of two files used in SSH public key authentication. In this authentication method, The possession of the private key is proof of identity. Only a private key that corresponds to a public key will be able to authenticate successfully. The private keys need to be stored and handled carefully, and no copies of the private key should be distributed.
Rationale
If an unauthorized user obtains the private SSH host key file, the host could be impersonated.
Audit Procedure
Command Line
Run the following command and verify Uid is 0/root and Gid is 0/root and permissions are 0600 or more restrictive:
find /etc/ssh -xdev -type f -name 'ssh_host_*_key' -exec stat {} \;
Expected Result
File: '/etc/ssh/ssh_host_rsa_key'
Size: 1675 Blocks: 8 IO Block: 4096 regular file
Access: (0600/-rw-------) Uid: ( 0/ root) Gid: ( 0/ root)
File: '/etc/ssh/ssh_host_ecdsa_key'
Size: 227 Blocks: 8 IO Block: 4096 regular file
Access: (0600/-rw-------) Uid: ( 0/ root) Gid: ( 0/ root)
File: '/etc/ssh/ssh_host_ed25519_key'
Size: 399 Blocks: 8 IO Block: 4096 regular file
Access: (0600/-rw-------) Uid: ( 0/ root) Gid: ( 0/ root)
File: '/etc/ssh/ssh_host_dsa_key'
Size: 672 Blocks: 8 IO Block: 4096 regular file
Access: (0600/-rw-------) Uid: ( 0/ root) Gid: ( 0/ root)
Remediation
Command Line
Run the following commands to set permissions, ownership, and group on the private SSH host key files:
find /etc/ssh -xdev -type f -name 'ssh_host_*_key' -exec chown root:root {} \;
find /etc/ssh -xdev -type f -name 'ssh_host_*_key' -exec chmod u-x,go-rwx {} \;
Default Value
Access: (0600/-rw-------) Uid: ( 0/ root) Gid: ( 0/ root)
References
- SSHD_CONFIG(5)
CIS Controls
Version 7
14.6 Protect Information through Access Control Lists - Protect all information stored on systems with file system, network share, claims, application, or database specific access control lists. These controls will enforce the principle that only authorized individuals should have access to the information based on their need to access the information as a part of their responsibilities.
Profile Applicability
- Level 1 - Server
- Level 1 - Workstation
Assessment Status
Automated