cis-gcp-cos-5-1-3

name	cis-gcp-cos-5.1.3
description	Ensure permissions on SSH public host key files are configured
category	cis-gcp-cos
version	1.2.0
author	cyberstrike-official
tags	["cis","gcp","container-optimized-os","ssh","sshd","permissions","authentication"]
cis_id	5.1.3
cis_benchmark	CIS Google Container-Optimized OS Benchmark v1.2.0
tech_stack	["gcp","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

5.1.3 Ensure permissions on SSH public host key files are configured (Automated)

Description

An SSH public key is one of two files used in SSH public key authentication. In this authentication method, a public key is a key that can be used for verifying digital signatures generated using a corresponding private key. Only a public key that corresponds to a private key will be able to authenticate successfully.

Rationale

If a public host key file is modified by an unauthorized user, the SSH service may be compromised.

Audit Procedure

Run the following command and verify Access does not grant write or execute permissions to group or other for all returned files:

# find /etc/ssh -xdev -type f -name 'ssh_host_*_key.pub' -exec stat {} \;

  File: '/etc/ssh/ssh_host_rsa_key.pub'
  Size: 382        Blocks: 8          IO Block: 4096   regular file
Device: ca01h/51713d    Inode: 8631758     Links: 1
Access: (0644/-rw-r--r--)   Uid: (    0/    root)   Gid: (    0/    root)
Access: 2018-10-22 18:24:56.861750616 +0000
Modify: 2018-10-22 18:24:56.861750616 +0000
Change: 2018-10-22 18:24:56.881750616 +0000
 Birth: -
  File: '/etc/ssh/ssh_host_ecdsa_key.pub'
  Size: 162        Blocks: 8          IO Block: 4096   regular file
Device: ca01h/51713d    Inode: 8631761     Links: 1
Access: (0644/-rw-r--r--)   Uid: (    0/    root)   Gid: (    0/    root)
Access: 2018-10-22 18:24:56.897750616 +0000
Modify: 2018-10-22 18:24:56.897750616 +0000
Change: 2018-10-22 18:24:56.917750616 +0000
 Birth: -
  File: '/etc/ssh/ssh_host_ed25519_key.pub'
  Size: 82         Blocks: 8          IO Block: 4096   regular file
Device: ca01h/51713d    Inode: 8631763     Links: 1
Access: (0644/-rw-r--r--)   Uid: (    0/    root)   Gid: (    0/    root)
Access: 2018-10-22 18:24:56.945750616 +0000
Modify: 2018-10-22 18:24:56.945750616 +0000
Change: 2018-10-22 18:24:56.961750616 +0000
 Birth: -

Expected Result

• All public host key files have Uid of 0/root
• All public host key files have Gid of 0/root
• Access is 0644/-rw-r--r-- (no write or execute for group or other)

Remediation

Run the following commands to set permissions and ownership on the SSH host public key files:

# find /etc/ssh -xdev -type f -name 'ssh_host_*_key.pub' -exec chmod 0644 {} \;
# find /etc/ssh -xdev -type f -name 'ssh_host_*_key.pub' -exec chown root:root {} \;

CIS Controls