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:

```bash
# 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:

```bash
# 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

| Controls Version | Control                                 | IG 1 | IG 2 | IG 3 |
| ---------------- | --------------------------------------- | ---- | ---- | ---- |
| v8               | 3.3 Configure Data Access Control Lists | x    | x    | x    |
| v7               | 5.1 Establish Secure Configurations     | x    | x    | x    |

## Profile

- Level 1 - Server