Using Ansible to Mitigate Network Vulnerabilities

April 16, 2018 by Sean Cavanaugh

Even Networks Aren’t Immune

Just like with Windows and Linux servers, networking devices can be exploited by vulnerabilities found in their operating systems. Many IT organizations do not have a comprehensive strategy for mitigating security vulnerabilities that span multiple teams (networking, servers, storage, etc.). Since the majority of network operations is still manual, the need to mitigate quickly and reliably across multiple platforms consisting of hundreds of network devices becomes extremely important.

In Cisco’s March 2018 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication, 22 vulnerabilities were detailed. While Red Hat does not report or keep track of individual networking vendors CVEs, Red Hat Ansible Engine can be used to quickly automate mitigation of CVEs based on instructions from networking vendors.

In this blog post we are going to walk through CVE-2018-0171 which is titled “Cisco IOS and IOS XE Software Smart Install Remote Code Execution Vulnerability.” This CVE is labeled as critical by Cisco, with the following headline summary:

“...a vulnerability in the Smart Install feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition, or to execute arbitrary code on an affected device.”


Gathering Information from Networks

Users leverage Ansible modules to access devices, retrieve information, execute commands and handle systems using specific keywords. One of the first things a CVE requires is collection of inventory. To mitigate a CVE, the networking platform and specific version of code is required. CVE-2018-0171 affects the IOS and IOS-XE network operating systems and Ansible can obtain this information easily. Let’s use the ios_facts module which returns key-value pairs for use in subsequent tasks. For example: ansible_net_model returns the model, and ansible_net_image returns the image file the device is running. For a full list see the ios_facts module documentation page.

- name: gather facts for ios platforms
  ios_facts:
    gather_subset: all

- name: output facts to terminal window
  debug:
    msg: >
      Device {{ansible_net_hostname}}, model 
{{ansible_net_model}}, running {{ansible_net_version}}

When executing the playbook we get nice output like this:

ok: [rtr1] => {
    "msg": "Device rtr1, model CSR1000V, running 16.05.02\n"
}
ok: [rtr2] => {
    "msg": "Device rtr2, model CSR1000V, running 16.05.02\n"
}
ok: [switch] => {
    "msg": "Device c3850-1, model WS-C3850-24T, running 16.06.01\n"
}        

This allows us to quickly grab useful information about our network, and check it against Cisco Security Advisory. In a demo on the GitHub network-automation project we show how to use network facts to quickly build a nice HTML report.

The vulnerability CVE-2018-0171 specifies that to see if a device is vulnerable we must run the show vstack config command. In my network, I have three devices running IOS-XE, two are CSR1000V devices, and one device is a 3850. The two CSR devices don’t have the command, while the 3850 switch does. To make my playbook robust enough to handle errors when a command doesn’t exist, I can use the ignore_errors parameter. Otherwise, the playbook would fail and exit when a target network node doesn’t have the ability to use that command. Alternatively, I could run the playbook only on switches by using a limit. For this example, let’s assume we are running the Cisco 3850 which has the show vstack config command.

- name: run show vstack config
    ios_command:
      commands:
        - show vstack config
    register: showvstack            

In the playbook above I used the register: showvstack. The showvstack is a user defined term (I chose it, it is not reserved). By registering this I can use the output from the show vstack config later in the playbook. We can use the debug module to look at the showvstack variable to see how it’s formatted:

ok: [switch] => {
    "showvstack": {
        "changed": false,
        "failed": false,
        "stdout": [
            "Capability: Director | Client\n Oper Mode: Disabled\n Role: NA\n Vstack Director IP address: 0.0.0.0\n\n *** Following configurations will be effective only on director ***\n Vstack default management vlan: 1\n Vstack start-up management vlan: 1\n Vstack management Vlans: none\n Join Window Details:\n\t Window: Open (default)\n\t Operation Mode: auto (default)\n Vstack Backup Details:\n\t Mode: On (default)\n\t Repository:"
        ],

<<rest of output removed for brevity>>

There is a stdout and a stdout_lines. To read more on the common return values refer to the documentation. Next, we will use my new favorite module, the assert module. This enables us to check if given expressions are true, failing the task if they are not. Cisco provides two outputs that we need to check for in the result of the show vstack config command :

switch1# show vstack config
Role: Client (SmartInstall enabled)

or

switch2# show vstack config
Capability: Client
Oper Mode: Enabled
Role: Client

We can use the assert module to check the text we saved in the showvstack variable:

- name: Check to make sure Cisco's Smart Install Client Feature is not enabled (1/2)
  assert:
    that:
      - "'SmartInstall enabled' not in showvstack.stdout"
      - "'Role' not in showvstack.stdout"
      - "'Client' not in showvstack.stdout" 

Each line in the assert module that is added means there is an implicit AND, meaning all three need to be true for the task to pass.

Similarly we can check the second statement:

- name: Check to make sure Cisco's Smart Install Client Feature is not enabled (1/1)
  assert:
    that:
      - "'Oper Mode' not in showvstack.stdout"
      - "'Enabled' not in showvstack.stdout"
      - "'Role' not in showvstack.stdout"
      - "'Client' not in showvstack.stdout"  

For this particular CVE it lists that there are no workarounds available. On some CVEs we could use the ios_command or ios_config modules to mitigate the CVE based on the instructions the vendor provided. For this particular CVE it links to the documentation on how to disable vstack using the command no vstack which could be sent using the ios_command module. It also recommends for older releases to block traffic on TCP port 4786, which could be pushed using the ios_config module. Since no workaround is provided on the CVE, a network operator needs to make an educated decision based on their environment. Alternatively, for CVE-2018-0150 there is a workaround provided, and the ios_config could simply send no username cisco to mitigate the CVE.

Red Hat Ansible Engine and Red Hat Ansible Tower can be used to help network operators and administrators scale repetitive tasks like checking these dozens of CVEs and make sure their network is safe from vulnerabilities. On the server side, when system administrators are using Red Hat Insights, they can automatically generate playbooks for Red Hat Enterprise Linux to help with vulnerabilities and proactively identify threats to security, performance, and stability. Ansible can be the common way to execute tasks across your entire IT infrastructure.

Link References

Ansible Documentation:

Module Documentation:

Cisco Security Advisories and Alerts:

  • CVE-2018-0171: Cisco IOS and IOS XE Software Smart Install Remote Code Execution Vulnerability
  • CVE-2018-0150: Cisco IOS XE Software Static Credential Vulnerability
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Topics:
Networks, Ansible Engine


 

Sean Cavanaugh

Sean is a Principal Product Marketing Manager, Ansible, where he brings over 10 years of experience building and automating computer networks. Sean previously worked for both Cumulus Networks and Cisco Systems where he helped customers deploy, manage and automate their network infrastructure. He resides in Chapel Hill, NC with his wife and children and tweets from @IPvSean.


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