深入分析SaltStack Salt命令注入漏洞

译文声明

本文是翻译文章，文章原作者thezdi，文章来源：thezdi.com原文地址：https://www.thezdi.com/blog/2020/11/24/detailing-saltstack-salt-command-injection-vulnerabilities

译文仅供参考，具体内容表达以及含义原文为准

 

一、概述

11月3日，SaltStack发布了Salt的安全补丁，修复了三个高危漏洞。其中有两个修复程序，是为了解决外部研究人员通过ZDI项目提交的五个漏洞。这些漏洞可导致在运行Salt应用程序的系统上实现未经身份验证的命令注入。ZDI-CAN-11143是由一位匿名研究人员提交给ZDI的，而其余的漏洞则是我们发现的ZDI-CAN-11143的变体。在这篇文章中，我们将详细研究这些漏洞的根本原因。

 

二、漏洞详情

漏洞影响应用程序的rest-cherrypy netapi模块。rest-cherrypy模块为Salt提供REST API。该模块来源于Python的CherryPy模块，在默认情况下未启用。如果要启用rest-cherrypy模块，主配置文件/etc/salt/master中必须加入以下行：

rest_cherrypy: 
Port: 8000 
Disable_ssl: true

其中，/run终端非常重要。它通过salt-ssh子系统发出命令，而salt-ssh子系统会使用SSH来执行Salt例程。发送到/run API的POST请求将调用salt.netapi.rest_cherrypy.app.Run类的POST()方法，这个类最终会调用salt.netapi.NetapiClient的run()方法：

class NetapiClient(object): 
# [... Truncated ...] 
salt.exceptions.SaltInvocationError( 
# "Invalid client specified: '{0}'".format(low.get("client")) 
"Invalid client specified: '{0}'".format(CLIENTS) 
) 
if not ("token" in low or "eauth" in low): 
raise salt.exceptions.EauthAuthenticationError( 
"No authentication credentials given" 
) 
if low.get("raw_shell") and not self.opts.get("netapi_allow_raw_shell"): 
raise salt.exceptions.EauthAuthenticationError( 
"Raw shell option not allowed." 
) 
l_fun = getattr(self, low["client"]) 
f_call = salt.utils.args.format_call(l_fun, low) 
return l_fun(*f_call.get("args", ()), **f_call.get("kwargs", {})) 
def local_batch(self, *args, **kwargs): 
""" 
Run :ref:`execution modules <all-salt.modules>` against batches of minions 
.. versionadded:: 0.8.4 
Wraps :py:meth:`salt.client.LocalClient.cmd_batch` 
:return: Returns the result from the execution module for each batch of 
returns 
""" 
local = salt.client.get_local_client(mopts=self.opts) 
return local.cmd_batch(*args, **kwargs) 
def ssh(self, *args, **kwargs): 
""" 
Run salt-ssh commands synchronously 
Wraps :py:meth:`salt.client.ssh.client.SSHClient.cmd_sync`. 
:return: Returns the result from the salt-ssh command 
""" 
ssh_client = salt.client.ssh.client.SSHClient( 
mopts=self.opts, disable_custom_roster=True 
) 
return ssh_client.cmd_sync(kwargs)

如上所示，run()方法负责验证client参数的值。client参数的有效值包括local、local_async、local_batch、local_subset、runner、runner_async、ssh、wheel和wheel_async。在验证了client参数后，它将检查请求中是否存在token或eauth参数。值得关注的是，这个方法无法验证token或eauth参数的值。因此，无论token或eauth参数是任何值，都可以通过检查。在通过检查后，该方法会根据client参数的值，去调用相应的方法。

如果client参数值为ssh时，将触发漏洞。在这种情况下，run()方法将调用ssh()方法。ssh()方法通过调用salt.client.ssh.client.SSHClient类的cmd_sync()方法同步执行ssh-salt命令，最终会调用_prep_ssh()方法。

class SSHClient(object): 
# [... Truncated] 
def _prep_ssh( 
self, tgt, fun, arg=(), timeout=None, tgt_type="glob", kwarg=None, **kwargs 
): 
""" 
Prepare the arguments 
""" 
opts = copy.deepcopy(self.opts) 
opts.update(kwargs) 
if timeout: 
opts["timeout"] = timeout 
arg = salt.utils.args.condition_input(arg, kwarg) 
opts["argv"] = [fun] + arg 
opts["selected_target_option"] = tgt_type 
opts["tgt"] = tgt 
return salt.client.ssh.SSH(opts) 
def cmd( 
self, tgt, fun, arg=(), timeout=None, tgt_type="glob", kwarg=None, **kwargs 
): 
ssh = self._prep_ssh(tgt, fun, arg, timeout, tgt_type, kwarg, **kwargs) #<-------------- calls ZDI-CAN-11143 
final = {} 
for ret in ssh.run_iter(jid=kwargs.get("jid", None)): #<------------- ZDI-CAN-11173 
final.update(ret) 
return final 
def cmd_sync(self, low): 
kwargs = copy.deepcopy(low) 
for ignore in ["tgt", "fun", "arg", "timeout", "tgt_type", "kwarg"]: 
if ignore in kwargs: 
del kwargs[ignore] 
return self.cmd( 
low["tgt"], 
low["fun"], 
low.get("arg", []), 
low.get("timeout"), 
low.get("tgt_type"), 
low.get("kwarg"), 
**kwargs 
)            #<------------------- calls

_prep_ssh()函数设置参数，并初始化SSH对象。

 

三、触发漏洞

触发漏洞的请求如下：

curl -i $salt_ip_addr:8000/run -H "Content-type: application/json" -d '{"client":"ssh","tgt":"A","fun":"B","eauth":"C","ssh_priv":"|id>/tmp/test #"}'

在这里，client参数的值为ssh，存在漏洞的参数是ssh_priv。在内部，ssh_priv参数在SSH对象初始化期间会用到，如下所示：

SSH(object): 
""" 
Create an SSH execution system 
""" 
ROSTER_UPDATE_FLAG = "#__needs_update" 
def __init__(self, opts): 
self.__parsed_rosters = {SSH.ROSTER_UPDATE_FLAG: True} 
pull_sock = os.path.join(opts["sock_dir"], "master_event_pull.ipc") 
if os.path.exists(pull_sock) and zmq: 
self.event = salt.utils.event.get_event( 
"master", opts["sock_dir"], opts["transport"], opts=opts, listen=False 
) 
else: 
self.event = None 
self.opts = opts 
if self.opts["regen_thin"]: 
self.opts["ssh_wipe"] = True 
if not salt.utils.path.which("ssh"): 
raise salt.exceptions.SaltSystemExit( 
code=-1, 
msg="No ssh binary found in path -- ssh must be installed for salt-ssh to run. Exiting.", 
) 
self.opts["_ssh_version"] = ssh_version() 
self.tgt_type = ( 
self.opts["selected_target_option"] 
if self.opts["selected_target_option"] 
else "glob" 
) 
self._expand_target() 
self.roster = salt.roster.Roster(self.opts, self.opts.get("roster", "flat")) 
self.targets = self.roster.targets(self.opts["tgt"], self.tgt_type) 
if not self.targets: 
self._update_targets() 
# If we're in a wfunc, we need to get the ssh key location from the 
# top level opts, stored in __master_opts__ 
if "__master_opts__" in self.opts: 
if self.opts["__master_opts__"].get("ssh_use_home_key") and os.path.isfile( 
os.path.expanduser("~/.ssh/id_rsa") 
): 
priv = os.path.expanduser("~/.ssh/id_rsa") 
else: 
priv = self.opts["__master_opts__"].get( 
"ssh_priv", 
os.path.join( 
self.opts["__master_opts__"]["pki_dir"], "ssh", "salt-ssh.rsa" 
), 
) 
else: 
priv = self.opts.get( 
"ssh_priv", os.path.join(self.opts["pki_dir"], "ssh", "salt-ssh.rsa") 
) 
if priv != "agent-forwarding": 
if not os.path.isfile(priv): 
try: 
salt.client.ssh.shell.gen_key(priv) 
except OSError: 
raise salt.exceptions.SaltClientError( 
"salt-ssh could not be run because it could not generate keys.nn" 
"You can probably resolve this by executing this script with " 
"increased permissions via sudo or by running as root.n" 
"You could also use the '-c' option to supply a configuration " 
"directory that you have permissions to read and write to." 
)

ssh_priv参数的值用于SSH私有文件。如果ssh_priv值对应的文件不存在，则调用/salt/client/ssh/shell.py的gen_key()方法来创建文件，并将ssh_priv作为path参数传递给该方法。基本上，gen_key()方法生成公钥和私钥密钥对，并将其存储在path参数定义的文件中。

def gen_key(path): 
""" 
Generate a key for use with salt-ssh 
""" 
cmd = 'ssh-keygen -P "" -f {0} -t rsa -q'.format(path) 
if not os.path.isdir(os.path.dirname(path)): 
os.makedirs(os.path.dirname(path)) 
subprocess.call(cmd, shell=True)

从上面的方法中我们可以看到，这里并没有清除路径，且在后续的Shell命令中使用了该路径来创建RSA密钥对。如果ssh_priv包含命令注入字符，则可以在通过subprocess.call()方法执行命令时执行用户控制的命令。这样就导致攻击者可以在运行Salt应用程序的系统上运行任意命令。

在进一步研究SSH对象初始化方法之后，可以观察到多个变量设置为用户控制的HTTP参数的值。随后，这些变量在shell命令中用作执行SSH命令的参数。在这里，user、port、remote_port_forwards和ssh_options变量非常容易受到攻击，如下所示：

class SSH(object): 
""" 
Create an SSH execution system 
""" 
ROSTER_UPDATE_FLAG = "#__needs_update" 
def __init__(self, opts): 
# [...] 
self.targets = self.roster.targets(self.opts["tgt"], self.tgt_type) 
if not self.targets: 
self._update_targets() 
# [...] 
self.defaults = { 
"user": self.opts.get( 
"ssh_user", salt.config.DEFAULT_MASTER_OPTS["ssh_user"] 
),  
"port": self.opts.get( 
"ssh_port", salt.config.DEFAULT_MASTER_OPTS["ssh_port"] 
),  # <------------- vulnerable parameter 
"passwd": self.opts.get( 
"ssh_passwd", salt.config.DEFAULT_MASTER_OPTS["ssh_passwd"] 
), 
"priv": priv, 
"priv_passwd": self.opts.get( 
"ssh_priv_passwd", salt.config.DEFAULT_MASTER_OPTS["ssh_priv_passwd"] 
), 
"timeout": self.opts.get( 
"ssh_timeout", salt.config.DEFAULT_MASTER_OPTS["ssh_timeout"] 
) 
+ self.opts.get("timeout", salt.config.DEFAULT_MASTER_OPTS["timeout"]), 
"sudo": self.opts.get( 
"ssh_sudo", salt.config.DEFAULT_MASTER_OPTS["ssh_sudo"] 
), 
"sudo_user": self.opts.get( 
"ssh_sudo_user", salt.config.DEFAULT_MASTER_OPTS["ssh_sudo_user"] 
), 
"identities_only": self.opts.get( 
"ssh_identities_only", 
salt.config.DEFAULT_MASTER_OPTS["ssh_identities_only"], 
), 
"remote_port_forwards": self.opts.get("ssh_remote_port_forwards"), # <------------- vulnerable parameter 
"ssh_options": self.opts.get("ssh_options"), # <------------- vulnerable parameter 
} 
def _update_targets(self): 
""" 
Update targets in case hostname was directly passed without the roster. 
:return: 
""" 
hostname = self.opts.get("tgt", "")  
if "@" in hostname: 
user, hostname = hostname.split("@", 1) # <------------- vulnerable parameter 
else: 
user = self.opts.get("ssh_user") # <------------- vulnerable parameter  
if hostname == "*": 
hostname = "" 
if salt.utils.network.is_reachable_host(hostname): 
hostname = salt.utils.network.ip_to_host(hostname) 
self.opts["tgt"] = hostname 
self.targets[hostname] = { 
"passwd": self.opts.get("ssh_passwd", ""), 
"host": hostname, 
"user": user, 
} 
if self.opts.get("ssh_update_roster"): 
self._update_roster()

_update_targets()方法设置user变量，该变量取决于tgt或ssh_user的值。如果HTTP参数tgt的值使用了“username@localhost”的格式，则会将“username”分配给用户变量。否则，user的值由ssh_user参数设置。port、remote_port_forwards和ssh_options的值分别由HTTP参数ssh_port、ssh_remote_port_forwards和ssh_options定义。

在初始化SSH对象后，_prep_ssh()方法通过handle_ssh()产生一个子进程，最终会执行salt.client.ssh.shell.Shell类的exec_cmd()方法。

def exec_cmd(self, cmd): 
""" 
Execute a remote command 
""" 
cmd = self._cmd_str(cmd) 
logmsg = "Executing command: {0}".format(cmd) 
if self.passwd: 
logmsg = logmsg.replace(self.passwd, ("*" * 6)) 
if 'decode("base64")' in logmsg or "base64.b64decode(" in logmsg: 
log.debug("Executed SHIM command. Command logged to TRACE") 
log.trace(logmsg) 
else: 
log.debug(logmsg) 
ret = self._run_cmd(cmd)  # <--------------- calls 
return ret 
def _cmd_str(self, cmd, ssh="ssh"): 
""" 
Return the cmd string to execute 
""" 
# TODO: if tty, then our SSH_SHIM cannot be supplied from STDIN Will 
# need to deliver the SHIM to the remote host and execute it there 
command = [ssh] 
if ssh != "scp": 
command.append(self.host) 
if self.tty and ssh == "ssh": 
command.append("-t -t") 
if self.passwd or self.priv: 
command.append(self.priv and self._key_opts() or self._passwd_opts()) 
if ssh != "scp" and self.remote_port_forwards: 
command.append( 
" ".join( 
[ 
"-R {0}".format(item) 
for item in self.remote_port_forwards.split(",") 
] 
) 
) 
if self.ssh_options: 
command.append(self._ssh_opts()) 
command.append(cmd) 
return " ".join(command) 
def _run_cmd(self, cmd, key_accept=False, passwd_retries=3): 
# [...] 
term = salt.utils.vt.Terminal( 
cmd, 
shell=True, 
log_stdout=True, 
log_stdout_level="trace", 
log_stderr=True, 
log_stderr_level="trace", 
stream_stdout=False, 
stream_stderr=False, 
) 
sent_passwd = 0 
send_password = True 
ret_stdout = "" 
ret_stderr = "" 
old_stdout = "" 
try: 
while term.has_unread_data: 
stdout, stderr = term.recv()

如上述代码所示，exec_cmd()首先调用the_cmd_str()方法来创建一个命令字符串，而不进行任何验证。然后，它通过显式调用系统Shell程序，调用_run_cmd()来执行命令。这会将命令注入字符视为Shell的元字符，而并非是命令的参数。一旦执行这个精心设计的命令字符串，就可能会导致任意命令注入。

 

四、总结

SaltStack发布了修复程序，以解决命令注入和身份验证绕过漏洞。同时，他们为这两个漏洞分配了编号CVE-2020-16846和CVE-2020-25592。CVE-2020-16846的修复原理是在执行命令时禁用系统Shell，以此来解决漏洞问题。禁用系统Shell意味着Shell元字符会被视为第一个命令的参数的一部分。

CVE-2020-25592的修复原理是添加对eauth和token参数的验证，以此来修复漏洞。这样一来，就仅允许有效用户通过rest-cherrypy netapi模块访问salt-ssh功能。这是我们的ZDI项目中收到的第一个SaltStack漏洞，后续的工作也非常重要。我们期待以后能接收到更多的提交报告。

大家可以关注我的Twitter @ nktropy，跟进团队研究进展，获取最新的漏洞利用技术和安全补丁。

本文翻译自 thezdi.com， 原文链接 。如若转载请注明出处。

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