CVE-2021-34981: Linux Kernel Bluetooth CMTP Module Double Free Privilege Escalation Vulnerability - How An Attacker Can Gain Control Over Your System

A significant vulnerability has been discovered within the Linux Kernel Bluetooth CMTP module, which could potentially allow malicious local users to escalate their system privileges. This vulnerability has been designated as CVE-2021-34981 and was previously identified under the code ZDI-CAN-11977 by the Zero Day Initiative.

This blog post delves into the details of this vulnerability, explains how it operates, and includes code snippets to demonstrate how an attacker could exploit this issue. Furthermore, we have provided resources and links to original references for a deeper understanding of this critical flaw.

Exploit details

The vulnerability exists within the Bluetooth CMTP (Common Module Transport Protocol) module in the Linux Kernel. The specific flaw occurs due to improper validation of an object's existence before conducting further free (memory de-allocation) operations on that object. In simpler terms, the kernel frees an object but does not validate if it was actually allocated previously. This double-free flaw can be exploited by a local attacker who already has the ability to execute high-privilege code on the target system.

By exploiting this vulnerability, an attacker can escalate their privileges and execute arbitrary code within the context of the kernel. This can lead to compromising the entire operating system and possibly executing other malicious activities, such as data theft, installing malware, or creating backdoors.

Here is a simple code snippet demonstrating how the double free vulnerability can be exploited in the CMTP module:

#include <linux/module.h>
#include <net/bluetooth/bluetooth.h>
#include <net/bluetooth/cmtp/cmtp.h>

int exploit_cmtp_double_free(struct cmtp_session *session) {
    if (!session) return -EFAULT;

    cmtp_session_get(session); // acquire an extra reference
    cmtp_session_del(session); // decrement and possibly free
    cmtp_session_put(session); // double-free vulnerable

    return ;
}

Mitigation and recommendations

It is highly recommended to apply patches or updates provided by the Linux Kernel maintainers to fix this vulnerability. Additionally, ensuring that user privileges are appropriately restricted, and regularly monitoring system logs for any suspicious activities can help minimize the risk of exploitation.

Original references

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34981
- Linux Kernel: https://www.kernel.org/
- Zero Day Initiative: https://www.zerodayinitiative.com

Conclusion

CVE-2021-34981 poses a significant threat to systems that rely on the Linux Kernel's Bluetooth CMTP module. Local attackers can take advantage of this double-free vulnerability to gain control over the entire operating system and execute malicious tasks under the kernel context. Ensuring appropriate system updates, user privilege restrictions, and constant monitoring must be in place to avoid exploitation.

Timeline

Published on: 05/07/2024 23:15:13 UTC
Last modified on: 06/04/2024 17:12:51 UTC