CVE-2021-33034 in Linux
Summary
by MITRE • 05/15/2021
In the Linux kernel before 5.12.4, net/bluetooth/hci_event.c has a use-after-free when destroying an hci_chan, aka CID-5c4c8c954409. This leads to writing an arbitrary value.
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Analysis
by VulDB Data Team • 08/10/2025
The vulnerability identified as CVE-2021-33034 represents a critical use-after-free condition within the Linux kernel's Bluetooth subsystem, specifically in the net/bluetooth/hci_event.c file. This flaw exists in kernel versions prior to 5.12.4 and manifests during the destruction of hci_chan objects, creating a scenario where freed memory can be accessed and manipulated by malicious actors. The vulnerability stems from improper memory management practices that allow an attacker to write arbitrary values to memory locations that should no longer be accessible, effectively creating a memory corruption issue that can be exploited for privilege escalation or system compromise.
The technical nature of this vulnerability aligns with CWE-416, which specifically addresses use-after-free conditions in software systems. This classification indicates that the vulnerability occurs when a program continues to use a pointer after the memory it points to has been freed, creating a dangerous state where subsequent memory allocations might reuse the freed memory space. The operational impact of this flaw is particularly severe within Bluetooth networking contexts where the HCI (Host Controller Interface) channel management is critical for device communication. When an hci_chan object is destroyed, the improper cleanup process leaves behind dangling pointers that can be manipulated to overwrite arbitrary memory locations, potentially allowing attackers to execute code with kernel-level privileges.
From an operational perspective, this vulnerability presents significant risks to systems running affected kernel versions, particularly those with Bluetooth capabilities such as smartphones, laptops, IoT devices, and embedded systems. The exploitation potential extends beyond simple memory corruption to include privilege escalation attacks that could allow unauthorized users to gain root access to affected systems. The ATT&CK framework categorizes this type of vulnerability under T1068, which involves exploiting legitimate credentials and privileges, while the technique T1059 covers command and script injection that could be facilitated through the memory corruption. The vulnerability's exploitation requires careful crafting of Bluetooth communication packets to trigger the specific memory management sequence that leads to the use-after-free condition, making it a sophisticated target for attackers who understand the kernel's Bluetooth subsystem internals.
Mitigation strategies for CVE-2021-33034 primarily focus on immediate kernel version updates to 5.12.4 or later, which contain the necessary patches to address the memory management issue in hci_event.c. System administrators should prioritize patching affected systems, particularly those handling sensitive data or operating in untrusted network environments where Bluetooth communication is prevalent. Additional defensive measures include implementing Bluetooth communication filtering at network boundaries, disabling unnecessary Bluetooth services when not actively required, and monitoring for anomalous Bluetooth traffic patterns that might indicate exploitation attempts. Organizations should also consider implementing kernel hardening measures such as stack canaries, address space layout randomization, and other exploit mitigations that can reduce the effectiveness of memory corruption attacks. The vulnerability highlights the importance of thorough code review and testing for memory management issues in kernel subsystems, particularly those handling device communication interfaces where improper resource cleanup can lead to severe security implications.