CVE-2026-99008 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: RFCOMM: Fix NULL tty_dev dereference in rfcomm_dev_shutdown
syzbot hit a NULL pointer dereference in rfcomm_dev_shutdown() when an RFCOMM tty is closed.
rfcomm_dev_add() puts the new device on rfcomm_dev_list first and only then registers the tty device and saves it in dev->tty_dev. If that registration fails, dev->tty_dev is never set at all. The device can be looked up in the meantime, so an open() of /dev/rfcommN gets through rfcomm_tty_install() and attaches to it. When that tty is closed again, rfcomm_dev_shutdown() uses dev->tty_dev without checking it:
Oops: general protection fault, probably for non-canonical address KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
RIP: 0010:rfcomm_dev_shutdown+0x48/0xb0 net/bluetooth/rfcomm/tty.c:131 Call Trace: tty_port_shutdown+0x1bf/0x220 drivers/tty/tty_port.c:372 tty_port_close+0x4e/0x150 drivers/tty/tty_port.c:705 tty_release+0x359/0x1670 drivers/tty/tty_io.c:1718 __fput+0x418/0xa50 fs/file_table.c:512
rfcomm_dev_destruct() already copes with a NULL dev->tty_dev. Add the same check to rfcomm_dev_shutdown(), so the tty device is only moved when it was actually registered.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel Bluetooth subsystem contains a critical flaw within the RFCOMM TTY implementation that leads to a null pointer dereference during device shutdown operations. This vulnerability arises from an improper initialization sequence in the rfcomm_dev_add function, where a newly created RFCOMM device is added to the global rfcomm_dev_list before its associated tty device structure is successfully registered and assigned to the dev->tty_dev field. If the registration process fails for any reason, such as resource exhaustion or configuration errors, the dev->tty_dev pointer remains uninitialized or null. However, because the device was already inserted into the lookup list, it becomes accessible to user space applications via standard character device interfaces like /dev/rfcommN. This creates a race condition window where an application can successfully open and attach to this partially initialized device structure despite its incomplete setup state.
The operational impact manifests when a user space process closes the file descriptor associated with the RFCOMM tty device, triggering the rfcomm_dev_shutdown routine. The shutdown function attempts to access dev->tty_dev without verifying whether it has been properly populated during initialization. Since the pointer is null in failure scenarios, this results in a general protection fault or kernel panic due to dereferencing a NULL address at offset 0x40 within the tty structure. This crash can be triggered locally by any unprivileged user who can access Bluetooth devices, leading to system instability and denial of service conditions for all users on the affected host. The vulnerability is particularly dangerous because it exploits the timing between device registration in kernel lists and actual resource allocation success, allowing attackers to force a crash through simple file descriptor lifecycle management without requiring elevated privileges or complex exploitation techniques beyond opening and closing devices rapidly under specific failure conditions.
From a technical classification perspective, this issue aligns with CWE-476 NULL Pointer Dereference, as the code fails to validate pointer validity before use. It also relates to CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization due to the race condition between device list insertion and initialization completion. In terms of MITRE ATT&CK mapping, this vulnerability falls under T1059 Command and Scripting Interpreter via local execution leading to system compromise through denial of service, specifically within the context of kernel exploitation techniques that disrupt availability rather than confidentiality or integrity directly. The root cause lies in inconsistent error handling paths where partial initialization states are exposed to external triggers without adequate defensive checks.
To mitigate this vulnerability, developers must ensure that device structures are not added to global lookup lists until all associated resources have been successfully allocated and initialized. Specifically, the rfcomm_dev_add function should defer list insertion until after dev->tty_dev has been confirmed as valid. Alternatively, adding explicit NULL pointer checks in both rfcomm_dev_shutdown and related cleanup routines provides immediate defense-in-depth against crashes resulting from partially constructed objects. System administrators running affected kernel versions should apply upstream patches that include these validation checks to prevent local denial of service attacks via Bluetooth device manipulation. Regular security auditing of driver initialization sequences can help identify similar patterns where resources are exposed before full readiness, ensuring robust handling of edge cases in hardware abstraction layers and network protocol stacks within the Linux kernel environment.