CVE-2026-64403: Linux Kernel Use-After-Free Vulnerability

CVE-2026-64403 Overview

CVE-2026-64403 is an out-of-bounds read vulnerability in the Linux kernel Bluetooth Logical Link Control and Adaptation Protocol (L2CAP) subsystem. The flaw resides in the l2cap_get_conf_opt() function, which derives an option length from the attacker-controlled opt->len field and immediately dereferences opt->val before verifying that the requested bytes are present in the buffer. Up to 4 bytes can be read past the end of the buffer during configuration option parsing.

The vulnerability affects Linux kernel Bluetooth stack processing of L2CAP configuration requests and responses. An adjacent attacker within Bluetooth range can trigger the flaw by sending crafted L2CAP configuration frames.

Critical Impact
Adjacent-network attackers can trigger out-of-bounds reads in kernel memory, potentially causing denial of service or contributing to further exploitation chains against Linux systems with Bluetooth enabled.

Affected Products

Linux kernel Bluetooth L2CAP subsystem (multiple stable branches, per fix commits)
Linux distributions shipping affected kernel versions with Bluetooth support enabled
Systems using net/bluetooth/l2cap_core.c prior to the applied patches

Discovery Timeline

2026-07-25 - CVE-2026-64403 published to NVD
2026-07-27 - Last updated in NVD database

Technical Details for CVE-2026-64403

Vulnerability Analysis

The vulnerability is a validate-after-use ordering bug in the L2CAP configuration option parser. The l2cap_get_conf_opt() helper reads the option length field from an untrusted buffer, then immediately dereferences opt->val as a u8, via get_unaligned_le16(), via get_unaligned_le32(), or as a raw pointer in the default case. No caller confirms that opt->len bytes are actually present before the dereference occurs.

Callers l2cap_parse_conf_req(), l2cap_parse_conf_rsp(), and l2cap_conf_rfc_get() only detect a malformed option after the fact, once a running length counter has gone negative. By that point the out-of-bounds read has already executed. An existing post-hoc length check prevents the garbage value from being consumed in the current control flow, so this is not a data leak today. However, up to 4 bytes are read past the end of the buffer, and the pattern is fragile to future changes in callers.

Root Cause

The root cause is missing bounds validation prior to memory access. The parser trusts the length field embedded in an incoming L2CAP configuration option without checking it against the buffer end pointer. This is a classic out-of-bounds read pattern where input length metadata is honored before being validated.

Attack Vector

Exploitation requires proximity to a target Bluetooth-enabled Linux host. An attacker within Bluetooth range sends a malformed L2CAP configuration request or response containing an option header whose declared length extends past the actual buffer boundary. Processing the frame triggers the out-of-bounds read inside the kernel. No authentication or user interaction is required beyond an established L2CAP connection context.

The upstream fix passes the end of the buffer into l2cap_get_conf_opt() and refuses to touch opt->val unless the full option (header plus value) fits. Each caller now computes an end pointer once before the loop and checks the return value directly instead of inferring the error from a negative length.

Detection Methods for CVE-2026-64403

Indicators of Compromise

Unexpected kernel warnings, KASAN reports, or oops messages referencing l2cap_get_conf_opt, l2cap_parse_conf_req, or l2cap_parse_conf_rsp in dmesg or journalctl -k output.
Repeated Bluetooth stack instability, HCI resets, or bluetoothd service crashes coinciding with nearby unknown Bluetooth devices.
Malformed L2CAP configuration frames observed in Bluetooth HCI capture logs (btmon, hcidump).

Detection Strategies

Enable Kernel Address Sanitizer (KASAN) on test systems to surface out-of-bounds reads in the L2CAP parser.
Correlate kernel ring buffer messages with Bluetooth pairing or connection events from adjacent devices.
Monitor the running kernel version against distribution security advisories referencing the fix commits (for example 687617555ced, 6b47bdaacfd0, 73abbaf91aa3).

Monitoring Recommendations

Forward kernel and bluetoothd logs to a centralized logging platform for anomaly review.
Track Bluetooth interface state transitions and connection attempts on endpoints where Bluetooth is not operationally required.
Alert on kernel panics or KASAN splats originating in net/bluetooth/ source paths.

How to Mitigate CVE-2026-64403

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in the stable tree commits, or install vendor kernel updates that incorporate the fix.
Disable the Bluetooth stack on systems that do not require it by stopping and masking the bluetooth.service unit and unloading the btusb and bluetooth modules.
Restrict Bluetooth discoverability and pairing on hosts where the service must remain enabled.

Patch Information

The fix is distributed across multiple Linux stable branches. Reference commits include 687617555ced, 6b47bdaacfd0, 73abbaf91aa3, 7d871e969b94, 98d93c226bdf, 996d3da39899, cca81b4bc672, and f70d4aa88068. Consult your distribution advisory for the corresponding kernel package version.

Workarounds

Blocklist the Bluetooth kernel modules on servers, virtual machines, and appliances that do not use Bluetooth.
Use physical shielding or operational controls to reduce adjacent-network attack surface in sensitive environments.
Where Bluetooth is required, limit the classes of devices permitted to establish L2CAP connections through host-based policy.

bash

# Disable and mask the Bluetooth service, then blocklist the modules
sudo systemctl stop bluetooth.service
sudo systemctl disable --now bluetooth.service
sudo systemctl mask bluetooth.service

echo "blacklist bluetooth" | sudo tee /etc/modprobe.d/disable-bluetooth.conf
echo "blacklist btusb"     | sudo tee -a /etc/modprobe.d/disable-bluetooth.conf
sudo modprobe -r btusb bluetooth