CVE-2026-63948 Overview
CVE-2026-63948 is a reference count leak in the Linux kernel's Bluetooth Logical Link Control and Adaptation Protocol (L2CAP) implementation. The flaw resides in the l2cap_chan_timeout() function, which handles delayed work for L2CAP channels. When the channel's conn pointer is NULL, the function takes an early return path that skips the required l2cap_chan_put() call, leaving the reference held by __set_chan_timer() unreleased. Repeated triggering of this code path leaks l2cap_chan objects, producing a memory leak in the kernel Bluetooth stack. The fix adds the missing l2cap_chan_put() before the early return.
Critical Impact
A local or proximate attacker able to trigger L2CAP channel timeouts on connectionless channels can leak kernel l2cap_chan references, gradually exhausting kernel memory and potentially degrading system availability.
Affected Products
- Linux kernel Bluetooth subsystem (net/bluetooth/l2cap_core.c)
- Multiple stable kernel branches receiving the backported fix
- Distributions shipping affected upstream Linux kernel versions
Discovery Timeline
- 2026-07-19 - CVE-2026-63948 published to NVD
- 2026-07-19 - Last updated in NVD database
Technical Details for CVE-2026-63948
Vulnerability Analysis
The defect is a memory/reference leak in the L2CAP channel timeout handler. In the Linux Bluetooth stack, __set_chan_timer() calls l2cap_chan_hold() to acquire an extra reference on the l2cap_chan object before scheduling delayed work. This reference guarantees the channel remains valid until the timer fires.
When the timer executes l2cap_chan_timeout(), the normal exit path releases the reference via l2cap_chan_put(). However, the function contains an early return when chan->conn is NULL, and that path never drops the held reference. Each occurrence permanently leaks one l2cap_chan object and its associated allocations.
While the immediate impact is resource exhaustion rather than memory corruption, sustained triggering can consume kernel slab memory and impair Bluetooth functionality or overall system stability.
Root Cause
The root cause is unbalanced reference counting on an error/short-circuit path. The hold performed by __set_chan_timer() is unconditional, but the matching put in l2cap_chan_timeout() sits after the !conn check. Any invocation where the channel's connection has already been torn down bypasses the release.
Attack Vector
Triggering the leak requires reaching a state where an L2CAP channel has an active timer while chan->conn is NULL. This can occur during connection teardown races between L2CAP timer scheduling and connection cleanup. An attacker with Bluetooth proximity or local ability to open and abort L2CAP connections repeatedly could induce the condition to slowly exhaust kernel memory. No verified public exploit is available, and the EPSS probability is 0.211%.
No verified public exploitation code is available for CVE-2026-63948. Review the upstream commits referenced in the Kernel Git Commit b5c59a5b469e and companion patches for the exact source-level change.
Detection Methods for CVE-2026-63948
Indicators of Compromise
- Steadily growing kernel slab usage attributable to l2cap_chan allocations without corresponding frees.
- Unusual patterns of short-lived L2CAP connections followed by abrupt disconnects from nearby or paired Bluetooth peers.
- dmesg entries related to L2CAP channel teardown coinciding with rising memory pressure.
Detection Strategies
- Monitor /proc/slabinfo for anomalous growth in Bluetooth-related caches on long-running hosts with Bluetooth enabled.
- Track kernel version and patch level across the fleet against the fixed commits listed in the NVD references.
- Correlate Bluetooth activity telemetry with host memory metrics to identify hosts leaking memory during L2CAP churn.
Monitoring Recommendations
- Alert on unexpected Bluetooth stack activity on servers and workstations where Bluetooth is not an operational requirement.
- Baseline kernel memory consumption per host and flag sustained upward drift that resists reclaim.
- Ingest kernel logs into a centralized platform to correlate L2CAP events with resource exhaustion symptoms.
How to Mitigate CVE-2026-63948
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the NVD entry, including commits b5c59a5b469e, 08d81fe96f80, 107c826e4ef9, 50f1bcaaaa3, 63cd225cc13d, 8894c2010435, 9dbd84990394, and e8a5baff5be2.
- Update to the distribution kernel package that incorporates the fix for l2cap_chan_timeout().
- Reboot affected hosts after patching to load the corrected kernel image.
Patch Information
The fix adds the missing l2cap_chan_put() before the early return in l2cap_chan_timeout() when chan->conn is NULL. The change is available in the mainline Linux tree and has been backported to multiple stable branches. Refer to the Kernel Git Commit 08d81fe96f80 and the additional stable-tree commits linked in the NVD advisory to identify the correct patch for each supported kernel line.
Workarounds
- Disable the Bluetooth kernel modules (bluetooth, btusb) on systems that do not require Bluetooth functionality.
- Blacklist Bluetooth modules via /etc/modprobe.d/ on servers and infrastructure hosts.
- Restrict physical and RF proximity to sensitive endpoints until the patched kernel is deployed.
# Disable and blacklist the Bluetooth stack until the patched kernel is applied
sudo systemctl stop bluetooth.service
sudo systemctl disable 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 rmmod btusb bluetooth 2>/dev/null || true
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
