CVE-2026-63803: Linux Kernel Use-After-Free Vulnerability

CVE-2026-63803 Overview

CVE-2026-63803 is a use-after-free vulnerability in the Linux kernel's hdlc_ppp driver. The flaw resides in how per-protocol timers for PPP control protocols (LCP, IPCP, IPV6CP) are handled during teardown of the HDLC state. The detach_hdlc_protocol() function frees the hdlc->state allocation containing struct ppp without synchronizing active timers. A running ppp_timer callback can dereference freed memory after kfree() completes, leading to memory corruption. The vulnerability was identified through static analysis and is fixed by introducing a .detach callback that invokes timer_shutdown_sync() on each per-protocol timer before deallocation.

Critical Impact
Local attackers with low privileges can trigger use-after-free conditions in the kernel, potentially resulting in privilege escalation, information disclosure, or denial of service.

Affected Products

Linux kernel versions containing the hdlc_ppp driver prior to the fix commits
Systems with HDLC PPP protocol attached to network devices
Distributions shipping affected upstream Linux kernel versions

Discovery Timeline

2026-07-19 - CVE-2026-63803 published to NVD
2026-07-24 - Last updated in NVD database

Technical Details for CVE-2026-63803

Vulnerability Analysis

The vulnerability exists in the Linux kernel's HDLC PPP subsystem, which handles Point-to-Point Protocol encapsulation over High-Level Data Link Control interfaces. Each PPP control protocol embedded in struct ppp registers a timer via timer_setup(). The struct ppp resides in the hdlc->state allocation, which detach_hdlc_protocol() frees with kfree() during both unregister_hdlc_device() and the re-attach path inside attach_hdlc_protocol().

The PPP protocol implementation never registered a .detach callback. As a result, detach_hdlc_protocol() performs no timer synchronization before freeing the underlying memory. The only existing cancellation, timer_delete(&proto->timer) in ppp_cp_event(), is partial because it does not wait for a running callback to finish. Additionally, ppp_stop() and ppp_close() do not synchronize timers either. This creates a classic use-after-free [CWE-416] window where a ppp_timer callback already executing (blocked on ppp->lock) survives the kfree() and then dereferences proto->state and ppp->lock in freed memory.

Root Cause

The root cause is missing timer synchronization during protocol detachment. Because attach_hdlc_protocol() uses kmalloc() rather than kzalloc(), the protos[i].timer field starts as uninitialized memory until the first timer_setup() call in ppp_start(), which only runs on NETDEV_UP. This means shutdown routines could operate on uninitialized timer_list structures if the interface was never brought up.

Attack Vector

A local attacker with the ability to configure network interfaces can trigger the race by rapidly attaching, starting, and detaching the PPP protocol on an HDLC device. If a timer callback is executing while detach_hdlc_protocol() calls kfree(), the callback will access freed memory upon resuming, corrupting kernel state. Exploitation requires local access with privileges sufficient to manipulate HDLC configuration.

No verified public exploit code is available. The vulnerability mechanism is described in the upstream patch commits linked in the references section.

Detection Methods for CVE-2026-63803

Indicators of Compromise

Kernel panics or oops messages referencing ppp_timer, ppp_cp_event, or detach_hdlc_protocol
KASAN reports indicating use-after-free in hdlc_ppp.c code paths
Unexpected system crashes on hosts with active HDLC PPP interfaces
Suspicious ioctl() sequences from unprivileged processes targeting HDLC devices

Detection Strategies

Enable KASAN (Kernel Address Sanitizer) on test kernels to identify use-after-free access patterns in hdlc_ppp
Monitor kernel logs (dmesg, journalctl -k) for BUG or Oops entries referencing PPP timer functions
Audit systems for the presence of vulnerable Linux kernel versions using package inventory tools
Track ioctl syscalls invoking HDLC protocol attach and detach operations from non-root contexts

Monitoring Recommendations

Collect kernel telemetry through auditd rules targeting network interface configuration syscalls
Alert on repeated crashes in the same driver module within short time windows
Baseline expected HDLC/PPP configuration activity and flag anomalies
Correlate kernel exceptions with process ancestry to identify potential exploitation attempts

How to Mitigate CVE-2026-63803

Immediate Actions Required

Update to a patched Linux kernel version containing the upstream fix commits referenced by kernel.org
Restrict access to HDLC device configuration to trusted administrators only
Disable the hdlc_ppp module on systems that do not require PPP over HDLC connectivity
Prioritize patching on multi-tenant systems where local unprivileged users are present

Patch Information

The upstream fix adds a .detach helper that calls timer_shutdown_sync() on every per-protocol timer. detach_hdlc_protocol() invokes proto->detach(dev) before kfree(hdlc->state), so timer_shutdown_sync() runs on both teardown paths. timer_shutdown_sync() is used instead of timer_delete_sync() because the keepalive path re-arms the timer through add_timer() and mod_timer(), and shutdown blocks any re-activation during teardown. Per-protocol timers are now initialized in ppp_ioctl() at attach time, and the redundant timer_setup() in ppp_start() is removed. Fix commits are available at Linux Kernel Commit 508a0139, Linux Kernel Commit c78a4e41, and additional stable branch commits.

Workarounds

Blacklist the hdlc_ppp kernel module where PPP over HDLC is not required
Remove the CAP_NET_ADMIN capability from untrusted user contexts to prevent HDLC configuration
Apply kernel lockdown or module signing policies to prevent loading of vulnerable driver versions
Deploy MAC frameworks such as SELinux or AppArmor to restrict network configuration syscalls

bash

# Blacklist the hdlc_ppp module until patching is complete
echo "blacklist hdlc_ppp" | sudo tee /etc/modprobe.d/blacklist-hdlc-ppp.conf
sudo modprobe -r hdlc_ppp

# Verify the module is not loaded
lsmod | grep hdlc_ppp