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CVE-2026-63803HIGH · CVSS 7.8

CVE-2026-63803

linux · linux kernel

What's the vulnerability?

In the Linux kernel, the following vulnerability has been resolved: hdlc_ppp: sync per-proto timers before freeing hdlc state Each PPP control protocol (LCP/IPCP/IPV6CP) embedded in struct ppp registers a timer via timer_setup(). That struct ppp is the hdlc->state allocation, which detach_hdlc_protocol() frees with kfree() in both teardown paths: unregister_hdlc_device() and the re-attach inside attach_hdlc_protocol(). The ppp proto never registered a .detach callback, so detach_hdlc_protocol() performs no timer synchronization before the kfree(). The only cancel, timer_delete(&proto->timer) in ppp_cp_event(), is partial (it does not wait for a running callback) and only runs on the ->CLOSED transition; ppp_stop()/ppp_close() do not sync either. A ppp_timer callback already executing (blocked on ppp->lock) survives the kfree and then dereferences proto->state / ppp->lock in freed memory, leading to a use-after-free. Fix this by adding a .detach helper that calls timer_shutdown_sync() on every per-proto timer. detach_hdlc_protocol() invokes proto->detach(dev) before kfree(hdlc->state), so timer_shutdown_sync() now runs on both free paths. timer_shutdown_sync() is used instead of timer_delete_sync() because the keepalive path re-arms the timer through add_timer()/mod_timer() and shutdown blocks any re-activation during teardown. Initialize the per-protocol timers in ppp_ioctl() when the protocol is attached, and remove the now-redundant timer_setup() from ppp_start(), so that the timers are initialized exactly once at attach time and ppp_timer_release() never operates on uninitialized timer_list structures. attach_hdlc_protocol() uses kmalloc() (not kzalloc), so struct ppp's protos[i].timer is uninitialized garbage until the first timer_setup(); without this init-at-attach, attaching the PPP protocol without ever bringing the device up would leave timer_shutdown_sync() operating on uninitialized memory in .detach. Moving the init out of ppp_start() (which only runs on NETDEV_UP) into the attach path makes the initialization unconditional and avoids initializing the same timer_list twice. This bug was found by static analysis.

R4IM analyst take

Business impact & how R4IM helps

This advisory is on our active-exploitation watchlist. Attackers are using it for initial access, privilege escalation or lateral movement in real-world intrusions. R4IM's offensive security and SOC teams already have detections, exploit replicas and remediation playbooks for this issue.

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Recommended remediation

  1. Inventory all assets running the affected vendor and product, including shadow IT and third-party hosted instances.
  2. Apply the vendor patch or mitigation referenced in the advisories below. Where no patch exists, isolate the asset or restrict network exposure.
  3. Hunt for indicators of prior compromise — exploitation of this class of bug often predates public disclosure.
  4. Deploy detections for the exploit primitives (network signature, EDR rule, WAF rule) and re-test after remediation.

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Vendor & research references