CVE-2026-98158

Medium · 5.5

CVSS

5.5

EPSS

0.1%

Published

Sep 25, 2026

CISA due date

—

Cloudanix analysis

Schedule a fix SSVC: Track

A bug in how the Linux kernel handles incoming network frames over certain serial connections can be triggered by an attacker to crash the entire machine.

Exploitation: Local Privilege
Attacker needs: Low Privilege
Complexity: Low
Impact: Denial Of Service
Component: OS kernel
Class: Memory Corruption
Remediation: Upgrade
Effort: Medium
Patch risk: Low

What it actually does

Improper skb headroom restoration in ppp_async process_input_packet() after an errored frame allows skb_push in ppp_receive_nonmp_frame() to write below skb->head, triggering skb_under_panic; the same root cause permits out-of-bounds reads in ppp_decompress_frame() when CCP is enabled.

Why it matters to the business

Remote denial-of-service that crashes the kernel on any Linux system using the ppp_async driver; 28 days after disclosure the flaw remains unpatched in released kernels, forcing organizations to accept uptime risk or disable PPP entirely until a fix ships.

How an attack unfolds

An attacker connected over a serial or pty link first sends a bad-FCS frame that triggers the error path and corrupts headroom, then immediately follows with a frame beginning ff 03; the next ppp_receive_nonmp_frame() call pushes a two-byte filter tag one byte below the buffer head, causing the kernel BUG and panic. The attack requires only that the victim has configured PPP and does not need authentication or code execution.

How to fix it

Apply the upstream patch that drops errored frames and clears ap->rpkt instead of attempting headroom restoration; monitor linux-stable and distribution kernels for the corrected version once released; until then, avoid PPP async configurations or isolate affected systems behind network controls that prevent untrusted peers from reaching the PPP endpoint.

How to confirm the fix worked

Confirm the presence of the fix by inspecting the ppp_async.c error path for skb_drop and ap->rpkt = NULL rather than skb_trim/skb_reserve; test with the supplied reproducer to verify the kernel no longer panics; scan running kernels for the original buggy skb_reserve(-skb_headroom(skb)) sequence.

Typical attack path

Establish PPP session over async link (serial/pty)
Transmit malformed frame with bad FCS to enter error path
Immediately send second frame starting with ff 03
Trigger skb_push of inbound filter tag resulting in under-panic

How to detect it

Search kernel logs and crash dumps for "skb_under_panic", "skb_panic", the RIP address of skb_push called from ppp_receive_nonmp_frame, and the call trace containing ppp_async_process and ppp_input; correlate with PPP interface activity immediately preceding the panic.

skb_under_panic: len:
skb_panic+0x
ppp_receive_nonmp_frame+0x
ppp_async_process+0x
skbuff: skb_under_panic

What your security team should do

Prioritize kernel upgrade as soon as a corrected package is published
Temporarily disable ppp_async interfaces on exposed systems
Add kernel panic monitoring for skb_panic signatures originating from ppp_* functions
Review PPP usage across the fleet and migrate to ppp_synctty or non-PPP alternatives where feasible

The record

In the Linux kernel, the following vulnerability has been resolved: ppp_async: drop the errored frame instead of resetting its headroom ppp_receive_nonmp_frame() prepends a two-byte direction tag before running the pass/active BPF filters: *(__be16 *)skb_push(skb, 2) = htons(PPP_FILTER_INBOUND_TAG); Nothing on the receive path guarantees those two bytes of headroom. The frame-error path in ppp_async's process_input_packet() resets a reused skb's headroom to zero while claiming to restore it to a freshly allocated state - but a fresh skb from dev_alloc_skb() carries NET_SKB_PAD: err: if (skb) { /* make skb appear as freshly allocated */ skb_trim(skb, 0); skb_reserve(skb, - skb_headroom(skb)); } ap->rpkt still points at that skb, so the next frame is reassembled into it with no headroom at all. A peer that sends a bad-FCS frame followed by one beginning ff 03 then leaves a single byte of headroom by the time the filter tag is pushed, which lands one byte below skb->head: skbuff: skb_under_panic: len:49 put:2 head:ffff888003c10000 data:ffff888003c0ffff tail:0x30 end:0x640 dev:<NULL> kernel BUG at net/core/skbuff.c:214! RIP: 0010:skb_panic+0x13e/0x230 Call Trace: skb_push+0xbd/0x100 ppp_receive_nonmp_frame+0x48a/0x1d10 ppp_input+0x4e9/0x2f80 ppp_async_process+0x2a/0xe0 tasklet_action_common+0x20f/0x8a0 handle_softirqs+0x18e/0x590 Kernel panic - not syncing: Fatal exception in interrupt Zeroing the headroom violates the NET_SKB_PAD guarantee that dev_alloc_skb() gives the rest of the receive path. Besides the filter panic above, when CCP compression is enabled ppp_decompress_frame() hands skb->data - 2 to ->decompress()/->incomp(), which then reads out of bounds before skb->head for the same reason. Rather than restore the headroom, drop the errored frame - as ppp_synctty already does on its error path - and clear ap->rpkt so the next frame is reassembled into a fresh skb with proper headroom. This is simpler and fixes both the filter under-panic and the CCP out-of-bounds read. The original V1 of this patch made room in ppp_receive_nonmp_frame() with skb_cow_head(); Eric pointed out that fixing the root cause in the transport is the right approach. Found by fuzzing the PPP receive path with a mutating peer on a pty; it is an interesting (remote) DoS: root configures PPP, the peer supplies two crashing frames. The reproducer (repro-ppp-skb.c, unchanged from v1) panics in about a second, and returns cleanly with this applied.

Affected packages

Ecosystem	Package	Introduced	Fixed