CVE-2026-98158 - Vulnerability Details

- ppp_async: drop the errored frame instead of resetting its headroom

Description

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.

Published: 2026-09-25

Score: n/a

EPSS: n/a

KEV: No

Impact:

Action:

Analysis

Impact

The flaw lies in the handling of erroneous PPP frames in the Linux kernel's asynchronous PPP driver. When a bad frame is received, the driver incorrectly resets the headroom of the socket buffer to zero, violating the NET_SKB_PAD guarantee. Subsequent frames are then reassembled into this corrupted buffer with insufficient space for the two‑byte inbound filter tag, causing a kernel panic. The panic is triggered by an out‑of‑bounds write to the buffer, resulting in a denial‑of‑service scenario that can crash the entire system.

Affected Systems

The defect exists in all versions of the Linux kernel that include the ppp_async process_input_packet routine unchanged from the reported code path. No specific version list is supplied, so any kernel prior to the patch that still uses the legacy headroom handling is affected. The issue is tied to the PPP protocol implementation in the kernel, so all distribution kernels shipping that code are considered vulnerable.

Risk and Exploitability

The CVE data does not include an EPSS score or a CVSS rating, but it does explicitly state that the vulnerability is not listed in the KEV catalog. Nevertheless, the bug can be triggered by an adversary that can inject two malformed PPP frames into a remote session. Based on the description, it is inferred that a PPP client or gateway that receives the bad frames could cause the kernel panic. The attack does not allow arbitrary code execution; instead it results in a denial‑of‑service that brings the host down.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Linux

unaffected

Version	Status	Constraints
`6722e78c90054101e6797d5944cdc81af9897a0a`	affected	< d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
`6722e78c90054101e6797d5944cdc81af9897a0a`	affected	< 0c53eb14975f029abd6b26896a460f0d2aaefe6b
`6722e78c90054101e6797d5944cdc81af9897a0a`	affected	< 717137221c7d90e7c98bda9a370c9da6cbf015e5
`6722e78c90054101e6797d5944cdc81af9897a0a`	affected	< 8dc5d98a16fa23c00999aecf10018c9f69fa5bf4

Linux

affected

Version	Status	Constraints
`2.6.15`	affected	—
`0`	unaffected	< 2.6.15
`6.12.111`	unaffected	≤ 6.12.*
`6.18.53`	unaffected	≤ 6.18.*
`7.2.7`	unaffected	≤ 7.2.*
`7.3-rc3`	unaffected	≤ *

No data.

Vendor Product Confidence Versions

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`[6722e78c90054101e6797d5944cdc81af9897a0a,d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da)`	affected	code_commit	—
`[6722e78c90054101e6797d5944cdc81af9897a0a,0c53eb14975f029abd6b26896a460f0d2aaefe6b)`	affected	code_commit	—
`[6722e78c90054101e6797d5944cdc81af9897a0a,717137221c7d90e7c98bda9a370c9da6cbf015e5)`	affected	code_commit	—
`[6722e78c90054101e6797d5944cdc81af9897a0a,8dc5d98a16fa23c00999aecf10018c9f69fa5bf4)`	affected	code_commit	—

Linux

Linux Kernel

99%

Version	Status	Scheme	Platform
`2.6.15`	affected	semver	—
`[0,2.6.15)`	unaffected	semver	—
`[6.12.111,6.13.0)`	unaffected	semver	—
`[6.18.53,6.19.0)`	unaffected	semver	—
`[7.2.7,7.3.0)`	unaffected	semver	—
`[7.3-rc3,*]`	unaffected	semver	—

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Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

Update the Linux kernel to a revision that includes the patch that replaces the headroom reset logic and drops the erroneous frame. The patch is available in the mainline kernel source; applying a backport or upgrading to the latest stable release will remediate the issue.
If an immediate kernel upgrade is not possible, restrict PPP access so that only trusted users or interfaces can establish PPP connections, reducing the surface area for the malformed‑frame attack.
Monitor system logs for kernel panic messages such as “skbuff: skb_under_panic” or “Kernel panic - not syncing” that may indicate the underlying issue still exists and that a faulty frame was received.

Generated by OpenCVE AI on September 25, 2026 at 13:46 UTC.

Tracking

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Advisories

No advisories yet.

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

References

Link	Providers
https://git.kernel.org/stable/c/0c53eb14975f029abd6b26896a460f0d2aaefe6b
https://git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5
https://git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4
https://git.kernel.org/stable/c/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da

History

Fri, 25 Sep 2026 14:15:00 +0000

Type	Values Removed	Values Added
Weaknesses		CWE-119

Fri, 25 Sep 2026 10:45:00 +0000

Type	Values Removed	Values Added
Description		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.
Title		ppp_async: drop the errored frame instead of resetting its headroom
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/0c53eb14975f029abd6b26896a460f0d2aaefe6b https://git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5 https://git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4 https://git.kernel.org/stable/c/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da

Subscriptions

Linux Linux Kernel

MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2026-09-25T10:36:27.608Z

Updated: 2026-09-25T10:36:27.608Z

Reserved: 2026-09-25T10:25:14.321Z

Link: CVE-2026-98158

Vulnrichment

No data.

NVD

Status : Received

Published: 2026-09-25T11:17:47.510

Modified: 2026-09-25T11:17:47.510

Link: CVE-2026-98158

Redhat

No data.

OpenCVE Enrichment

Updated: 2026-09-25T14:13:34Z

Weaknesses

CWE-119
Improper Restriction of Operations within the Bounds of a Memory Buffer

Impact

Affected Systems

Risk and Exploitability

Tracking

Subscriptions

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