# CVE-2026-98158: ppp_async: drop the errored frame instead of resetting its headroom

> A remote attacker can cause a kernel panic and denial of service by sending malformed PPP frames that trigger an out-of-bounds memory access.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-104685, BELL-CVE-2026-98158, DEBIAN-CVE-2026-98158, GHSA-qfj9-3jj2-w6g8, UBUNTU-CVE-2026-98158

## What is CVE-2026-98158?

CVE-2026-98158 is a medium-severity memory corruption vulnerability in Linux kernel, published on 25 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-98158?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-98158?

Apply the vendor's patch.

## Is CVE-2026-98158 being exploited?

There is no known exploitation of CVE-2026-98158 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98158 and fix it if it is affected.

What it is: CVE-2026-98158 is a medium-severity memory corruption vulnerability in Linux kernel, published on 25 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-98158 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-98158/ (Markdown: https://insights.sbomify.com/CVE-2026-98158/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98158
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98158
- OSV: https://osv.dev/vulnerability/CVE-2026-98158

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98158/
Summaries are generated and may contain mistakes; the official description above is the record.