# CVE-2026-23398: icmp: fix NULL pointer dereference in icmp_tag_validation()

> A remote attacker can cause a kernel panic and denial of service by sending crafted ICMP packets when hardened PMTU mode is enabled.

- **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:** 26 Mar 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-80847, BELL-CVE-2026-23398, DEBIAN-CVE-2026-23398, GHSA-x3j5-9ghr-8cwm, ROOT-OS-DEBIAN-11-CVE-2026-23398, ROOT-OS-DEBIAN-12-CVE-2026-23398, ROOT-OS-DEBIAN-13-CVE-2026-23398, ROOT-OS-UBUNTU-2204-CVE-2026-23398, ROOT-OS-UBUNTU-2404-CVE-2026-23398, UBUNTU-CVE-2026-23398

## What is CVE-2026-23398?

CVE-2026-23398 is a medium-severity vulnerability in Linux kernel, published on 26 Mar 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:

icmp: fix NULL pointer dereference in icmp_tag_validation()

icmp_tag_validation() unconditionally dereferences the result of
rcu_dereference(inet_protos[proto]) without checking for NULL.
The inet_protos[] array is sparse -- only about 15 of 256 protocol
numbers have registered handlers. When ip_no_pmtu_disc is set to 3
(hardened PMTU mode) and the kernel receives an ICMP Fragmentation
Needed error with a quoted inner IP header containing an unregistered
protocol number, the NULL dereference causes a kernel panic in
softirq context.

 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN NOPTI
 KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
 RIP: 0010:icmp_unreach (net/ipv4/icmp.c:1085 net/ipv4/icmp.c:1143)
 Call Trace:
  <IRQ>
  icmp_rcv (net/ipv4/icmp.c:1527)
  ip_protocol_deliver_rcu (net/ipv4/ip_input.c:207)
  ip_local_deliver_finish (net/ipv4/ip_input.c:242)
  ip_local_deliver (net/ipv4/ip_input.c:262)
  ip_rcv (net/ipv4/ip_input.c:573)
  __netif_receive_skb_one_core (net/core/dev.c:6164)
  process_backlog (net/core/dev.c:6628)
  handle_softirqs (kernel/softirq.c:561)
  </IRQ>

Add a NULL check before accessing icmp_strict_tag_validation. If the
protocol has no registered handler, return false since it cannot
perform strict tag validation.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2026-23398?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23398?

Apply the vendor's patch.

## Is CVE-2026-23398 being exploited?

There is no known exploitation of CVE-2026-23398 in the wild and no public exploit code.

Public exploit code:

- icmp_tag_validation() NULL deref (github_poc): https://github.com/JohannesLks/CVE-2026-23398
- Functional LAB with CVE-2026-23398 PoC  (github_poc): https://github.com/zpol/cve-2026-23398-poc

## 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-2026-23398 and fix it if it is affected.

What it is: CVE-2026-23398 is a medium-severity vulnerability in Linux kernel, published on 26 Mar 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-2026-23398 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-2026-23398/ (Markdown: https://insights.sbomify.com/CVE-2026-23398/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23398
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23398
- OSV: https://osv.dev/vulnerability/CVE-2026-23398

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23398/
Summaries are generated and may contain mistakes; the official description above is the record.