# CVE-2026-23397: nfnetlink_osf: validate individual option lengths in fingerprints

> A missing validation in the Linux kernel's netfilter OS fingerprinting module allows a remote attacker to cause a system crash via crafted TCP packets.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/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-80850, BELL-CVE-2026-23397, DEBIAN-CVE-2026-23397, GHSA-3rcm-5vqm-53w6, ROOT-OS-DEBIAN-11-CVE-2026-23397, ROOT-OS-DEBIAN-12-CVE-2026-23397, ROOT-OS-DEBIAN-13-CVE-2026-23397, ROOT-OS-UBUNTU-2204-CVE-2026-23397, ROOT-OS-UBUNTU-2404-CVE-2026-23397, UBUNTU-CVE-2026-23397

## What is CVE-2026-23397?

CVE-2026-23397 is a high-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:

nfnetlink_osf: validate individual option lengths in fingerprints

nfnl_osf_add_callback() validates opt_num bounds and string
NUL-termination but does not check individual option length fields.
A zero-length option causes nf_osf_match_one() to enter the option
matching loop even when foptsize sums to zero, which matches packets
with no TCP options where ctx->optp is NULL:

 Oops: general protection fault
 KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
 RIP: 0010:nf_osf_match_one (net/netfilter/nfnetlink_osf.c:98)
 Call Trace:
  nf_osf_match (net/netfilter/nfnetlink_osf.c:227)
  xt_osf_match_packet (net/netfilter/xt_osf.c:32)
  ipt_do_table (net/ipv4/netfilter/ip_tables.c:293)
  nf_hook_slow (net/netfilter/core.c:623)
  ip_local_deliver (net/ipv4/ip_input.c:262)
  ip_rcv (net/ipv4/ip_input.c:573)

Additionally, an MSS option (kind=2) with length < 4 causes
out-of-bounds reads when nf_osf_match_one() unconditionally accesses
optp[2] and optp[3] for MSS value extraction.  While RFC 9293
section 3.2 specifies that the MSS option is always exactly 4
bytes (Kind=2, Length=4), the check uses "< 4" rather than
"!= 4" because lengths greater than 4 do not cause memory
safety issues -- the buffer is guaranteed to be at least
foptsize bytes by the ctx->optsize == foptsize check.

Reject fingerprints where any option has zero length, or where an MSS
option has length less than 4, at add time rather than trusting these
values in the packet matching hot path.

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

## Which versions are affected by CVE-2026-23397?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23397?

Apply the vendor's patch.

## Is CVE-2026-23397 being exploited?

There is no known exploitation of CVE-2026-23397 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-2026-23397 and fix it if it is affected.

What it is: CVE-2026-23397 is a high-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-2026-23397 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-2026-23397/ (Markdown: https://insights.sbomify.com/CVE-2026-23397/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23397
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23397
- OSV: https://osv.dev/vulnerability/CVE-2026-23397

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23397/
Summaries are generated and may contain mistakes; the official description above is the record.