# CVE-2026-97420: bpf: NUL-terminate replaced sysctl value

> A missing NUL terminator in BPF sysctl replacement allows out-of-bounds reads, potentially causing kernel crashes or information leaks for privileged local attackers.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Sep 2026
- **Updated:** 3 Oct 2026
- **Also known as:** AZL-103875, BELL-CVE-2026-97420, DEBIAN-CVE-2026-97420, GHSA-xwmh-p524-56rv, UBUNTU-CVE-2026-97420

## What is CVE-2026-97420?

CVE-2026-97420 is a memory corruption vulnerability in Linux, published on 24 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:

bpf: NUL-terminate replaced sysctl value

When writing to sysctls, proc_sys_call_handler() guarantees that the
buffer passed to proc handlers is NUL-terminated. If
bpf_sysctl_set_new_value() replaces the pending sysctl value, it can
hand a replacement buffer directly to proc handlers. However, the
helper currently copies only buf_len bytes into that buffer without
appending a NUL terminator, leaving downstream parsers vulnerable to
out-of-bounds access.

Fix this by appending a '\0' after the replaced value to restore the
expected sysctl semantics. Since the helper already rejects buf_len
greater than PAGE_SIZE - 1, there is always room for the extra byte.

Reproduced in a QEMU x86_64 guest booted with KASAN while exercising
the sysctl replacement path with a cgroup/sysctl BPF program. The
reproducer targets `/proc/sys/net/core/flow_limit_cpu_bitmap`, fills
the original user write buffer with non-zero bytes, and overrides the
sysctl value so the replacement buffer lacks a terminating NUL. Under
that setup, the pre-fix kernel reported:

  BUG: KASAN: slab-out-of-bounds in strnchrnul+0x72/0x90
  Read of size 1 at addr ffff88800de57000 by task repro_patch3/66
  CPU: 0 UID: 0 PID: 66 Comm: repro_patch3 Not tainted 7.1.0-rc3-00269-g8370ca1f87cc #6 PREEMPT(lazy)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x68/0xa0
   print_report+0xcb/0x5e0
   ? __virt_addr_valid+0x21d/0x3f0
   ? strnchrnul+0x72/0x90
   ? strnchrnul+0x72/0x90
   kasan_report+0xca/0x100
   ? strnchrnul+0x72/0x90
   strnchrnul+0x72/0x90
   bitmap_parse+0x37/0x2e0
   flow_limit_cpu_sysctl+0xc6/0x840
   ? __pfx_flow_limit_cpu_sysctl+0x10/0x10
   ? __kvmalloc_node_noprof+0x5ba/0x870
   proc_sys_call_handler+0x31d/0x480
   ? __pfx_proc_sys_call_handler+0x10/0x10
   ? selinux_file_permission+0x39f/0x500
   ? lock_is_held_type+0x9e/0x120
   vfs_write+0x98e/0x1000
   ...
   </TASK>
  The buggy address is located 0 bytes to the right of
  allocated 4096-byte region [ffff88800de56000, ffff88800de57000)
With this fix applied, rerunning the same sysctl-targeted path yields
no corresponding KASAN reports.

## Which versions are affected by CVE-2026-97420?


## How do I fix CVE-2026-97420?

Apply the vendor's patch.

## Is CVE-2026-97420 being exploited?

There is no known exploitation of CVE-2026-97420 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-97420 and fix it if it is affected.

What it is: CVE-2026-97420 is a memory corruption vulnerability in Linux, published on 24 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-2026-97420 in the wild and no public exploit code.

Affected product: Linux. 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-2026-97420/ (Markdown: https://insights.sbomify.com/CVE-2026-97420/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-97420
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-97420
- OSV: https://osv.dev/vulnerability/CVE-2026-97420

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-97420/
Summaries are generated and may contain mistakes; the official description above is the record.