# CVE-2026-98840: vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF

> A use-after-free in the Linux kernel's vcs_write function allows local attackers to execute arbitrary code or escalate privileges via a race condition.

- **Severity:** Not scored
- **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:** 18 Oct 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-109056, BELL-CVE-2026-98840, DEBIAN-CVE-2026-98840, GHSA-vpw3-5qwq-wxvw, UBUNTU-CVE-2026-98840

## What is CVE-2026-98840?

CVE-2026-98840 is an use after free vulnerability in Linux, published on 18 Oct 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:

vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF

The reload of 'vc' added by commit 8fb9ea65c9d1 ("vc_screen: reload load
of struct vc_data pointer in vcs_write() to avoid UAF") sits after the
'if (ret)' block, so the copy-failure break path exits the loop without
reloading vc. If the vc was kfree()'d via vc_port_destruct during the
unlocked copy_from_user() window, the post-loop
'if (written && vc) vcs_scr_updated(vc)' is reached with a stale
non-NULL vc. The '&& vc' guard from commit a287620312dc ("vc_screen:
fix null-ptr-deref in vcs_notifier() during concurrent vcs_write") only
handles the NULL case, not this stale-non-NULL case; vcs_notifier() then
reads param->vc->vc_num from freed memory:

  BUG: KASAN: slab-use-after-free in vcs_notifier+0x7c/0xd0
  Read of size 2 at addr ffff888007149190
  Call Trace:
   vcs_notifier+0x7c/0xd0
   atomic_notifier_call_chain+0x70/0xa0
   vcs_scr_updated+0x77/0xa0
   vcs_write+0x71b/0x7e0
  Allocated by task: vc_allocate -> con_install -> tty_open
  Freed by task: kfree <- vt_ioctl (VT_DISALLOCATE -> vc_port_destruct)

Move the reload to immediately after console_lock(), before 'if (ret)',
so every break path below passes a fresh vc to the post-loop
vcs_scr_updated().

## Which versions are affected by CVE-2026-98840?


## How do I fix CVE-2026-98840?

Apply the vendor's patch.

## Is CVE-2026-98840 being exploited?

There is no known exploitation of CVE-2026-98840 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98840 and fix it if it is affected.

What it is: CVE-2026-98840 is an use after free vulnerability in Linux, published on 18 Oct 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-2026-98840 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-2026-98840/ (Markdown: https://insights.sbomify.com/CVE-2026-98840/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98840
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98840
- OSV: https://osv.dev/vulnerability/CVE-2026-98840

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98840/
Summaries are generated and may contain mistakes; the official description above is the record.