# CVE-2026-98713: KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

> A flaw in Linux KVM arm64 allows a guest to trigger a kernel warning or denial of service by executing specific TLBI instructions on hardware without FEAT_FGT.

- **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:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-98713, DEBIAN-CVE-2026-98713, GHSA-854r-xw42-gj8x, UBUNTU-CVE-2026-98713

## What is CVE-2026-98713?

CVE-2026-98713 is a denial of service 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:

KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

FGUs don't actually require hardware support, so don't gate fgt trap
config insertion on FGT. This is the only action required to allow FGUs
always, as kvm_calculate_traps() already computes the FGU bits with or
without FEAT_FGT.

This also fixes a spurious WARN when a TLBI OS is run on a guest in vEL1
without FEAT_TLBIOS on non FEAT_FGT hardware. In this configuration the
course-grained HCR_EL2.TTLBOS trap reaches the handler handle_tlbi_el1(),
which expects an EL1 TLBI only from vEL2. With FGUs enabled the UNDEF
will be injected without needing the specific handler.

[oliver: Take Wei-Lin's suggested changelog]

## Which versions are affected by CVE-2026-98713?


## How do I fix CVE-2026-98713?

Apply the vendor's patch.

## Is CVE-2026-98713 being exploited?

There is no known exploitation of CVE-2026-98713 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98713 and fix it if it is affected.

What it is: CVE-2026-98713 is a denial of service 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-2026-98713 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-2026-98713/ (Markdown: https://insights.sbomify.com/CVE-2026-98713/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98713
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98713
- OSV: https://osv.dev/vulnerability/CVE-2026-98713

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98713/
Summaries are generated and may contain mistakes; the official description above is the record.