# CVE-2026-98551: perf/core: Fill branch entries with a single assignment

> Local attackers can read uninitialized kernel memory via perf branch entries on arm64 systems due to missing field initialization.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **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:** AZL-109389, BELL-CVE-2026-98551, DEBIAN-CVE-2026-98551, GHSA-r5w5-qjwh-6pq3, UBUNTU-CVE-2026-98551

## What is CVE-2026-98551?

CVE-2026-98551 is a 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:

perf/core: Fill branch entries with a single assignment

perf_clear_branch_entry_bitfields() clears the bitfields of struct
perf_branch_entry one by one and leaves from/to alone, since callers
overwrite those straight away. The list has to be kept in sync with the
struct by hand and has already fallen behind: new_type and priv were
added to perf_branch_entry and never added here.

Only BRBE writes those two, and neither for every record.
brbe_set_perf_entry_type() leaves new_type alone for a branch type it
does not recognise, and priv is not set for source-only records.
arm_pmuv3.c allocates the per-CPU branch stack with kmalloc(), so such a
record reaches userspace with whatever the slot held: uninitialised
kmalloc() data on the first pass over the buffer, the previous record's
values after that. Nothing under arch/x86/events/ writes either field,
so only arm64 is affected.

Assign the whole entry at each site instead. Everything not named is
then zero, and there is no list to keep in sync. The bitfields add up to
exactly 64 bits, so the struct has no padding to leave undefined.

perf_clear_branch_entry_bitfields() has no callers left, so remove it.
perf_entry_from_brbe_regset() assigns an empty literal instead, since it
fills from/to conditionally. PERF_BR_SPEC_NA is 0, so dropping the
explicit spec assignment changes nothing.

## Which versions are affected by CVE-2026-98551?


## How do I fix CVE-2026-98551?

Apply the vendor's patch.

## Is CVE-2026-98551 being exploited?

There is no known exploitation of CVE-2026-98551 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98551 and fix it if it is affected.

What it is: CVE-2026-98551 is a 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-2026-98551 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-2026-98551/ (Markdown: https://insights.sbomify.com/CVE-2026-98551/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98551
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98551
- OSV: https://osv.dev/vulnerability/CVE-2026-98551

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98551/
Summaries are generated and may contain mistakes; the official description above is the record.