# CVE-2026-98299: tcp: do not let tcp_rmem be set below 4096

> A local attacker with high privileges can cause a kernel crash by setting the TCP receive buffer size to an extremely low value, triggering a division by zero error.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-106307, BELL-CVE-2026-98299, DEBIAN-CVE-2026-98299, GHSA-3ffw-pmr7-c5jp, UBUNTU-CVE-2026-98299

## What is CVE-2026-98299?

CVE-2026-98299 is a denial of service vulnerability in Linux, published on 6 Oct 2026. An attacker needs administrator access to exploit it, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

tcp: do not let tcp_rmem be set below 4096

We can hit a division by zero crash in tcp_rcvbuf_grow()
and tcp_rcv_space_adjust():

divide error: 0000 [#1] PREEMPT SMP
RIP: 0010:tcp_rcvbuf_grow+0x187/0x450 net/ipv4/tcp_input.c:939
...
grow = div_u64(((u64)rcvwin << 1) * (newval - oldval), oldval);

The division uses oldval = tp->rcvq_space.space as divisor.
When tp->rcvq_space.space is zero, this leads to a divide-by-zero
exception.

tp->rcvq_space.space is initialized in tcp_init_buffer_space():
    tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,
                                (u32)TCP_INIT_CWND * tp->advmss);

If tcp_rmem[1] is configured to very small values (such as 1),
sk->sk_rcvbuf is initialized to 1. Then tcp_full_space(sk), which
computes (sk->sk_rcvbuf * scaling_ratio) >> 8, truncates to 0.
This sets tp->window_clamp = 0, tp->rcv_ssthresh = 0, and
tp->rcvq_space.space = 0. Later, when data arrives and DRS is invoked,
tcp_rcvbuf_grow() divides by oldval == 0.

Back in 2015, commit b1cb59cf2efe ("net: sysctl_net_core: check SNDBUF
and RCVBUF for min length") ensured that net.core.rmem_default and
net.core.rmem_max cannot be set below SOCK_MIN_RCVBUF. Similarly,
SO_RCVBUF setsockopt enforces max_t(int, val * 2, SOCK_MIN_RCVBUF).

However, net.ipv4.tcp_rmem still had .extra1 = SYSCTL_ONE, allowing
arbitrarily small values.

Because SOCK_MIN_RCVBUF depends on sizeof(struct sk_buff) and cacheline
alignment, its value varies across architectures and configuration options.
Using a fixed constant of 4096 ensures a predictable, architecture-
independent lower bound that is safely above SOCK_MIN_RCVBUF everywhere
and matches the documented 4K default.

Fix this by setting tcp_rmem.extra1 to 4096 and updating the documentation.

## Which versions are affected by CVE-2026-98299?


## How do I fix CVE-2026-98299?

Apply the vendor's patch.

## Is CVE-2026-98299 being exploited?

There is no known exploitation of CVE-2026-98299 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98299 and fix it if it is affected.

What it is: CVE-2026-98299 is a denial of service vulnerability in Linux, published on 6 Oct 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-98299 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-2026-98299/ (Markdown: https://insights.sbomify.com/CVE-2026-98299/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98299
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98299
- OSV: https://osv.dev/vulnerability/CVE-2026-98299

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98299/
Summaries are generated and may contain mistakes; the official description above is the record.