# CVE-2026-19735: Predictable TCP initial sequence numbers when the RFC 6528 secret key generation fails silently in the Zephyr TCP stack

> Attackers can predict TCP sequence numbers if entropy generation fails, enabling blind connection spoofing and data injection in Zephyr RTOS.

- **Severity:** Medium 4.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 11 Oct 2026
- **Updated:** 16 Oct 2026
- **Also known as:** GHSA-37mc-2q9v-fp64

## What is CVE-2026-19735?

CVE-2026-19735 is a medium-severity vulnerability in zephyrproject zephyr, published on 11 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The RFC 6528 initial-sequence-number implementation in subsys/net/ip/tcp.c derived every TCP ISN from SHA-256(unique_key || four-tuple) plus a uptime-derived offset, where unique_key is a 128-bit secret filled once by sys_csrand_get(). The return value of that call was discarded and the once guard was latched to true even when the call failed. Because sys_csrand_get() leaves the destination buffer untouched on failure (it deliberately propagates the entropy-driver error rather than filling the buffer), a single failed call left unique_key as its all-zero BSS content for the remainder of the boot, with no retry, no log message and no fallback.

A failure of the cryptographic random source is required to reach the weak state — for example -ENODEV/-EIO from the entropy driver in subsys/random/random_entropy_device.c (the in-tree comment notes that the hardware RNG "might still be gathering entropy during early boot situations"), or psa_generate_random() failing in subsys/random/random_psa.c when PSA crypto is not initialised or is backed by a Bluetooth-HCI entropy device that is not yet up. The first TCP connection is what triggers key generation, so a remote peer that reaches a listening port immediately after boot, or that can provoke a reboot, has indirect influence over whether generation coincides with that window. tcp_init_isn() is called for every passive open in tcp_conn_new() and every active open in net_tcp_connect(), so the poisoned key governs all TCP connections for that boot.

With an all-zero key the ISN becomes a public function of the connection four-tuple plus a device-wide time offset. An off-path attacker can compute the hash term offline for any four-tuple and recover the shared time offset from a single observed ISN, after which the ISN the device will pick for other four-tuples is predictable. That defeats exactly the protection RFC 6528 provides: blind TCP connection spoofing against peers that trust the source address, and blind data injection into or reset of connections whose four-tuple can be guessed. Devices whose entropy source never errors were never in the weak state.

The fix moves key generation into a single guarded helper that latches only on success, logs the error otherwise, and makes tcp_init_isn() fall back to sys_rand32_get() — the behaviour already used when CONFIG_NET_TCP_ISN_RFC6528 is disabled — instead of hashing with a known-constant key.

Weakness: CWE-330 (Use of Insufficiently Random Values)

## Which versions are affected by CVE-2026-19735?


## How do I fix CVE-2026-19735?

No fix has been published for CVE-2026-19735 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-19735 being exploited?

There is no known exploitation of CVE-2026-19735 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-19735 and fix it if it is affected.

What it is: CVE-2026-19735 is a medium-severity vulnerability in zephyrproject zephyr, published on 11 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-19735 in the wild and no public exploit code.

Affected product: zephyrproject zephyr. 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-2026-19735/ (Markdown: https://insights.sbomify.com/CVE-2026-19735/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-19735
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-19735
- OSV: https://osv.dev/vulnerability/CVE-2026-19735

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-19735/
Summaries are generated and may contain mistakes; the official description above is the record.