# CVE-2026-15893: Zephyr IPv6 Neighbor Discovery zero reachable time from crafted Router Advertisement causes assertion/DoS

> A remote attacker can cause a denial of service in Zephyr RTOS by sending a crafted Router Advertisement that triggers a kernel assertion or degrades Neighbor Discovery.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-8v32-9xf8-r765

## What is CVE-2026-15893?

CVE-2026-15893 is a medium-severity denial of service vulnerability in zephyrproject zephyr, published on 14 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

net_if_ipv6_calc_reachable_time() in subsys/net/ip/net_if.c derives a randomized ND reachable time from ipv6->base_reachable_time as min_reachable + sys_rand32_get() % (max_reachable - min_reachable), where min_reachable = base/2 and max_reachable = 3*base/2 using integer division. When base_reachable_time is 1, both min_reachable and the modulus collapse so the function returns 0, and net_if_ipv6_set_reachable_time() stores that 0 into ipv6->reachable_time.

The base_reachable_time is attacker-controlled: handle_ra_input() in subsys/net/ip/ipv6_nbr.c accepts the Reachable Time field of an incoming Router Advertisement whenever it is nonzero and <= MAX_REACHABLE_TIME, so a single unauthenticated, link-local RA carrying a Reachable Time of 1 drives the computed reachable time to 0. Router Advertisements are unauthenticated by default and require only adjacency to the target link.

When a neighbor is subsequently confirmed reachable, net_ipv6_nbr_set_reachable_timer() reads the value and executes NET_ASSERT(time, "Zero reachable timeout!"). On builds with CONFIG_ASSERT enabled this triggers a fatal kernel assertion — a remote denial of service; on builds without assertions the reachable timer is armed with K_MSEC(0) and fires immediately, forcing reachable neighbors into perpetual re-solicitation (STALE), degrading Neighbor Discovery. The impact is limited to availability; there is no memory-safety, confidentiality, or integrity consequence.

Weakness: CWE-617 (Reachable Assertion)

## Which versions are affected by CVE-2026-15893?


## How do I fix CVE-2026-15893?

No fix has been published for CVE-2026-15893 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-15893 being exploited?

There is no known exploitation of CVE-2026-15893 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-15893 and fix it if it is affected.

What it is: CVE-2026-15893 is a medium-severity denial of service vulnerability in zephyrproject zephyr, published on 14 Sep 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-2026-15893 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-2026-15893/ (Markdown: https://insights.sbomify.com/CVE-2026-15893/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-15893
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-15893
- OSV: https://osv.dev/vulnerability/CVE-2026-15893

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-15893/
Summaries are generated and may contain mistakes; the official description above is the record.