# CVE-2026-7656: Broken IPv6 Neighbor Discovery input validation allows spoofed RA/NS/NA acceptance in Zephyr net stack

Status: published. Published: 2026-06-29. Updated: 2026-11-04.

The IPv6 Neighbor Discovery handlers in subsys/net/ip/ipv6_nbr.c (handle_ra_input, handle_ns_input, handle_na_input) used an incorrect boolean expression that combined the RFC 4861 validity checks with the ICMPv6 code check using the wrong operator precedence: the form was ((length/hop/source/target checks) && (icmp_hdr->code != 0)). Because every legitimate ND message carries ICMPv6 code 0, an attacker setting code == 0 (the normal value) caused the entire predicate to evaluate false, so the packet was never dropped and all of the other checks were silently skipped. The bypassed checks include the mandatory Hop Limit == 255 verification (which proves an ND packet originated on-link and was not forwarded) and, for Router Advertisements, the requirement that the source be a link-local address, as well as multicast-target sanity checks. As a result, an adjacent on-link attacker — and, because the Hop-Limit-255 guard is bypassed, potentially a remote/off-link attacker whose packets would otherwise be rejected — can have forged Router Advertisement, Neighbor Solicitation, and Neighbor Advertisement messages accepted. A forged RA lets the attacker reconfigure the victim's default router, on-link prefixes (SLAAC), MTU, reachable/retransmit timers, and (with CONFIG_NET_IPV6_RA_RDNSS) DNS servers, while forged NS/NA enable neighbor-cache poisoning, enabling man-in-the-middle, traffic redirection, and denial of service. The flaw is an input-validation/authentication weakness rather than a memory-safety issue: the underlying packet-parsing primitives (net_pkt_get_data, net_pkt_read, net_pkt_skip) are independently bounds-safe and the validated length is the true buffer length, so skipping the length check causes no out-of-bounds access. The defect has existed since the logic was introduced in 2018 and shipped in all releases through v4.4.0; it is fixed by splitting the condition so any failing check drops the packet.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: zephyr) — https://github.com/zephyrproject-rtos/zephyr/commit/095f064c94b95f9c35e05602e693dc268f0cb865
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: vulnerabilities@zephyrproject.org) — https://github.com/zephyrproject-rtos/zephyr/commit/095f064c94b95f9c35e05602e693dc268f0cb865
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: vulnerabilities@zephyrproject.org) — https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: 134c704f-9b21-4f2e-91b3-4a467353bcc0) — https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f

## Affected products and versions

- zephyrproject / zephyr (zephyr): ≥ 1.14.0; < 4.6.0 (affected, semver). Source: CNA: zephyr

## NVD configuration conditions

Affected CPE matches depend on the following AND/OR and negation conditions:

```json
[
  {
    "negate": false,
    "nodes": [
      {
        "cpeMatch": [
          {
            "criteria": "cpe:2.3:o:zephyrproject:zephyr:*:*:*:*:*:*:*:*",
            "versionEndIncluding": "4.4.1",
            "vulnerable": true
          }
        ],
        "negate": false,
        "operator": "OR"
      }
    ],
    "operator": "OR"
  }
]
```

## Severity and prioritization

- CVSS 3.1: 8.1 (high); CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H; source: CNA: zephyr

- CVSS 3.1: 6.8 (medium); CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H; source: NVD: nvd@nist.gov

- CVSS 3.1: 8.1 (high); CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H; source: NVD: vulnerabilities@zephyrproject.org

- CISA KEV: No listing in collected sources

- EPSS: 0.33% as of 2026-11-03; percentile 26.10%

- SSVC: exploitation poc; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-290, CWE-670

## Exploit evidence

- exploit: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (ADP: CISA-ADP)
- exploit: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (vulnerabilities@zephyrproject.org)
- exploit: https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (134c704f-9b21-4f2e-91b3-4a467353bcc0)

## References

- https://github.com/zephyrproject-rtos/zephyr/commit/095f064c94b95f9c35e05602e693dc268f0cb865 (CNA: zephyr; patch)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (CNA: zephyr)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (ADP: CISA-ADP; exploit)

- https://github.com/zephyrproject-rtos/zephyr/commit/095f064c94b95f9c35e05602e693dc268f0cb865 (vulnerabilities@zephyrproject.org; Patch)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (vulnerabilities@zephyrproject.org; Exploit, Patch, Vendor Advisory)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-cpjw-rvwx-ph9f (134c704f-9b21-4f2e-91b3-4a467353bcc0; Exploit, Patch, Vendor Advisory)

## Provenance

- CNA: zephyr: https://www.cve.org/CVERecord?id=CVE-2026-7656; retrieved 2026-11-04; source updated 2026-11-04

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-7656; retrieved 2026-11-04; source updated 2026-11-04

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-7656; retrieved 2026-11-04; source updated Not reported

Revision: 1b70c5a53806e1b0aa017f58b2eebd3244d3ae58616bdd804bf298fc67cd5f0b

Canonical: https://y2.dev/cve/CVE-2026-7656

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