# CVE-2026-5855: Contiki-NG LwM2M TLV Parser Out-of-Bounds Read via Unchecked Buffer Length in lwm2m_tlv_read

> Attackers can read adjacent heap memory, including keys, via crafted CoAP packets in Contiki-NG LwM2M TLV parser.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Aug 2026
- **Updated:** 14 Aug 2026
- **Also known as:** GHSA-ch5q-j8ff-mq34

## What is CVE-2026-5855?

CVE-2026-5855 is a high-severity vulnerability in Contiki-NG, published on 6 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Contiki-NG's LwM2M TLV parser lwm2m_tlv_read() in os/services/lwm2m/lwm2m-tlv.c ignores its caller-supplied buffer length argument and reads up to six bytes from the input buffer with no bounds check. The caller in lwm2m-engine.c iterates while there is at least one byte remaining, so a crafted CoAP WRITE to any LwM2M endpoint whose final TLV supplies exactly one byte triggers up to five out-of-bounds reads of heap memory adjacent to the CoAP input buffer, disclosing memory contents (including key material and peer addresses) through the parsed tlv->id, tlv->length, and tlv->value fields. Corrupted tlv_len derived from the out-of-bounds memory further corrupts the caller's parse offset. In LwM2M NoSec mode, the default for constrained devices, no authentication is required.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-5855?


## How do I fix CVE-2026-5855?

No fix has been published for CVE-2026-5855 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-5855 being exploited?

There is no known exploitation of CVE-2026-5855 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-5855 and fix it if it is affected.

What it is: CVE-2026-5855 is a high-severity vulnerability in Contiki-NG, published on 6 Aug 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-2026-5855 in the wild and no public exploit code.

Affected product: Contiki-NG. 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-2026-5855/ (Markdown: https://insights.sbomify.com/CVE-2026-5855/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5855
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5855
- OSV: https://osv.dev/vulnerability/CVE-2026-5855

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-5855/
Summaries are generated and may contain mistakes; the official description above is the record.