# CVE-2026-5857: Contiki-NG MQTT Client Out-of-Bounds Write in PUBLISH Topic Parser via Persistent State Between TCP Segments

> An attacker-controlled MQTT broker can trigger an out-of-bounds write in Contiki-NG, leading to remote code execution or denial of service on embedded devices.

- **Severity:** Critical 9.2 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/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-x5g5-4jq8-vj28

## What is CVE-2026-5857?

CVE-2026-5857 is a critical memory corruption 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 MQTT client parse_publish_vhdr() in os/net/app-layer/mqtt/mqtt.c sets topic_len_received=1 before checking topic_len against the 64-byte limit, so an over-length topic returns early but leaves the flag set. On the next TCP segment, tcp_input() re-invokes the parser with topic_received==0, and the persisted topic_len_received==1 skips the length-reading block containing the guard, falling through directly to a memcpy() that uses the unvalidated 16-bit topic_len as the copy length. The 65-byte topic[] destination overruns into adjacent struct fields including the payload_chunk pointer, which subsequent MQTT code dereferences, giving a compromised or attacker-controlled broker an arbitrary-pointer-write primitive. Contiki-NG's MQTT implementation has no TLS support so the connection is plaintext. Impact ranges from information disclosure and denial of service to remote code execution on embedded targets without memory protection.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-5857?


## How do I fix CVE-2026-5857?

No fix has been published for CVE-2026-5857 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-5857 being exploited?

There is no known exploitation of CVE-2026-5857 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-5857 and fix it if it is affected.

What it is: CVE-2026-5857 is a critical memory corruption 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-2026-5857 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-2026-5857/ (Markdown: https://insights.sbomify.com/CVE-2026-5857/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5857
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5857
- OSV: https://osv.dev/vulnerability/CVE-2026-5857

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-5857/
Summaries are generated and may contain mistakes; the official description above is the record.