---
cve: "CVE-2026-5857"
severity: "CRITICAL"
cvss: 9.2
epss: "0.5%"
vendor: "contiki-ng"
kev: false
exploited: false
published: "2026-08-06 18:52:40"
tags: [cve, security, critical]
source: tsecurity.de CVE-Dossier
exported: "2026-09-20T23:11:27+02:00"
---

# CVE-2026-5857

> 9.2 CRITICAL · 🧪 PoC

## Beschreibung

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.

## CVSS-Vektor

```
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
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Hoch | good |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |

## Patch verfügbar (OSV)

- a34a2dbdc8bea784bd2ae5079aa4be520cd74f2d (Commit)

## Referenzen

- <https://github.com/contiki-ng/contiki-ng>
- <https://github.com/contiki-ng/contiki-ng/pull/3163>
- <https://github.com/contiki-ng/contiki-ng/commit/a34a2dbdc8bea784bd2ae5079aa4be520cd74f2d>
- <https://y637f9qq2x.com/posts/cve-2026-5857/>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-5857) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_