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

Contiki-NG's MQTT client parsepublishvhdr() in os/net/app-layer/mqtt/mqtt.c sets topiclenreceived=1 before checking topiclen against the 64-byte limit, so an over-length topic returns early but leaves the flag set. On the next TCP segment, tcpinput() re-invokes the parser with topicreceived==0, and the persisted topiclenreceived==1 skips the length-reading block containing the guard, falling through directly to a memcpy() that uses the unvalidated 16-bit topiclen as the copy length. The 65-byte topic[] destination overruns into adjacent struct fields including the payloadchunk 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.