SMTP is the key: BPFDoor and AVERAT hitting the network edge

SMTP is the key: BPFDoor and AVERAT hitting the network edge

SMTP is the key: BPFDoor and AVERAT hitting the network edge

Rapid7 tracked a set of Linux samples that blend into the software and device conventions of the telecom environments they target. The set spans a newly observed BPFDoor variant, a BPF Rekoobe build seen against South Korean targets, a dropper, and six builds of a Linux implant we track as AVERAT, deployed against Taiwanese appliances. Additionally, we provide source code details of the Rapid7 BPFDoor controller introduced in our April 2026 blog, Stealthy BPFDoor Variants are a Needle That Looks Like Hay.

The chain uses two binaries. A dropper writes a shell script to the appliance's storage mount and executes it. The script stages both payloads into /sbin under the names ntpdate and udevds, launches them, and deletes each file ten seconds later while the processes continue running. One of those payloads is the dropper itself, re-executing as a resident watchdog, leaving both processes running without an on-disk image.

The dropper derives its encryption key from the string ShareTech and lives in the appliance's own add-on package directory. The BPFDoor variants seen against South Korean systems impersonate the PID file of SpamSniper, a Korean anti-spam product, and rotate through ten Linux daemon names. Across the samples, each component adopts names and conventions designed to look unremarkable in the environment it targets.