FortiGuard documents C0XMO, a cross-platform Gafgyt variant propagating through a five-year-old DD-WRT UPnP flaw

FortiGuard Labs analysed C0XMO, a new Gafgyt-derived DDoS botnet that propagates by exploiting an old stack buffer overflow in the UPnP/SSDP parser of DD-WRT router firmware — sending an oversized ST value in a crafted M-SEARCH packet to UDP 1900 to drop its payload (FortiGuard Labs, 2026-06-03; BleepingComputer, 2026-06-07). FortiGuard attributes the DD-WRT flaw to CVE-2021-27137, an identifier that does not currently resolve on NVD or MITRE (flagged in § 7). The operationally interesting part is the engineering: C0XMO ships builds for seven architectures (ARM, MIPS, m68k, PowerPC, SuperH, x86, AMD64), splits its scanning/exploitation logic into a standalone Python propagator so it can be updated independently of the core bot, terminates rival malware on the host, and supports 19 DDoS methods including Cloudflare-bypass HTTP floods and game-server-specific floods. Persistence is via cron (15-minute interval) and shell-profile modification; payloads stage to hidden .sys files under /tmp, /var/tmp and /dev/shm.

Why it matters to us: the direct exposure is low for hardened public-sector cores, but self-managed SOHO/branch gateways and any DD-WRT devices below changeset 45723 are recruitable — and a compromised edge device becomes both a DDoS source and a foothold. Defender concepts: block or restrict outbound UDP 1900 / inbound SSDP at the perimeter and disable UPnP where it is not required; on Linux gateways, hunt for cron entries spawning processes from hidden dot-directories and for shell-profile modifications (Sysmon-for-Linux / auditd execve on /tmp/.sys-class paths). No IOCs are reproduced here.