Hackers breach F5 BIG-IP APM devices to deploy Linux rootkit

A Linux rootkit targeting devices in F5 BIG-IP APM environments can intercept PHP file loading and inject a fileless web shell directly into memory, avoiding the need to write malicious code to disk.

The malware shows signs of being a second-stage payload that was likely deployed after exploiting CVE-2025-53521, a critical remote code execution (RCE) flaw that F5 Networks reclassified from a DoS problem in March.

Sophos researchers analyzed a sample and noted that, while it enables “on-demand server-side code execution” typically associated with webshells, it achieves this through “deeper Linux- and Apache-specific tradecraft.”

During the research, Sophos learned that the same malware was analyzed by ESET, who identifies it as ‘PoisonedRefresh.’

In technical analysis published this week, Sophos says that the payload was deployed by a distinct installer or propagation component that had infected the Apache /usr/sbin/httpd executable used on BIG-IP APM systems.