A stealthy Linux rootkit has been observed compromising F5 BIG-IP Access Policy Manager (APM) systems and maintaining access through fileless PHP web shells. Instead of creating a persistent malicious PHP file on disk, the implant modifies the PHP component loaded by the Apache process and injects web-shell functionality directly into memory. The activity affects BIG-IP APM webtop environments that use Apache and PHP. F5 has associated related exploitation activity with CVE-2025-53521, an unauthenticated remote-code-execution vulnerability that has been exploited against exposed BIG-IP APM systems. Sophos researchers identified the rootkit during forensic investigations and assessed that its architecture is consistent with a targeted post-exploitation payload. No sufficient evidence was identified to attribute the activity to a specific threat actor. The implant gains control early during application startup by manually loading the legitimate server executable and intercepting a Linux initialization function. It subsequently monitors Apache's module-loading activity and activates when the PHP module, libphp, is loaded. The rootkit hooks selected file and memory operations within PHP and targets three APM webtop scripts. When these scripts are loaded, it generates modified in-memory copies containing the legitimate content alongside the web shell, while leaving the corresponding files on disk unchanged. The shell processes specially crafted requests, decrypts their contents and executes commands on the compromised appliance. Responses use HTTP status 201 while presenting CSS-like content, potentially allowing the malicious traffic to blend with legitimate web activity. The implant also creates a Unix-domain socket capable of providing an authenticated Bash shell without exposing a conventional TCP listening port. Additional persistence mechanisms may include modification of the HTTP server binary, changes to SELinux configuration and tampering with BIG-IP upgrade images, meaning a routine service restart may not fully remove the compromise. Organizations investigating potential exploitation of CVE-2025-53521 should preserve volatile evidence before remediation and examine process memory, Apache/libphp activity, unexpected Unix sockets, Bash child processes and anomalous PHP responses. Teams should also review upgrade images and SELinux configurations and follow F5's latest remediation and compromise-assessment guidance for affected BIG-IP APM versions.
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