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PoisonedRefresh: A Fileless Linux Rootkit That Injects PHP Web Shells Into F5 BIG-IP APM Server Memory
Sophos details PoisonedRefresh, a fileless Linux rootkit that injects a memory-resident PHP web shell into F5 BIG-IP APM after exploitation of CVE-2025-53521.
On September 8, 2026, SophosLabs published an analysis of Linux/Agnt-IC (dubbed PoisonedRefresh by ESET) found in compromised F5 BIG-IP Access Policy Manager environments. Initial access leverages CVE-2025-53521, an unauthenticated RCE in BIG-IP APM when an access policy is configured on a virtual server; F5 confirmed exploitation and links the activity to cluster c05d5254, while Shadowserver observed 795 exposed vulnerable endpoints. A first stage hidden in a modified umount binary modifies /usr/sbin/httpd and SELinux configuration and embeds itself in BIG-IP upgrade images for persistence. The second-stage ELF intercepts __libc_start_main, hooks apr_dso_load, and injects a PHP web shell into libphp memory via mmap manipulation, leaving no disk artifacts, and exposes a /bin/bash shell via a local UNIX domain socket instead of a TCP port.
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Understanding Angler Exploit Kit
Unit 42 examines Angler EK operations, including rapid zero-day adoption, fileless Bedep infections, and ransomware payloads like TeslaCrypt and CryptXXX.
Unit 42 published the second part of its Angler EK analysis, covering the kit's history since 2013, its SaaS rental model, and its focus on Flash, Internet Explorer, and Silverlight exploits. Angler integrated the CVE-2015-5119 Flash zero-day from the Hacking Team leak within hours and later added exploits for CVE-2015-2419 and CVE-2016-0034 roughly a month after Microsoft patched them. Campaigns use Angler to deliver ransomware such as CryptoWall, TeslaCrypt, and CryptXXX, plus banking trojans and stealers via EITest. Since August 2014, Angler has used fileless, in-memory execution, most often for Bedep, which later downloads CryptXXX and click-fraud malware.
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