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Stealth rootkit targeting F5 BIG-IP could expose enterprise identity gateways

Sophos analyzed a stealth Linux rootkit that hides a web shell in memory on compromised F5 BIG-IP APM identity gateways, evading file-based detection.

Sophos analyzed a second-stage Linux rootkit implant found in compromised F5 BIG-IP APM environments running Apache and PHP, linked to exploitation of CVE-2025-53521, an unauthenticated RCE. The implant hooks Apache's PHP-loading process and serves modified in-memory versions of three legitimate webtop PHP files (apm_css.php3, full_wt.php3, webtop_popup_css.php3), leaving on-disk files untouched so hashes and integrity checks pass. It also establishes access via an authenticated local UNIX socket that can provide an interactive /bin/bash session. Experts warn compromised APM appliances, which handle federated SSO and terminate TLS at enterprise perimeters, could enable SSO token theft and lateral movement to trusted downstream applications.

CSO Online · 5d agoMalware in the wild 4 sourcesCVE-2025-53521

Related CVEs

  • CVE-2025-535219.32%
    Stack-Based Buffer Overflow in F5 BIG-IP APM Enables Unauthenticated RCE
    CVE-2025-53521 is a stack-based buffer overflow (CWE-121) in F5 BIG-IP's Access Policy Manager (APM) that allows remote code execution. The flaw is triggered when a virtual server is configured with an APM access policy and receives specific malicious network traffic; no authentication or user interaction is required per the CVSS 4.0 vector. An attacker who exploits it gains remote code execution on the BIG-IP device, and in observed intrusions attackers have deployed a fileless Linux rootkit (dubbed 'PoisonedRefresh') that injects a PHP web shell into server memory to evade disk-based scanning. Any F5 BIG-IP deployment with an APM access policy configured on a virtual server is potentially affected; versions that have reached End of Technical Support are not evaluated by the vendor. The vulnerability is being actively exploited in the wild — it was added to CISA's Known Exploited Vulnerabilities catalog on 2026-03-27, with active exploitation and in-memory malware campaigns confirmed in press reports, though ransomware use is unknown.
    · F5 BIG-IP (BIG-IP Access Policy Manager / APM) KEVlarge

Appears with

organization BeyondTrust 1organization Sophos 1product F5 BIG-IP APM 1vendor F5 1vendor Sophos 1
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