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HTTP/2 CONTINUATION Flood technique can be exploited in DoS attacks
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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.
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Ivanti Warns of Critical Zero-Day Flaw Being Actively Exploited in Sentry Software
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F5 BIG-IP APM Malware Injects a PHP Web Shell Into Memory, Evading Disk Scans
Sophos found F5 BIG-IP APM malware that injects a PHP web shell into Apache's memory, evading disk scans, linked to exploited CVE-2025-53521.
Sophos's September 7 analysis describes malware tied to F5's c05d5254 activity that hooks apr_dso_load, modifies the libphp PHP module in memory, and injects a PHP web shell when Apache loads apm_css.php3, full_wt.php3 or webtop_popup_css.php3, leaving on-disk files clean. A separate installer infects /usr/sbin/httpd and umount, runs before Apache starts, disables SELinux (per ESET's related PoisonedRefresh analysis), and opens a local socket at /run/bigtlog.pipe for shell access. The activity is linked to CVE-2025-53521 in BIG-IP APM, rated 9.8 CVSS 3.1, patched in October 2025, added to CISA KEV on March 27, 2026.
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Hackers deploy Linux rootkit on F5 BIG-IP APM devices, hiding web shell in memory
Sophos analyzed a Linux rootkit, tracked as PoisonedRefresh, on hacked F5 BIG-IP APM devices hiding an in-memory web shell linked to exploited CVE-2025-53521.
Sophos and ESET analyzed an implant (named PoisonedRefresh by ESET) deployed on compromised F5 BIG-IP APM appliances, which serves enterprise, financial, government, and public sector deployments. The second-stage payload intercepts __libc_start_main, hooks Apache's APR module loader (apr_dso_load), RC4-encodes key strings, and rewrites PHP file reads in memory to embed a web shell inside legitimate scripts like apm_css.php3, full_wt.php3, and webtop_popup_css.php3, leaving on-disk files unchanged. A related installer component infects /usr/sbin/httpd, persists across BIG-IP upgrade images, modifies SELinux configuration, and deploys the payload. The malware also creates a Unix domain socket at /run/bigtlog.pipe that hands authenticated local connections to /bin/bash; F5 links the activity to CVE-2025-53521, an unauthenticated RCE in BIG-IP APM already exploited in the wild.
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Top 15 Vulnerabilities Attackers Exploited Millions of Times to Hack Linux Systems
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