UPDATE: Dirty Frag — Microsoft confirms limited in-the-wild exploitation; Red Hat, NCSC.ch, CCB Belgium publish coordinated advisories

UPDATE (originally covered 2026-05-09): Microsoft Threat Intelligence published Active attack: Dirty Frag Linux vulnerability expands post-compromise risk on 2026-05-08 reporting "limited in-the-wild activity where privilege escalation involving su is observed." The attack chain observed: SSH initial access → shell spawn → execution of an ELF binary that triggers the LPE primitive in either CVE-2026-43284 (xfrm-ESP page-cache write) or CVE-2026-43500 (RxRPC page-cache write). This is the first formal "exploited in the wild" attribution since the V4bel write-up published on 2026-05-07.

Red Hat published RHSB-2026-003 covering both CVEs on 2026-05-07 and updated it on 2026-05-09, with backported errata rolling out to RHEL 8/9/10 and OpenShift 4 (Red Hat RHSB-2026-003). NCSC.ch issued Security Hub post 12547 on 2026-05-08 noting "Proof of Concept Available" and advising temporary blacklisting of the esp4, esp6 and rxrpc kernel modules pending distribution backports. Belgium's CCB issued a parallel advisory (CCB Belgium, 2026-05-08).

The upstream xfrm-ESP fix merged on 2026-05-07 (kernel commit referenced by V4bel and corroborated by Red Hat); the RxRPC fix was still pending in the netdev tree at time of writing. AlmaLinux backported kernels on 2026-05-08; Ubuntu noted fixes will arrive via the kernel image package. Defender hunt focus: outbound SSH-to-unprivileged-shell-to-ELF-execution chains immediately followed by setuid(0) or su invocations, plus suspicious setsockopt(AF_ALG) patterns on the esp4/esp6/rxrpc modules followed by splice() syscalls into the page cache of read-only files. The Microsoft post emphasises that the page-cache write primitive bypasses on-disk file integrity monitoring (AIDE / IMA-EVM / auditd watch rules) — post-incident forensics must compare in-memory page contents against on-disk checksums, not just md5sum of the file.

Mitigation note (carried from 2026-05-09): on Ubuntu where unprivileged user namespaces are blocked by default, the esp4/esp6 path is harder to reach because CAP_NET_ADMIN is required — but the RxRPC path remains exploitable without user-namespaces; the two CVEs are designed to complement each other. Where IPsec is in use, Red Hat suggests kernel.unprivileged_userns_clone=0 (sysctl) as a less disruptive mitigation than full esp4/esp6 module blacklisting. AFS users cannot blacklist rxrpc without losing AFS — wait for the distribution backport.