Dirty Frag — Linux kernel RxRPC page-cache write primitive, LPE chain (ITW, patch pending)

Dirty Frag (CVE-2026-43284 xfrm-ESP + CVE-2026-43500 RxRPC) — Microsoft confirmed ITW, RxRPC distro patches still propagating

If you did nothing this week: any Linux host (workload, container host, on-premises server, public-cloud VM) where the kernel ships xfrm-ESP enabled (default on virtually every distribution) is exposed to a single-command unprivileged-to-root privilege escalation with public PoC; Microsoft confirmed limited in-the-wild exploitation on 2026-05-08 and tracked further activity into 2026-05-11 (Microsoft Security Blog, 2026-05-08; daily 2026-05-11 UPDATE; Wiz Research). Patch propagation is substantially complete: AlmaLinux 8/9/10, Ubuntu, Debian, Fedora, openSUSE all ship CVE-2026-43284 kernels as of 2026-05-07–10, with KernelCare live-patches generally available (AlmaLinux blog).

CVE-2026-43500 (RxRPC) patch propagation is uneven. AlmaLinux 8 is not affected (rxrpc module not built); RHEL 9 errata are rolling; Ubuntu and Debian shipped patches; the lagging configurations are systems that have the optional kernel-modules-partner package installed (typical on AFS-using estates and some research-network deployments). The interim mitigation — modprobe -r esp4 esp6 rxrpc — breaks IPsec VPNs and AFS file-system access, so production rollout requires impact testing rather than blanket application. Detection focus: Sysmon EID 1 / auditd execve events showing unusual parent-process chains from non-root users spawning root-effective shells.

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

UPDATE · originally covered CVE-2026-43284 / CVE-2026-43500 — Linux "Dirty Frag": deterministic LPE chain via page-cache write primitives in xfrm-ESP and RxRPC, active exploitation confirmed (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.

CVE-2026-43284 / CVE-2026-43500 — Linux "Dirty Frag": deterministic LPE chain via page-cache write primitives in xfrm-ESP and RxRPC, active exploitation confirmed

Researcher Hyunwoo Kim disclosed "Dirty Frag" on 2026-05-07/08 after a third party inadvertently broke embargo by reverse-engineering the upstream patch. The chain exploits two page-cache write primitives: CVE-2026-43284 (xfrm-ESP/IPsec subsystem, introduced ~2017, kernel mainline patch merged 2026-05-08) and CVE-2026-43500 (RxRPC subsystem, introduced ~2023, patch still pending at disclosure). Unlike race-condition kernel exploits, this chain is deterministic and near-100% reliable: both primitives allow userland code to write arbitrary values into read-only page-cache pages (e.g., /etc/passwd, /usr/bin/su, setuid binaries) via memory aliasing caused by DMA remapping. The combined primitive produces a stable root primitive without timing windows. Exploitation requires CAP_NET_ADMIN — available by default in Linux user namespaces on Ubuntu, Fedora, and most Arch-based distributions; restricted on RHEL 8/9 and some hardened configs. Public PoC was published alongside disclosure. Microsoft Defender telemetry confirms limited active campaigns in which threat actors escalated from SSH-compromised user accounts, modified LDAP authentication files, exfiltrated PHP session contents, and disrupted active sessions (Microsoft Security Blog, 2026-05-08 · Wiz Research, 2026-05-08 · NCSC-CH advisory 12547, 2026-05-08).

Affected distributions with confirmed exposure: Ubuntu 22.04/24.04/24.10, RHEL 8/9/10, Fedora, CentOS Stream, AlmaLinux, openSUSE Tumbleweed. Red Hat published RHSB-2026-003 (Red Hat security bulletin); Ubuntu published a fixes-available blog (Ubuntu blog). Mitigation until patches land: modprobe -r esp4 esp6 rxrpc (breaks IPsec VPNs and AFS filesystems). This is a distinct chain from CVE-2026-31431 ("Copy Fail"), also by Kim; the two vulnerabilities are not the same primitive.