CriticalCVE-2021-22555

    Heap Out-Of-Bounds Write in Netfilter IP6T_SO_SET_REPLACE (CVE-2021-22555)

    Published Last modified In CISA KEV since Sources: OSV, pwn.watch, CVE.org, BSI

    What is affected, what to do

    A heap out-of-bounds write vulnerability in the Linux Kernel allows an attacker to escalate privileges or cause a DoS via heap memory corruption through user namespace. This affects Linux since version 2.6.19-rc1 and is actively exploited. Android devices with the affected kernel version are vulnerable. Immediate patching to the latest secure kernel version is critical to mitigate this vulnerability.

    The vulnerability is being actively exploited; treat it as a priority. Update :linux_kernel: to version :2021-10-05. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Linux Kernel eq 2.6.19-rc1..2.6.19-rc1

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.3

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    8.3

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: OSV, pwn.watch, CVE.org, BSI

    78.7 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.59

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    220.85 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.3 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.3
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    78.7 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.59
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-787

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw requires local, low-privileged access but yields full compromise of confidentiality, integrity and availability. Reliable, publicly documented exploits for root escalation and container escape have circulated since 2021, and its exploitation likelihood ranks among the very highest. The late KEV addition points to current use as a post-exploitation building block: attackers gain an initial foothold through a web app, stolen credentials or a CI job, then escalate on the host. CISA does not flag ransomware use. Treat container platforms, Kubernetes clusters, CI runners, shared hosting and long-lived appliances on old kernels as critical (patch within 72 h). Isolated single-purpose servers without third-party shell access can follow the regular patch cycle, provided user namespaces are disabled.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory kernel versions: Collect uname -r from every Linux host via Ansible or your fleet tooling. Affected kernels run from 2.6.19 up to the fixed stable releases. Container hosts, Kubernetes nodes, CI/CD runners and multi-user systems come first.
      • Deploy the fixed kernel and reboot: Target at least 4.4.267, 4.9.267, 4.14.231, 4.19.188, 5.4.113 or 5.10.31, or your distribution's backport. The underlying fix is in net/netfilter/x_tables.c (compat match/target padding). For the 5.11 line and for distribution kernels (RHEL, Ubuntu, SLES, Debian): patch available from the vendor — look up the exact version in the vendor advisory. Use livepatching (kpatch, Canonical Livepatch, kGraft) to bridge the gap until you can reboot.
      • Disable unprivileged user namespaces now wherever nothing depends on them:
        • generic: sysctl -w user.max_user_namespaces=0
        • Debian/Ubuntu: sysctl -w kernel.unprivileged_userns_clone=0
        • Persist the setting in /etc/sysctl.d/. Check rootless containers, Flatpak and Chromium sandboxing first.
      • Check Broadcom Fabric OS (SAN switches): Patch available from the vendor — look up the exact version in the vendor advisory. Until then, restrict management access to a dedicated admin network.
      • Review tenant and shell access: Identify local accounts with shell access on unpatched hosts. Lock unused accounts and rotate third-party SSH keys if you see suspicious activity.

    2. Runbook step 2

      Mitigation layers

      • Container hardening: Enforce the Kubernetes RuntimeDefault seccomp profile or Docker's default profile, which blocks unshare/clone(CLONE_NEWUSER) without CAP_SYS_ADMIN. In addition:
        • no privileged: true pods
        • allowPrivilegeEscalation: false
        • enforcement via Pod Security Admission ("restricted" level) or Kyverno/Gatekeeper
      • Restrict the 32-bit compat path: Public exploits use the compat setsockopt path (IPT_SO_SET_REPLACE from a 32-bit process). Where no 32-bit binaries are needed, block 32-bit ELF execution through seccomp or AppArmor/SELinux policy.
      • Least privilege and tenant isolation: Run untrusted workloads (build jobs, customer code) only on patched, dedicated nodes or behind VM-grade isolation (gVisor, Kata Containers, Firecracker). Until the kernel is patched, a shared kernel is a trust boundary with a known hole.
      • Module hardening: On hosts that don't need iptables/x_tables (e.g. pure nftables hosts without iptables-legacy), prevent autoloading via /etc/modprobe.d/: install ip_tables /bin/false, install x_tables /bin/false. Verify Docker, kube-proxy and firewalld dependencies first.
      • Access control: The attack vector is local, so shrink local code-execution opportunities. Use an MFA-protected SSH bastion, mount /tmp, /dev/shm and /var/tmp with noexec, and remove compilers (gcc) from production systems.

    3. Runbook step 3

      Detection rules

      • auditd – namespace creation and compat setsockopt:
        • -a always,exit -F arch=b64 -S unshare -F a0&0x10000000 -F auid>=1000 -k userns_create
        • -a always,exit -F arch=b32 -S setsockopt,socketcall -F auid>=1000 -k compat_sockopt
        • Alert when both keys fire from the same non-root process within a short window.
      • Process telemetry (Sysmon for Linux EID 1 / EDR): Watch for 32-bit ELF binaries executing from /tmp, /dev/shm or home directories on x86_64 hosts. Also flag the chain unshare -Urn → unknown binary → shell with euid=0. Sigma shape: Image|startswith: '/tmp/' AND ParentImage|endswith: '/unshare'.
      • Privilege transitions without a legitimate path: Look for UID 0 processes whose parent ran as non-root and that did not originate from sudo, su, pkexec or sshd. KQL sketch: DeviceProcessEvents | where AccountName == "root" and InitiatingProcessAccountName != "root" and InitiatingProcessFileName !in ("sudo","su","sshd","pkexec").
      • Kernel log anomalies: Ship journalctl -k to the SIEM and match on general protection fault, BUG: unable to handle, slab-out-of-bounds, kernel BUG at mm/slub.c, or oopses with xt_compat or msg_msg in the call trace. Failed exploitation attempts frequently leave panics or oopses behind.
      • Container escape (Falco/Tetragon): Enable the Falco "Container Escape" and "Change thread namespace" rules. Further indicators are container processes touching host paths (/proc/1/root, /var/run/docker.sock) and new root processes in the host PID namespace that trace back to a container.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A heap out-of-bounds write affecting Linux since v2.6.19-rc1 was discovered in net/netfilter/x_tables.c. This allows an attacker to gain privileges or cause a DoS (via heap memory corruption) through user name space

    Source: OSV (Open Source Vulnerabilities), pwn.watch, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE), BSI CSAF, BSI WID Portal

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    —Linux Kernel
    • 2.6.19-rc1
    • —
    netappcloud_backup
    • —
    • —
    netapphci_management_node
    • —
    • —
    netappsolidfire
    • —
    • —
    netappsolidfire_baseboard_management_controller
    • —
    • —
    OracleJD Edwards
    • 9.2.26.3
    • 9.2
    • —
    • —
    android:linux_kernel:
    • :0
    • Kernel
    • :2021-10-05
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxlinuxlinux_kernel—
    Othernetappaff_500f_firmware—
    Othernetappaff_a250_firmware—
    Othernetappaff_a400_firmware—
    Othernetappc250_firmware—
    Othernetappc400_firmware—
    Otherbroadcomfabric_operating_system—
    Otherbrocadefabric_operating_system—
    Othernetappfas_8300_firmware—
    Othernetappfas_8700_firmware—
    Othernetapph300s_firmware—
    Othernetapph410c_firmware—
    Othernetapph410s_firmware—
    Othernetapph500s_firmware—
    Othernetapph610c_firmware—
    Othernetapph610s_firmware—
    Othernetapph615c_firmware—
    Othernetapph700s_firmware—

    Weakness classes (CWE)

    • CWE-787Out-of-bounds Write

      Base

      The product writes data past the end, or before the beginning, of the intended buffer.

      Entry at MITRE CWE-787

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    Packet StormLinux Kernel Netfilter Heap Out Of Bounds Write (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormKernel Live Patch Security Notice LSN 0080 1 (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormKernel Live Patch Security Notice LSN 0081 1 (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormNetfilter x_tables Heap Out Of Bounds Write Privilege Escalation (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormKernel Live Patch Security Notice LSN 0083 1 (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Product identifiers remappedNVD

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Product configurationchanged
      OR *cpe:2.3:o:brocade:fabric_operating_system:-:*:*:*:*:*:*:*
      OR *cpe:2.3:o:broadcom:fabric_operating_system:-:*:*:*:*:*:*:*

      Source identifier: nvd@nist.gov

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