CriticalCVE-2026-52991

    Linux Kernel: Time-of-check Time-of-use (TOCTOU) Race Condition (CVE-2026-52991)

    Published Last modified Sources: BSI, Ubuntu, Red Hat, SUSE, CVE.org, Msrc Update Guide

    What is affected, what to do

    A flaw was found in the Linux kernel's Pressure Stall Information (PSI) subsystem. A race condition exists between the file release and pressure write operations, specifically concerning the `priv` member of `struct kernfs_open_file`.

    Update Linux Kernel to version 6.1.191, Linux Kernel to version 6.6.160, Linux Kernel to version 6.12.114.

    Affected
    • Linux Kernel lt 5.2..6.1.191
    • Linux Kernel lt 6.2..6.6.160
    • Linux Kernel lt 6.7..6.12.114
    • Linux Kernel lt 6.13..6.18.33
    • Linux Kernel lt 6.19..7.0.10
    Fixed in
    • Linux Kernel 6.1.191
    • Linux Kernel 6.6.160
    • Linux Kernel 6.12.114
    • Linux Kernel 6.18.33
    • Linux Kernel 7.0.10
    • Linux Kernel 7.1

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    7.8

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: BSI, Ubuntu, Red Hat, SUSE, CVE.org, Msrc Update Guide

    0.2 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 3.9

    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
    58.43 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
    no public PoC known
    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
    9.8 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
    7.8
    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
    0.2 %
    Low — model estimates < 1% exploitation likelihood.
    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
    3.9
    Low — this CVE sits in the lower 10% of all CVEs scored today.
    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
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 16 October 2026
    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-367

    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
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    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
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    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 flaw was found in the Linux kernel's Pressure Stall Information (PSI) subsystem. A race condition exists between the file release and pressure write operations, specifically concerning the `priv` member of `struct kernfs_open_file`. This can lead to a use-after-free vulnerability or a NULL dereference, potentially causing system instability or a denial of service. The issue arises when the `priv` member is deallocated during a cgroup file release while a pressure write operation attempts to access it, or when it becomes NULL before being accessed.

    Source: BSI WID Portal, BSI CSAF, Ubuntu Security Notices, redhat_csaf, suse_csaf, cvelistv5, NVD (NIST), CVE.org (MITRE), Microsoft Security Update Guide

    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
    LinuxLinux
    • 6.7 – < 6.12.114
    • 5.2 – < 6.1.191
    • —
    • 6.2 – < 6.6.160
    • 6.13 – < 6.18.33
    • 6.19 – < 7.0.10
    • 6.12.114
    • 6.1.191
    • 7.1
    • 6.6.160
    • 6.18.33
    • 7.0.10
    IBMDataPower Gateway
    • < 11.0.0.3
    • < 10.6.0.11
    • < 10.5.0.23
    • 11.0.0.3
    • 10.6.0.11
    • 10.5.0.23
    IBMQRadar SIEM
    • < 7.6.0.3
    • <7.5.0 UP16 IF01
    • 7.6.0.3
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxlinuxlinux_kernel7.1
    Linuxlinuxlinux_kernel—

    Weakness classes (CWE)

    • CWE-367Time-of-check Time-of-use (TOCTOU) Race Condition

      Base

      The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check.

      Entry at MITRE CWE-367

    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. Record modifiedLinux

      Show 2 original changes
      Individual changes of this event as published by NVD
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      Affected versions (publisher)added
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      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/52xxx/CVE-2026-52991.json">CVE-2026-52991</a>
      Referenceadded
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      https://git.kernel.org/stable/c/58e6ca11e45c9d6e52255d1135e7dbff0a360334

      Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

    2. Record modifiedLinux

      Show 3 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
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      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/52xxx/CVE-2026-52991.json">CVE-2026-52991</a>
      Referenceadded
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      https://git.kernel.org/stable/c/af0638534bfae7980582c6482531a0872f8ee341
      Referenceadded
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      https://git.kernel.org/stable/c/d8ddbc3330fa6851c56b046e630a8fc64830a0ce

      Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

    3. Record modifiedCISA ADP

      Weakness type: CWE-367

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Weakness type (CWE)added
      —
      CWE-367

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    4. Record modifiedLinux

      Show 9 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
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      https://git.kernel.org/stable/c/03dc070fa0fc3cb4068693f468ccd5f8a7e58282
      Referenceadded
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      https://git.kernel.org/stable/c/a5b98009f16d8a5fb4a8ff9a193f5735515c38fa
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      https://git.kernel.org/stable/c/d4352c0709bfd38c752fccbde7fd72a82ac78f23
      Referenceremoved
      https://git.kernel.org/stable/c/03dc070fa0fc3cb4068693f468ccd5f8a7e58282
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      Referenceremoved
      https://git.kernel.org/stable/c/a5b98009f16d8a5fb4a8ff9a193f5735515c38fa
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      Referenceremoved
      https://git.kernel.org/stable/c/d4352c0709bfd38c752fccbde7fd72a82ac78f23
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      Reference typeremoved
      https://git.kernel.org/stable/c/03dc070fa0fc3cb4068693f468ccd5f8a7e58282 Types: Patch
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      Reference typeremoved
      https://git.kernel.org/stable/c/a5b98009f16d8a5fb4a8ff9a193f5735515c38fa Types: Patch
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      Reference typeremoved
      https://git.kernel.org/stable/c/d4352c0709bfd38c752fccbde7fd72a82ac78f23 Types: Patch
      —

      Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

    5. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: none known, automatable: no

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentadded
      —
      {"id":"CVE-2026-52991","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-10-16T15:24:15.317006Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Show 3 older changes

    1. Record modifiedRed Hat ADP

      Show 17 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
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      https://access.redhat.com/errata/RHSA-2026:57251
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      https://access.redhat.com/errata/RHSA-2026:57252
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      https://access.redhat.com/errata/RHSA-2026:57253
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      https://access.redhat.com/errata/RHSA-2026:57254
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      https://access.redhat.com/security/cve/CVE-2026-52991
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      https://bugzilla.redhat.com/show_bug.cgi?id=2492403
      Referenceadded
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      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52991.json
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:57251
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      https://access.redhat.com/errata/RHSA-2026:57252
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      https://access.redhat.com/errata/RHSA-2026:57253
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      https://access.redhat.com/errata/RHSA-2026:57254
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      https://access.redhat.com/security/cve/CVE-2026-52991
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      https://bugzilla.redhat.com/show_bug.cgi?id=2492403
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      Referenceremoved
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52991.json
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      Reference typeremoved
      https://access.redhat.com/security/cve/CVE-2026-52991 Types: Third Party Advisory
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      Reference typeremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=2492403 Types: Issue Tracking, Third Party Advisory
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      Reference typeremoved
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52991.json Types: Third Party Advisory
      —

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    2. Record modifiedRed Hat ADP

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
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      https://access.redhat.com/errata/RHSA-2026:57251
      Referenceadded
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      https://access.redhat.com/errata/RHSA-2026:57252
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      https://access.redhat.com/errata/RHSA-2026:57253
      Referenceadded
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      https://access.redhat.com/errata/RHSA-2026:57254

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    3. ReceivedLinux

      Description added or changed

      Show 5 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
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      Linux, Linux
      Descriptionadded
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      In the Linux kernel, the following vulnerability has been resolved: sched/psi: fix race between file release and pressure write A potential race condition exists between pressure write and cgroup file release regarding the priv member of struct kernfs_open_file, which triggers the uaf reported in [1]. Consider the following scenario involving execution on two separate CPUs: CPU0 CPU1 ==== ==== vfs_rmdir() kernfs_iop_rmdir() cgroup_rmdir() cgroup_kn_lock_live() cgroup_destroy_locked() cgroup_addrm_files() cgroup_rm_file() kernfs_remove_by_name() kernfs_remove_by_name_ns() vfs_write() __kernfs_remove() new_sync_write() kernfs_drain() kernfs_fop_write_iter() kernfs_drain_open_files() cgroup_file_write() kernfs_release_file() pressure_write() cgroup_file_release() ctx = of->priv; kfree(ctx); of->priv = NULL; cgroup_kn_unlock() cgroup_kn_lock_live() cgroup_get(cgrp) cgroup_kn_unlock() if (ctx->psi.trigger) // here, trigger uaf for ctx, that is of->priv The cgroup_rmdir() is protected by the cgroup_mutex, it also safeguards the memory deallocation of of->priv performed within cgroup_file_release(). However, the operations involving of->priv executed within pressure_write() are not entirely covered by the protection of cgroup_mutex. Consequently, if the code in pressure_write(), specifically the section handling the ctx variable executes after cgroup_file_release() has completed, a uaf vulnerability involving of->priv is triggered. Therefore, the issue can be resolved by extending the scope of the cgroup_mutex lock within pressure_write() to encompass all code paths involving of->priv, thereby properly synchronizing the race condition occurring between cgroup_file_release() and pressure_write(). And, if an live kn lock can be successfully acquired while executing the pressure write operation, it indicates that the cgroup deletion process has not yet reached its final stage; consequently, the priv pointer within open_file cannot be NULL. Therefore, the operation to retrieve the ctx value must be moved to a point *after* the live kn lock has been successfully acquired. In another situation, specifically after entering cgroup_kn_lock_live() but before acquiring cgroup_mutex, there exists a different class of race condition: CPU0: write memory.pressure CPU1: write cgroup.pressure=0 =========================== ============================= kernfs_fop_write_iter() kernfs_get_active_of(of) pressure_write() cgroup_kn_lock_live(memory.pressure) cgroup_tryget(cgrp) kernfs_break_active_protection(kn) ... blocks on cgroup_mutex cgroup_pressure_write() cgroup_kn_lock_live(cgroup.pressure) cgroup_file_show(memory.pressure, false) kernfs_show(false) kernfs_drain_open_files() cgroup_file_release(of) kfree(ctx) of->priv = NULL cgroup_kn_unlock() ... acquires cgroup_mutex ctx = of->priv; // may now be NULL if (ctx->psi.trigger) // NULL dereference Consequently, there is a possibility that of->priv is NULL, the pressure write needs to check for this. Now that the scope of the cgroup_mutex has been expanded, the original explicit cgroup_get/put operations are no longer necessary, this is because acquiring/releasing the live kn lock inherently executes a cgroup get/put operation. [1] BUG: KASAN: slab-use-after-free in pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011 Call Trace: pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011 cgroup_file_write+0x36f/0x790 kernel/cgroup/cgroup.c:43 ---truncated---
      Referenceadded
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      https://git.kernel.org/stable/c/03dc070fa0fc3cb4068693f468ccd5f8a7e58282
      Referenceadded
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      https://git.kernel.org/stable/c/a5b98009f16d8a5fb4a8ff9a193f5735515c38fa
      Referenceadded
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      https://git.kernel.org/stable/c/d4352c0709bfd38c752fccbde7fd72a82ac78f23

      Source identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

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