CriticalCVE-2025-21796

    Linux Kernel: Use-After-Free (CVE-2025-21796)

    Published Last modified Sources: Siemens ProductCERT, BSI, SUSE, Debian LTS, CVE.org, EUVD, Red Hat, Msrc Update Guide

    What is affected, what to do

    In the Linux kernel, the following vulnerability has been resolved: nfsd: clear acl_access/acl_default after releasing them If getting acl_default fails, acl_access and acl_default will be released simultaneously. However, acl_access will still retain a pointer pointing to the released posix_acl, which will trigger a WARNING in nfs3svc_release_getacl like this: ------------[ cut here ]------------ refcount_t: underflow; use-after-free.

    Update Linux Kernel to version 5.10.235, Linux Kernel to version 5.15.179, Linux Kernel to version 6.1.129.

    Affected
    • Linux Kernel lt 2.6.13..5.10.235
    • Linux Kernel lt 5.11..5.15.179
    • Linux Kernel lt 5.16..6.1.129
    • Linux Kernel lt 6.2..6.6.79
    • Linux Kernel lt 6.7..6.12.16
    • Linux Kernel lt 6.13..6.13.4
    Fixed in
    • Linux Kernel 5.10.235
    • Linux Kernel 5.15.179
    • Linux Kernel 6.1.129
    • Linux Kernel 6.6.79
    • Linux Kernel 6.12.16
    • Linux Kernel 6.13.4
    • Linux Kernel 6.14

    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

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: Siemens ProductCERT, BSI, SUSE, Debian LTS, CVE.org, EUVD, Red Hat, Msrc Update Guide

    0.7 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 52.1

    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
    78.70 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
    9.8
    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
    0.7 %
    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
    52.1
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    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 27 February 2025
    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-416

    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

    In the Linux kernel, the following vulnerability has been resolved: nfsd: clear acl_access/acl_default after releasing them If getting acl_default fails, acl_access and acl_default will be released simultaneously. However, acl_access will still retain a pointer pointing to the released posix_acl, which will trigger a WARNING in nfs3svc_release_getacl like this: ------------[ cut here ]------------ refcount_t: underflow; use-after-free. WARNING: CPU: 26 PID: 3199 at lib/refcount.c:28 refcount_warn_saturate+0xb5/0x170 Modules linked in: CPU: 26 UID: 0 PID: 3199 Comm: nfsd Not tainted 6.12.0-rc6-00079-g04ae226af01f-dirty #8 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 RIP: 0010:refcount_warn_saturate+0xb5/0x170 Code: cc cc 0f b6 1d b3 20 a5 03 80 fb 01 0f 87 65 48 d8 00 83 e3 01 75 e4 48 c7 c7 c0 3b 9b 85 c6 05 97 20 a5 03 01 e8 fb 3e 30 ff <0f> 0b eb cd 0f b6 1d 8a3 RSP: 0018:ffffc90008637cd8 EFLAGS: 00010282 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff83904fde RDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88871ed36380 RBP: ffff888158beeb40 R08: 0000000000000001 R09: fffff520010c6f56 R10: ffffc90008637ab7 R11: 0000000000000001 R12: 0000000000000001 R13: ffff888140e77400 R14: ffff888140e77408 R15: ffffffff858b42c0 FS: 0000000000000000(0000) GS:ffff88871ed00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000562384d32158 CR3: 000000055cc6a000 CR4: 00000000000006f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> ? refcount_warn_saturate+0xb5/0x170 ? __warn+0xa5/0x140 ? refcount_warn_saturate+0xb5/0x170 ? report_bug+0x1b1/0x1e0 ? handle_bug+0x53/0xa0 ? exc_invalid_op+0x17/0x40 ? asm_exc_invalid_op+0x1a/0x20 ? tick_nohz_tick_stopped+0x1e/0x40 ? refcount_warn_saturate+0xb5/0x170 ? refcount_warn_saturate+0xb5/0x170 nfs3svc_release_getacl+0xc9/0xe0 svc_process_common+0x5db/0xb60 ? __pfx_svc_process_common+0x10/0x10 ? __rcu_read_unlock+0x69/0xa0 ? __pfx_nfsd_dispatch+0x10/0x10 ? svc_xprt_received+0xa1/0x120 ? xdr_init_decode+0x11d/0x190 svc_process+0x2a7/0x330 svc_handle_xprt+0x69d/0x940 svc_recv+0x180/0x2d0 nfsd+0x168/0x200 ? __pfx_nfsd+0x10/0x10 kthread+0x1a2/0x1e0 ? kthread+0xf4/0x1e0 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x34/0x60 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Kernel panic - not syncing: kernel: panic_on_warn set ... Clear acl_access/acl_default after posix_acl_release is called to prevent UAF from being triggered.

    Source: siemens_advisory, BSI WID Portal, BSI CSAF, suse_csaf, Debian LTS Advisories, NVD (NIST), CVE.org (MITRE), ENISA EUVD, redhat_csaf, cvelistv5, 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
    • —
    • 5.16 – < 6.1.129
    • 2.6.13 – < 5.10.235
    • 5.11 – < 5.15.179
    • 6.2 – < 6.6.79
    • 6.13 – < 6.13.4
    • 6.7 – < 6.12.16
    • 6.14
    • 6.1.129
    • 5.10.235
    • 5.15.179
    • 6.6.79
    • 6.13.4
    • 6.12.16
    DellSecure Connect Gateway
    • < 5.30.0.14
    • 5.30.0.14
    IBMSecurity Verify Access
    • < 10.0.9.2
    • 10.0.9.2
    SiemensSIMATIC S7
    • 1500 TM MFP V1.1.1 GNU/Linux subsystem
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianlinuxbullseye
    Linuxdebianlinux-6.1bullseye
    Linuxlinuxlinux_kernel6.14
    Linuxlinuxlinux_kernel—

    Weakness classes (CWE)

    • CWE-416Use After Free

      Variant

      The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

      Entry at MITRE CWE-416

    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 21 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://git.kernel.org/stable/c/1fd94884174bd20beb1773990fd3b1aa877688d9
      Referenceadded
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      https://git.kernel.org/stable/c/2e59b2b68782519560b3d6a41dd66a3d01a01cd3
      Referenceadded
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      https://git.kernel.org/stable/c/55d947315fb5f67a35e4e1d3e01bb886b9c6decf
      Referenceadded
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      https://git.kernel.org/stable/c/6f7cfee1a316891890c505563aa54f3476db52fd
      Referenceadded
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      https://git.kernel.org/stable/c/7faf14a7b0366f153284db0ad3347c457ea70136
      Referenceadded
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      https://git.kernel.org/stable/c/8a1737ae42c928384ab6447f6ee1a882510e85fa
      Referenceadded
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      https://git.kernel.org/stable/c/f8d871523142f7895f250a856f8c4a4181614510
      Referenceremoved
      https://git.kernel.org/stable/c/1fd94884174bd20beb1773990fd3b1aa877688d9
      —
      Referenceremoved
      https://git.kernel.org/stable/c/2e59b2b68782519560b3d6a41dd66a3d01a01cd3
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      Referenceremoved
      https://git.kernel.org/stable/c/55d947315fb5f67a35e4e1d3e01bb886b9c6decf
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      Referenceremoved
      https://git.kernel.org/stable/c/6f7cfee1a316891890c505563aa54f3476db52fd
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      Referenceremoved
      https://git.kernel.org/stable/c/7faf14a7b0366f153284db0ad3347c457ea70136
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      Referenceremoved
      https://git.kernel.org/stable/c/8a1737ae42c928384ab6447f6ee1a882510e85fa
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      Referenceremoved
      https://git.kernel.org/stable/c/f8d871523142f7895f250a856f8c4a4181614510
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/1fd94884174bd20beb1773990fd3b1aa877688d9 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/2e59b2b68782519560b3d6a41dd66a3d01a01cd3 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/55d947315fb5f67a35e4e1d3e01bb886b9c6decf Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/6f7cfee1a316891890c505563aa54f3476db52fd Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/7faf14a7b0366f153284db0ad3347c457ea70136 Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/8a1737ae42c928384ab6447f6ee1a882510e85fa Types: Patch
      —
      Reference typeremoved
      https://git.kernel.org/stable/c/f8d871523142f7895f250a856f8c4a4181614510 Types: Patch
      —

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

    2. Record modifiedCVE Program

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html
      Referenceadded
      —
      https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html
      —
      Referenceremoved
      https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
      —

      Source identifier: af854a3a-2127-422b-91ae-364da2661108

    3. Record modifiedSiemens ADP

      Show 9 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2025/21xxx/CVE-2025-21796.json">CVE-2025-21796</a>
      Referenceadded
      —
      https://cert-portal.siemens.com/productcert/html/ssa-019113.html
      Referenceadded
      —
      https://cert-portal.siemens.com/productcert/html/ssa-082556.html
      Referenceadded
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      https://cert-portal.siemens.com/productcert/html/ssa-265688.html
      Referenceadded
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      https://cert-portal.siemens.com/productcert/html/ssa-457936.html
      Referenceadded
      —
      https://cert-portal.siemens.com/productcert/html/ssa-784472.html
      Referenceremoved
      https://cert-portal.siemens.com/productcert/html/ssa-019113.html
      —
      Referenceremoved
      https://cert-portal.siemens.com/productcert/html/ssa-082556.html
      —
      Referenceremoved
      https://cert-portal.siemens.com/productcert/html/ssa-265688.html
      —

      Source identifier: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e

    4. Record modifiedLinux

      CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      CVSSadded
      —
      AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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

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