Security readout for executives and security teams
Plain-English summary
A Linux NFS server flaw can leave a stale pointer after an access-control-list lookup fails. Later cleanup may reuse freed memory, producing a kernel warning and potentially a panic when panic-on-warning is enabled. Affected NFS servers could suffer serious disruption; broader compromise is scored as possible but is not demonstrated in the supplied evidence.
Executive priority
Prioritize internet-facing or broadly reachable NFS servers for urgent assessment and patching. The documented failure can crash systems configured to panic on warnings, making availability risk credible. Treat the 9.8 score seriously, while recognizing that the supplied evidence does not establish active exploitation or reliable full-system compromise.
Technical view
When retrieval of acl_default fails, acl_access and acl_default are released together, but acl_access retains a pointer to the freed posix_acl. nfs3svc_release_getacl can then decrement the stale object's reference count, causing a CWE-416 use-after-free and refcount underflow. The stable fixes clear both pointers after release.
Likely exposure
Exposure is concentrated on systems running affected Linux kernels with the kernel NFS server enabled. The supplied CVSS vector treats the flaw as remotely reachable without privileges or user interaction. Exact vulnerable package boundaries are unclear from the branch-oriented version data, so assess the running kernel using distribution guidance and fix inclusion.
Exploitation context
CISA KEV status is false, and the supplied sources do not report active exploitation or a public exploit. The evidence shows a kernel warning and panic under panic_on_warn. Reliable remote code execution, data theft, or data modification is not demonstrated, despite the CVSS 9.8 confidentiality, integrity, and availability ratings.
Researcher notes
The root cause and corrective pointer clearing are well supported. Practical exploitability is less certain: the supplied trace demonstrates refcount underflow and panic, not controlled code execution. Version information appears branch-oriented and ambiguous; resolve exposure by matching the exact kernel tree or vendor package against the applicable stable commit.
Mitigation direction
- Upgrade to a distribution kernel containing the CVE-2025-21796 fix, then reboot into that kernel.
- For custom kernels, backport the relevant Linux stable fix and complete normal kernel regression testing.
- Until updated, restrict NFS service exposure to trusted networks using existing network controls.
- Consult distribution or product-vendor advisories for exact fixed package versions and operational guidance.
Validation and detection
- Inventory hosts running the kernel NFS server and record their active kernel builds.
- Confirm the running kernel includes the stable fix applicable to its maintained branch.
- Review kernel logs for refcount underflow, use-after-free warnings, nfs3svc_release_getacl traces, or panic events.
- After updating, run standard NFS ACL regression tests in a controlled environment.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-416: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2025-21796 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
SSVC decision data
CVSS vector scores
2 official scoresWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9LinuxCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9CISA-ADPVulnerability scoring details
Base CVSS 3.1 score
9.8CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Base CVSS 3.1 score
7.8HighVector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- https://git.kernel.org/stable/c/8a1737ae42c928384ab6447f6ee1a882510e85faCVE reference, Linux
- https://git.kernel.org/stable/c/6f7cfee1a316891890c505563aa54f3476db52fdCVE reference, Linux
- https://git.kernel.org/stable/c/2e59b2b68782519560b3d6a41dd66a3d01a01cd3CVE reference, Linux
- https://git.kernel.org/stable/c/55d947315fb5f67a35e4e1d3e01bb886b9c6decfCVE reference, Linux
- https://git.kernel.org/stable/c/f8d871523142f7895f250a856f8c4a4181614510CVE reference, Linux
- https://git.kernel.org/stable/c/1fd94884174bd20beb1773990fd3b1aa877688d9CVE reference, Linux
- https://git.kernel.org/stable/c/7faf14a7b0366f153284db0ad3347c457ea70136CVE reference, Linux
- https://lists.debian.org/debian-lts-announce/2025/05/msg00030.htmlCVE reference, CVE
- https://lists.debian.org/debian-lts-announce/2025/03/msg00028.htmlCVE reference, CVE
- https://cert-portal.siemens.com/productcert/html/ssa-265688.htmlCVE reference, siemens-SADP
- https://cert-portal.siemens.com/productcert/html/ssa-082556.htmlCVE reference, siemens-SADP
- https://cert-portal.siemens.com/productcert/html/ssa-019113.htmlCVE reference, siemens-SADP
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Use After Free
Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.