Security readout for executives and security teams
Plain-English summary
A Linux NFSv4 locking flaw can freeze affected file operations when an NFS server reboots while an application closes a deleted-but-still-open file. The documented impact is loss of availability, not data theft or modification.
Executive priority
Prioritize normal high-severity patching for NFS-dependent production systems, especially where a deadlock could interrupt critical services. Accelerate remediation for environments with frequent NFS server failovers or reboots. This is not supported as an emergency internet-wide exploitation event by the supplied evidence.
Technical view
During NFSv4 state recovery, an open reclaim can race with close processing for a sillyrenamed, delete-on-close file. A synchronous delegation return may then deadlock because it lacks privileged handling. The fix catches that return and schedules it asynchronously. The record assigns CVSS 7.5 and CWE-667.
Likely exposure
Exposure is limited to Linux systems using affected NFSv4 code and encountering the specific server-reboot, state-reclaim, and concurrent-close conditions. The supplied version data names 6.11, 6.12.18, 6.13.6, and 6.14 but is ambiguous; confirm exact affected ranges with the kernel or distribution vendor.
Exploitation context
The bundle marks this CVE as absent from KEV and provides no evidence of active exploitation. Although the CVSS vector is network-based, the documented trigger depends on a particular NFSv4 recovery race. Treat it primarily as an availability risk until stronger exploitation evidence appears.
Researcher notes
The affected-version representation is internally unclear, including an unexplained "0" entry and repeated commit identifiers marked unaffected. Avoid deriving precise vulnerable ranges from this bundle alone. Validate ancestry or vendor backports against the three stable-kernel commits. The described security consequence is availability loss through improper locking.
Mitigation direction
- Apply a vendor-supported kernel update confirmed to contain the linked upstream fix.
- Prioritize NFSv4 clients supporting business-critical or high-availability workloads.
- If immediate updating is impossible, consult the Linux distribution vendor for supported mitigations.
- Schedule maintenance carefully because kernel remediation normally requires activating the updated kernel.
Validation and detection
- Inventory Linux kernel versions on systems mounting or serving NFSv4 workloads.
- Compare installed kernels with current distribution advisories and the CVE record.
- Confirm the installed kernel includes the applicable linked stable fix.
- After updating, verify the running kernel version, not only installed packages.
- Test server-reboot and NFS recovery behavior in a non-production 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-667: 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-2025-21900 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
- High
- CVSS
- 7.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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:N/I:N/A:H3.93.6LinuxCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H1.83.6CISA-ADPVulnerability scoring details
Base CVSS 3.1 score
7.5HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Base CVSS 3.1 score
5.5MediumVector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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/4fe4ae6c2e01d028856b73b6328b12b8945df871CVE reference, Linux
- https://git.kernel.org/stable/c/f41a60bc43e7abbc636fee78bed0d74c31e738b0CVE reference, Linux
- https://git.kernel.org/stable/c/8f8df955f078e1a023ee55161935000a67651f38CVE reference, Linux
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.
Improper Locking
Improper Locking represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.