Security readout for executives and security teams
Plain-English summary
This is a Linux kernel CIFS client flaw. A local low-privileged user may be able to trigger a denial of service by supplying an oversized acdirmax mount option. The disclosed impact is availability only; the sources do not report data theft, privilege escalation, or active exploitation.
Executive priority
Treat as a routine but real availability-risk kernel update. Prioritize shared Linux servers, file-access hosts, and environments where unprivileged users can mount CIFS shares or influence mount automation.
Technical view
The CIFS code converts the u32 acdirmax mount parameter from seconds to jiffies before validating its upper bound. That order can cause integer overflow. CVSS 3.1 is 5.5 with local attack vector, low complexity, low privileges, no user interaction, and high availability impact.
Likely exposure
Exposure is most relevant to Linux systems using CIFS/SMB client mounts where local users or automation can influence mount options. Kernel branch and distribution package status should be checked against vendor advisories and the listed stable fixes.
Exploitation context
The bundle marks KEV as false and provides no cited evidence of in-the-wild exploitation. Exploitation requires local access and a path to provide the vulnerable mount parameter. Public evidence is incomplete on practical crash behavior across distributions.
Researcher notes
The root issue is CWE-190 integer overflow in acdirmax processing. The source bundle names upstream stable commits but does not provide exploit details. Avoid assuming broader impact than local availability loss unless vendor advisories add evidence.
Mitigation direction
- Apply kernel updates from your Linux distribution or vendor.
- Track the listed Linux stable commits for upstream fixed versions.
- Apply Debian LTS guidance where Debian LTS systems are in scope.
- Restrict who can create CIFS mounts or control mount options.
- Review automation that builds CIFS mount parameters from untrusted input.
Validation and detection
- Inventory Linux kernel versions across CIFS-capable systems.
- Identify systems using CIFS or SMB client mounts.
- Check whether local users can influence CIFS mount options.
- Compare installed kernels with distribution advisories and stable fixes.
- Confirm patched kernel packages are deployed after maintenance.
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-190: 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-2025-21963 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
- Medium
- CVSS
- 5.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:L/AC:L/PR:L/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
1 official scoreWe 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: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
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/0c26edf477e093cefc41637f5bccc102e1a77399CVE reference, Linux
- https://git.kernel.org/stable/c/39d086bb3558da9640ef335f97453e01d32578a1CVE reference, Linux
- https://git.kernel.org/stable/c/9e438d0410a4002d24f420f2c28897ba2dc0af64CVE reference, Linux
- https://git.kernel.org/stable/c/2809a79bc64964ce02e0c5f2d6bd39b9d09bdb3cCVE reference, Linux
- https://git.kernel.org/stable/c/6124cbf73e3dea7591857dd63b8ccece28952afdCVE reference, Linux
- https://git.kernel.org/stable/c/5b29891f91dfb8758baf1e2217bef4b16b2b165bCVE reference, Linux
- https://lists.debian.org/debian-lts-announce/2025/05/msg00045.htmlCVE reference, CVE
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.