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CVE Record

CVE-2025-21955: ksmbd: prevent connection release during oplock break notification

In the Linux kernel, the following vulnerability has been resolved: ksmbd: prevent connection release during oplock break notification ksmbd_work could be freed when after connection release. Increment r_count of ksmbd_conn to indicate that requests are not finished yet and to not release the connection.

HighCVSS 8.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A Linux kernel flaw in the ksmbd SMB file-server component can release a connection while related work is still running. A network attacker with low privileges could potentially trigger memory corruption affecting confidentiality, integrity, and availability. Risk is concentrated on systems actively using and exposing ksmbd.

Executive priority

Prioritize exposed ksmbd file servers and systems accessible from untrusted or broadly privileged networks. Remediate promptly because the severity is high and the stated impact spans data confidentiality, integrity, and service availability. Systems not using ksmbd are lower priority after configuration is verified.

Technical view

The flaw is a use-after-free condition: ksmbd_work may remain active after its ksmbd_conn is released during an oplock-break notification. The upstream correction increments the connection reference count while requests remain unfinished. The supplied CVSS 3.1 score is 8.8, with network reachability, low complexity, low privileges, and no user interaction.

Likely exposure

Exposure requires a Linux system using ksmbd, with its SMB service reachable by the attacker. The bundle flags Linux 5.15 and listed 6.x releases through 6.14 as affected. Distribution kernels may contain backported fixes, so version numbers alone are insufficient for confirmation.

Exploitation context

The supplied record is not in KEV, and the source bundle provides no evidence of active exploitation or a public exploit. The CVSS vector indicates that exploitation is network-accessible but requires low privileges. The available evidence does not establish reliable code execution outcomes.

Researcher notes

The core issue is connection lifetime management during oplock-break notification. Review the referenced stable commits and distribution backports for exact branch coverage. The affected-version representation in the supplied bundle is not detailed enough to determine every vulnerable or fixed downstream kernel solely from its release string.

Mitigation direction

  • Install a vendor-supported kernel containing the applicable upstream stable fix.
  • Check distribution security guidance because vendors may backport fixes without changing the main kernel version.
  • Restrict network access to ksmbd services while remediation is pending.
  • Disable ksmbd where it is not operationally required, following vendor guidance.

Validation and detection

  • Confirm whether the ksmbd module or service is enabled and actively serving SMB.
  • Identify every network path that can reach the affected SMB service.
  • Verify the installed vendor kernel includes the applicable referenced stable commit.
  • After updating, confirm the fixed kernel is running rather than merely installed.
Prepared
Confidence
medium
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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

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CVE-2025-21955 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
8.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Official CVE source material

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.

1CVSS vectors
3Timeline events
0ADP providers
5Source links

CVSS vector scores

1 official score

We 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.

ScoreVersionSeverityVectorExploitImpactSource
8.8CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H2.85.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

8.8High
CVSS 3.1 vector shape for CVE-2025-21955Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux0626e6641f6b467447c81dd7678a69c66f7746cf, 0626e6641f6b467447c81dd7678a69c66f7746cf, 0626e6641f6b467447c81dd7678a69c66f7746cf, 0626e6641f6b467447c81dd7678a69c66f7746cfunaffected
LinuxLinux5.15, 0, 6.6.84, 6.12.20, 6.13.8, 6.14affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.