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CVE Record

CVE-2026-31538: smb: server: make use of smbdirect_socket.recv_io.credits.available

In the Linux kernel, the following vulnerability has been resolved: smb: server: make use of smbdirect_socket.recv_io.credits.available The logic off managing recv credits by counting posted recv_io and granted credits is racy. That's because the peer might already consumed a credit, but between receiving the incoming recv at the hardware and processing the completion in the 'recv_done' functions we likely have a window where we grant credits, which don't really exist. So we better have a decicated counter for the available credits, which will be incremented when we posted new recv buffers and drained when we grant the credits to the peer. This fixes regression Namjae reported with the 6.18 release.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A race in the Linux kernel’s SMB Direct receive-credit accounting can grant a peer credits that are not actually available. On affected systems, a remote, unauthenticated peer may disrupt availability. The issue is rated 7.5 High and is described as a regression introduced with Linux 6.18.

Executive priority

Treat exposed affected SMB servers as a high-priority availability risk. Accelerate inventory and supported kernel updates, especially for business-critical file services. This is not currently supported as an actively exploited emergency, but unauthenticated network reachability and potential service disruption justify prompt remediation.

Technical view

The SMB server counts posted receive buffers and granted credits asynchronously. A peer may consume a credit after hardware reception but before completion processing, creating a window where nonexistent credits are granted. The fix introduces a dedicated available-credit counter, incremented when receive buffers are posted and drained when credits are granted.

Likely exposure

Exposure is most likely where an affected Linux kernel runs the kernel SMB server with SMB Direct functionality reachable over a network. The source identifies a 6.18 regression and lists affected version boundaries through 7.0, but does not provide an unambiguous distribution-specific fixed-version map.

Exploitation context

The CVSS vector indicates network reachability, low attack complexity, no required privileges, no user interaction, and high availability impact. The supplied bundle marks KEV false and provides no evidence of active exploitation or a public exploit.

Researcher notes

The record describes a receive-credit accounting race rather than memory corruption, confidentiality loss, or integrity compromise. The stated security impact is availability only. The cited kernel commits are the authoritative remediation references, but the supplied metadata does not clearly map each commit to a kernel branch or identify distribution package versions.

Mitigation direction

  • Inventory Linux systems providing kernel SMB Direct services, prioritizing externally or broadly reachable deployments.
  • Apply the distribution-supported kernel update incorporating the applicable cited stable fix.
  • If an update is unavailable, obtain supported mitigation guidance from the kernel distributor or vendor.
  • Limit network access to affected SMB services where operationally acceptable until remediation is confirmed.

Validation and detection

  • Confirm the running kernel version and distribution package release on each SMB server.
  • Determine whether the kernel SMB server and SMB Direct functionality are enabled and network-accessible.
  • Verify the installed kernel contains the applicable cited stable commit or vendor backport.
  • After updating, confirm the repaired kernel is running and SMB availability remains stable.
Prepared
Confidence
medium
Sources
5

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

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CVE-2026-31538 mapping review

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Vulnerability profileCVE Program record
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

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
4Source 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
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2026-31538Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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
LinuxLinux89b021a72663c4d96d8a8b85272bb42d991a1c6f, 89b021a72663c4d96d8a8b85272bb42d991a1c6f, 89b021a72663c4d96d8a8b85272bb42d991a1c6funaffected
LinuxLinux6.18, 0, 6.18.11, 6.19.1, 7.0affected
Weakness

CWE details

No CWE listed

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