CVE-2025-37956 ksmbd: Patch Linux SMB Server and Audit Microsoft Artifacts

A small, defensive change in the Linux kernel’s in‑kernel SMB server, ksmbd, has been tracked as CVE‑2025‑37956 and fixed upstream — but Microsoft’s public wording that “Azure Linux includes this open‑source library and is therefore potentially affected” is a product‑scoped attestation, not proof that no other Microsoft product can include the same vulnerable code. This distinction matters for defenders: treat Microsoft’s Azure Linux attestation as an authoritative, actionable signal for that product family while continuing to inventory and verify other Microsoft‑distributed kernel artifacts (WSL kernels, linux‑azure builds, AKS node images, Marketplace images, etc.) until they are explicitly attested or patched.

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Background / Overview​

CVE‑2025‑37956 was published in May 2025 and is described as a ksmbd issue where a client can supply an empty new‑name string during a rename operation; when that input reaches the kernel the flawed path can trigger a kernel oops originating in d_alloc. The upstream fix is straightforward: validate the newname (reject empty strings) and return an error instead of allowing the code path that triggers an oops. Multiple public trackers and distro advisories record the same description and show patches or fixed kernel package versions rolling out.
Why ksmbd? ksmbd is the kernel’s SMB3 server implementation (the kernel alternative to userspace Samba). It lives under fs/smb/server in the kernel source tree and is selected at kernel build time via configuration options (the config name is commonly exposed as CONFIG_SMB_SERVER / CONFIG_KSMBD in many kernels). Whether a given kernel image includes ksmbd is a build‑time decision — it can be built in, built as a module, or not built at all. That artifact‑level nature of inclusion is the technical root of the practical answer to the user’s question.

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What the CVE actually is (technical summary)​

The bug, in plain terms​

• Component: ksmbd (Linux kernel SMB server).
• Fault: A code path that processes rename requests fails to validate that the supplied new name is non‑empty before using it. A crafted or malformed request with an empty new name leads to a kernel memory allocator path (d_alloc) being invoked in an invalid way and produces a kernel oops (crash). This is a stability/availability issue (denial‑of‑service) rather than a direct confidentiality or integrity compromise by design, though kernel oopses can sometimes be a stepping stone to more serious exploitation in specific environments.

Upstream action and patch status​

• Upstream kernel maintainers merged a small defensive patch to return an error when a rename request contains an empty new name, preventing the d_alloc path from receiving invalid input.
• Public vulnerability trackers (NVD/OSV/distro trackers) list the CVE and point to the stable‑tree fixes and to the kernel commits that correct the behavior. Distribution vendors (Debian, Ubuntu, SUSE, Oracle Linux, etc.) map the kernel fix into package updates and state fixed or fixed‑in versions per their release policies.

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Microsoft’s statement and what it means (CSAF / VEX context)​

Microsoft’s Security Response Center (MSRC) has adopted a machine‑readable approach for attesting which Microsoft products include particular open‑source components. That rollout — publishing Common Security Advisory Framework (CSAF) and Vulnerability Exploitability eXchange (VEX) attestations — started as Azure Linux (formerly CBL‑Mariner) as a first product family to be inventoried and published in machine‑readable form in October 2025. Microsoft’s MSRC CVE entries often contain a short FAQ line indicating which Microsoft product(s) have been identified to ship the implicated open‑source component and promise to update the CVE/VEX mapping if more Microsoft products are discovered to carry the component.
Operationally, that wording is deliberate:

• It is an authoritative attestation for the product named (Azure Linux): Microsoft has t family and confirmed the presence of the implicated upstream code. Treat the attestation as the canonical signal for Azure Linux customers.
• It is not an exhaustive inventory of Microsoft’s entire portfolio. Microsoft explicitly frames VEX/CSAF as a phased rollout, and the absence of a VEX entry for another Microsoft product is not a guarantee that product does not include the component. You should treat "no attestation yet" as unknown, not safe.

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Is Azure Linux the only Microsoft product that could be affected?​

Short answer: No — Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include ksmbd for this CVE, but other Microsoft artifacts may include ksmbd depending on how each kernel or image was built. The attestation is a product‑scoped truth, not a blanket exclusivity statement.
Longer explanation and examples

• Microsoft produces multiple kernel artifacts and Linux‑adjacent images that could, in principle, include ksmbd if the kernel configuration and packaging enable it:
• WSL2 kernel binaries maintained by Microsoft (the WSL2 kernel configuration is in the WSL2‑Linux‑Kernel repository and can be inspected). Whether ksmbd is present depends on the configuration used to build those binaries.
• linux‑azure kernel packages used for some Azure VM SKUs and specialized Azure images.
• AKS node images and curated Marketplace images that may be built from Microsoft‑maintained kernel trees or vendor forks.
• Third‑party Marketplace appliances or partner images hosted in Azure (these are the publisher’s responsibility and not covered by Microsoft’s VEX unless the vendor publishes their own attestations).
• Because inclusion of ksmbd is determined by kernel build flags (CONFIG_SMB_SERVER / CONFIG_KSMBD and related crypto/FS options), two Microsoft kernel artifacts derived from the same upstream base can differ in whether ksmbd is present (built‑in, module, or omitted). Inspecting the kernel config or runtime modules is therefore the only reliable artifact‑level verification.

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How to verify whether a given Linux artifact includes ksmbd (practical checks)​

When you need to confirm whether a host or image is actually vulnerable, do the following artifact‑level checks. These are short, deterministic inspections you can automate.

• On a running Linux host (physical/VM/container host):
• Check loaded modules: lsmod | egrep -i 'ksmbd|smb'
• Inspect the running kernel config: grep -i KSMBD /boot/config-$(uname -r) || zgrep -i KSMBD /proc/config.gz
• Look for module file: modinfo ksmbd (returns module metadata if installed)
• Check kernel logs for ksmbd activity / oopses: dmesg | egrep -i 'ksmbd|smb|oops|ksmbd'
• For images or kernels you can inspect offline:
• Unpack the image and inspect /boot/config-* or the kernel build config file for CONFIG_SMB_SERVER / CONFIG_KSMBD entries.
• If you control the build pipeline, inspect the Kconfig used for the kernel build or the build artifacts produced by your CI system.
• For WSL:
• Microsoft publishes the WSL2 kernel sources and the Microsoft/config-wsl config file; check that repository to see whether the WSL kernel config enables ksmbd (it typically does not enable many server subsystems by default, but this must be verified for the specific WSL kernel version used).
• Example of the exact checks to run (single‑line variants you can script):
• grep -i CONFIG_SMB_SERVER /boot/config-$(uname -r) || zgrep -i CONFIG_SMB_SERVER /proc/config.gz
• lsmod | grep -i ksmbd
• dmesg | tail -n 200 | egrep -i 'ksmbd|smb'

If any of these checks indicate ksmbd is present and active, the host should be prioritized for patching.

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Mitigation and remediation guidance​

If you find ksmbd on a Microsoft distributed artifact (Azure Linux or any other image), take the following immediate and staged actions:
Immediate (minutes to hours)

• Patch first: Apply the vendor kernel update that contains the ksmbd fix. Distribution trackers and vendors list which package versions include the fix; for example, Debian and Ubuntu trackers map fixed kernel package versions or backports. Prioritize reboot windows for kernel updates on production hosts.
• If you cannot patch immediately:
• Unload the module (if ksmbd is a loadable module): sudo rmmod ksmbd (if no userspace component depends on it). Note: removing a builtin component is not possible without a reboot; unloading only works if ksmbd is built as a module.
• Restrict network access to SMB ports (TCP 445, TCP 139) at the network perimeter and host firewall to prevent untrusted network actors from reaching any active ksmbd service.
• If ksmbd is paired with a userspace daemon (ksmbd.mountd / ksmbd.tools), stop/disable the userspace components until patched.
• Temporary host hardening:
• Enforce strict network segmentation for hosts that must continue running unpatched kernels.
• Use kernel lockdown / sysctl hardening where appropriate to limit unprivileged access to local IPC paths that could reach ksmbd.

Planned (days)

• Schedule kernel package upgrades and reboots for affected hosts. Use vendor advisories and distribution trackers to identify the minimal fixed kernel versions for your distribution lines. For cloud images (Azure VMs), apply the vendor‑supplied image updates or redeploy from patched images.

Longer term (weeks to months)

• Integrate Microsoft VEX/CSAF feeds into your vulnerability management automation so an attestation — such as the one Microsoft published for Azure Linux — triggers inventory and remediation workflows automatically. Microsoft’s VEX rollout aims to provide machine‑readable, product‑scoped attestations for precisely this purpose.
• Maintain an artifact inventory of Microsoft‑distributed kernels and images in your estate and verify kernel configs during build/release pipelines so third‑party CVEs like this are triaged at build time, not after production deployment.

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Practical prioritization — what defenders should do now​

• If you run Azure Linux instances: treat Microsoft’s attestation as authoritative. Patch those instances immediately with the vendor’s kernel updatered. Microsoft’s VEX/CSAF entry is intentionally product‑scoped and intended for automation.
• For mixed estates that include WSL, linux‑azure packages, AKS node images, or Marketplace images:
• Inventory all Microsoft kernel artifacts in use.
• Perform the runtime checks above to determin- Prioritize remediation for any artifact where ksmbd is present and reachable by untrusted users/networks.
• If you use WSL on developer machines, check whether the WSL kernel binary in use was built with ksmbd enabled; it often isn’t, but you must verify rather than assume. Microsoft’s WSL kernel tree and config are public and can be inspected.
• Track vendor and distro advisories (Debian/Ubuntu/SUSE/Oracle/Red Hat) for fixed package versions and backports; many distributions have already published mapping entries. Automate mapping from CVE → package → installed version to accelerate triage.

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Critical analysis — strengths of Microsoft’s approach and residual risks​

Strengths​

• Product‑level transparency is valuable. Microsoft’s decision to publish machine‑readable VEX/CSAF attestations for Azure Linux gives customers a deterministic, automatable signal about which Microsoft product families were inventoried and found to carry particular upstream components. That reduces noisy triage and speeds patching for Azure Linux customers.
• Procedural clarity. Microsoft’s FAQ wording that it will update the CVE if impact to additional products is identified is a clear commitment to expand coverage rather than leaving customers guessing. That commitment allows security teams to plan for a phased update model and incorporate VEX feeds into their pipelines over time.

Limitations and residual risk​

• Phased rollouts create windows of uncertainty. Because VEX/CSAF attestations are published product‑by‑product, there is a measurable interval during which other Microsoft artifacts remain unverified. An organization that treats the absence of an attestation as evidence of safety risks leaving uninspected assets exposed. In practice this means defenders musact‑level verification for every Microsoft kernel artifact in their estate.
• Build‑time variability is the wildcard. Two kernel binaries from the same upstream release can differ radically depending on the CONFIG flags used. A ksmbd‑enabled azure‑linux kernel does not imply the same kernel features are present in every Microsoft kernel binary shipped for other purposes. Only direct inspection and inventory closes this gap.
• Communication cadence matters. Microsoft’s promise to update VEX entries when more products are identified depends on the vendor’s internal inventory pace. Customers should not substitute that cadencication processes; treat vendor VEX as authoritative for the attested products and as a helpful but incomplete dataset otherwise.

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Practical detection rules and monitoring suggestions​

• IDS/IPS and EDR rules:
• Alert on unexpected SMB server behavior on Linux hosts where ksmbd is not expected.
• Flag ksmbd module loads on endpoints that historically do not run SMB services.
• SIEM and logging:
• Search dmesg and kernel logs for ksmbd oops, KASAN traces, or warnings referencing d_alloc, opinfo, or other ksmbd internals.
• Correlate kernel crash reports with recent SMB protocol activity from remote IPs.
• Asset inventory automation:
• Map CVE → distribution package → installed kernel versions; automatically flag hosts where installed kernel versions are older than the vendor’s fixed release.
• In clouds, track and nel artifact they use (for example, “linux-azure-5.15.90-azure” vs a custom kernel build).

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When to treat Microsoft attestation as sufficient (and when it isn’t)​

• Sufficient: If you run only Azure Linux images from Microsoft and your environment is small, Microsoft’s Azure Linux VEX attestation is the primary authoritative signal to drive remediation — patch and reboot those images promptly.
• Not sufficient: In heterogeneous fleets, or where Microsoft‑distributed artifacts other than Azure Linux are present (WSL2, linux‑azure, Marketplace images, AKS nodes, partner appliances), you must verify each artifact because the attestation is product‑scoped. Absence of attestation equals “not yet verified,” not “not affected.”

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Final takeaways and a short checklist​

• CVE‑2025‑37956 (ksmbd: prevent rename with empty string) is a real kernel stability issue fixed upstream; distributions and vendors have mapped and begun to backport the fix. Prioritize patching for any host that includes ksmbd.
• Microsoft’s MSRC statement that “Azure Linux includes this open‑source library and is therefore potentially affected” is an authoritative product attestation for Azure Linux and should prompt immediate remediation for customers running that product, but it does not prove exclusivity across all Microsoft products. Treat other Microsoft kernel artifacts as unverified until inspected or attested.
• Operational checklist:
• Inventory Microsoft-distributed kernel artifacts in your estate.
• Run the quick artifact checks (grep CONFIG_SMB_SERVER, lsmod, dmesg).
• Patch Azure Linux instances immediately (vendor packages).
• For any unpatched ksmbd hosts, apply mitigations: unload module (if possible), restrict SMB ports, and segment networks.
• Integrate Microsoft VEX/CSAF feeds into your automation to pick up future attestations and changes to product mappings.

Microsoft’s move to publish machine‑readable attestations is a positive step for transparency and automation; it anchors remediation for attested products like Azure Linux. But defenders must still do the fundamental work of artifact verification and inventory across their environments — the kernel’s feature set is decided at build time, and only direct inspection or an explicit vendor attestation can prove whether a particular Microsoft artifact includes ksmbd and therefore required the CVE mitigation.

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Conclusion: Treat the Azure Linux VEX attestation as a high‑confidence trigger to patch Azure Linux images now. Do not stop there — inventory other Microsoft‑supplied kernels in your environment, verify ksmbd presence by inspecting kernel configs or modules, and apply the mitigation checklist above for any artifact that contains ksmbd but is not yet patched.

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Source: MSRC Security Update Guide - Microsoft Security Response Center