CVE-2025-37776: ksmbd Use-After-Free Fix and Azure Linux Attestation

A recently assigned Linux-kernel CVE, CVE-2025-37776, fixes a subtle but important use‑after‑free in the in‑kernel SMB server (ksmbd) — and Microsoft’s public attestation that “Azure Linux includes this open‑source library and is therefore potentially affected” should be read as an authoritative, product‑scoped inventory statement, not as categorical proof that no other Microsoft product can contain the same vulnerable code.

Background / Overview​

ksmbd is the Linux kernel’s in‑kernel SMB/CIFS server implementation used to serve file shares from kernel space. It exposes SMB server functionality directly from the kernel, which offers performance benefits but raises stakes for memory‑safety bugs because they run in kernel mode. The bug tracked as CVE‑2025‑37776 was fixed upstream by changing the locking discipline inside smb_break_all_levII_oplock() so a read lock protects the entire loop and prevents a race that could lead to a use‑after‑free. Upstream commit notes and vendor vulnerability trackers document the fix and the affected code paths.
Why this matters operationally: a use‑after‑free inside a kernel SMB server can lead to memory corruption, crashes, or more severe escalation paths if an attacker is able to influence the vulnerable path. Even when a patch is small and surgical, kernel memory errors are high‑impact by nature — they affect availability, integrity, and potentially confidentiality on hosts that expose the vulnerable component.

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What Microsoft actually said — and why customers asked “Is Azure Linux the only product?”​

Microsoft’s Security Response Center (MSRC) mapped CVE‑2025‑37776 to its public product inventory and published the now‑familiar short attestation: “Azure Linux includes this open‑source library and is therefore potentially affected by this vulnerability.” That one‑line is a product‑level inventory result: it states that Microsoft inspected the Azure Linux build(s) for the relevant upstream code and found it present. Microsoft also documented that it began publishing machine‑readable CSAF/VEX attestations starting in October 2025 and has committed to update CVE entries when additional Microsoft products are identified as carrying the implicated library.
The operational question many administrators asked is reasonable: does that attestation mean Azure Linux is the only Microsoft product that includes ksmbd? The short and accurate answer is: no — Azure Linux is the only Microsoft product Microsoft has explicitly attested (at the time of the advisory) to include the affected ksmbd code, but that attestation is not a technical guarantee that other Microsoft artifacts are free of the component. Whether another Microsoft product includes the vulnerable ksmbd code depends on artifact‑level details (kernel version, upstream commit range, and the kernel CONFIG options used to build the binary). Microsoft’s VEX/CSAF rollout is phased and product‑scoped, so absence of an attestation for a different Microsoft product is simply absence of attestation, not proof of absence.

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Technical summary of the vulnerability​

• Affected component: ksmbd (Linux in‑kernel SMB server).
• Vulnerable routine: smb_break_all_levII_oplock(). The bug arises when the loop unlocking logic can release locks while iterating, creating a race where a freed object may be accessed later in the loop — a classic use‑after‑free scenario. Upstream maintainers changed the locking scheme to hold a read lock for the duration of the loop to eliminate the race.
• Upstream patch: landed in the stable upstream tree; vendors and trackers reference the kernel commit(s) that implement the read‑lock change.

What the public trackers say about exploitability and severity: the vulnerability is tracked by NVD and other vulnerability aggregators; some sources assign a high impact score given the kernel‑level nature of ksmbd bugs, but public advisories vary on whether the bug is remotely exploitable in practice. The presence and exposure of ksmbd on a host — whether the kernel was built with ksmbd enabled and whether SMB server ports are reachable — will determine whether remote exploitation is a plausible attack path. Treat the vulnerability as high‑priority on any host where ksmbd is present and reachable.

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Is Azure Linux the only Microsoft product affected? Practical answer for defenders​

Short answer for operations and security teams: treat Microsoft’s Azure Linux attestation as au for Azure Linux deployments, but do not assume other Microsoft‑supplied kernels or images are automatically safe. You must verify each Microsoft artifact you run.
Why:

• Inclusion of ksmbd is a build‑time decision. Kernel artifacts differ by upstream source, commit ranges, and CONFIG options. Two kernels built from the same upstream tree can diverge in included subsystems. Whether ksmbd appears in a kernel depends on the kernel configuration (CONFIG_KSMBD as module or built‑in) used by that artifact.
• Microsoft ships and publishes several distinct Linux artifacts: Azure Linux images (the specific product Microsoft attested), kernels used by WSL2 (a Microsoft‑maintained WSL kernel), kernels embedded in Azure VM images, AKS node images, and sometimes specialized kernels for other services. Any of these might include the vulnerable component depending on build choices. Microsoft’s initial VEX/CSAF focus began with Azure Linux and will expand, per the MSRC blog and advisories.
• Absence of an entry in Microsoft’s VEX set for a product is not proof that the product is “not affected” — it means the artifact hasn’t yet been invento it was inventoried and found not to include the component (which would be explicitly stated). Microsoft has pledged to update its VEX/CSAF files and the CVE entry if additional products are found to include the component.

Operational takeaway: Azure Linux is the most direct signal and highest‑priority Microsoft product to patch for CVE‑2025‑37776 where it is used. Other Microsoft artifacts should be verified on an artifact‑by‑artifact basis: check the kernel version, kernel config, and whether ksmbd is present. Do not assume a Microsoft‑packaged Linux kernel is safe unless it is explicitly attested.

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How to check whether your hosts are actually affected (practical steps)​

Below are pragmatic verification steps administrators can use to determine whether a particular Linux host — whether an Azure VM, an on‑prem image, or a WSL instance — includes ksmbd and therefore should be considered in scope for remediation.

• Identify the kernel artifact in question
• For cloud VMs or images, record the image SKU (Azure Linux, distro name, kernel package).
• For WSL2, list installed distributions and the kernel binary (Windows exposes a WSL kernel binary that Microsoft builds).
• Check the running kernel version and config
• uname -a to capture kernel identity.
• Inspect the kernel config (often /boot/config-$(uname -r) or /proc/config.gz): search for ksmbd flags, e.g., grep -i ksmbd /boot/config-$(uname -r) or zgrep KSMBD /proc/config.gz. If CONFIG_KSMBD is set to y or m, the artifact includes the component.
• Search for ksmbd modules or symbols
• lsmod | grep ksmbd or modinfo ksmbd (if modules are present).
• grep -R "ksmbd" /lib/modules/$(uname -r) to find compiled modules in the kernel module tree.
• Check whether the SMB server is enabled or reachable
• If ksmbd is present but not loaded, check whether any distribution packages configure it to start.
• Network exposure: confirm whether SMB ports are reachable (e.g., tcp/445) from untrusted networks; if they are, prioritize remediation.
• If uncertain, contact vendor support and consult MSRC VEX/CSAF for the product
• Microsoft has begun publishing machine‑readable attestations — check MSRC VEX for Azure Linux and watch for updates to other product attestations. Microsoft has committed to update CVE records if additional products are found to ship the library.

These verification steps let you determine artifact‑level exposure quickly so you can prioritize patches and mitigations.

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Recommended mitigations and remediation options​

• Patch promptly: install the vendor kernel update or distribution package that includes the upstream fix. Upstream commits were backported by vendors and distributors; consult your distro vendor for the correct package and release that contains the fix. Public trackers list references to the kernel commits and the upstream patch.
• Disable or unload ksmbd if you do not need in‑kernel SMB:
• If the kernel is configured with ksmbd as a module and your environment does not require in‑kernel SMB serving, consider blacklisting the module or removing the package that enables it until an updated kernel is available.
• Reduce exposure: ensure SMB ports are not reachable from untrusted networks; use network ACLs, NSGs (in cloud), firewalls, or segmentation to limit SMB exposure to trusted subnets only.
• Verify WSL and other Microsoft kernels: if you run WSL2, check the WSL kernel build Microsoft supplies. If ksmbd is present in the WSL kernel build you use, consider patching or applying mitigations consistent with your risk tolerance.
• Use vendor attestations as a triage signal, not a final scope decision: treat MSRC’s Azure Linux attestation as a high‑confidence signal for Azure Linux images, but inventory and verify other Microsoft artifacts you run. Microsoft’s VEX/CSAF program will expand over time; in the meantime, don’t rely solely on absence of attestation.

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Cross‑checking the technical record (verification of facts)​

To ensure accuracy, the ksmbd fix and description have been cross‑referenced with multiple independent sources:

• The National Vulnerability Database (NVD) entry for CVE‑2025‑37776 summarizes the issue as a ksmbd use‑after‑free in smb_break_all_levII_oplock() and records the upstream resolution. The NVD summary reflects the upstream change and places the bug in context as a kernel memory‑safety fix.
• Public vulnerability aggregators and trackers (for example, cvefeed and security aggregators) record the tion and reference the kernel commit(s) that implemented the read‑lock change. These trackers also surface vendor advisories and patch references, giving administrators multiple places to verify patch availability.
• Microsoft’s MSRC product attestation and its October 2025 VEX/CSAF blog explain the company’s phased approach to publishing machine‑readable vendorship attestations starting with Azure Linux; MSRC explicitly states it will update the CVE/VEX records as additional Microsoft products are identified as shipping the implicated component. That public commitment is why Azure Linux is named first and why Microsoft’s statement is best read as product‑scoped.

Where a public claim could not be independently verified (for example, whether a particular Microsoft packaged kernel in WSL at a given date included the vulnerable commit), the correct operational posture is to perform artifact inspection or ask the vendor for an explicit attestation for that product SKU. MSRC’s VEX/CSAF program will produce those attestations over time; in the meantime, artifact inspection is the pragmatic verification step.

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Risks and criticisms: what this attestation model helps and what it leaves exposed​

Strengths of Microsoft’s VEX/CSAF rollout​

• Machine‑readable attestations let large enterdors automate triage decisions and reduce false positives when mapping CVEs to product inventories. Microsoft’s phased approach, starting with Azure Linux, produces a concrete inventory signal customers can act on immediately. That signal is especially valuable for cloud tenants running the attested image family.

Limitations and residual risks​

• Phased attestation creates short windows where other artifacts remain unverified. Microsoft’s initial focus on a single product family (Azure Linux) means other Microsoft‑shipped kernels (WSL2, Azure VM kernels, AKS node images, appliance images) might not yet be inventoried publicly. Absence of attestation is not an assurance of non‑inclusion. Administrators must therefore perform artifact checks and not rely solely on the vendor’s current attestations.
• Vendor attestations are only as fast as inventory workflows allow. In complex organizations that publish many kernel artifacts, the attestation cadence can trail upstream disclosure — so waiting solely on vendor attestations can delay mitigation for assets under your control.
• Differences in kernel configuration matter. Two otherwise identical kernels can differ by the inclusion of a single subsystem like ksmbd. Artifact‑level verification is therefore required for high‑confidence triage.

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Practical guidance for Windows‑centric environments (WSL, Azure, and hybrid)​

• WSL2 customers: WSL2 runs a Microsoft‑maintained Linux kernel. Check the WSL kernel binary and kernel config used by your WSL distributions. If you operate WSL in a shared or developer‑workstation environment, the risk profile differs from cloud VMs, but the verification steps above still apply. If ksmbd is present and you do not require in‑kernel SMB, prefer to blacklist the module or update the WSL kernel once Microsoft publishes a patched build.
• Azure customers: patch Azure Linux images according to Microsoft soft’s attestation to drive priority. If you run other Azure marketplace images or partner appliances, verify those artifacts separately. Network segmentation and NSG policies remain valuable compensating controls until you confirm artifact status.
• Enterprise patching practice: treat kernel memory‑safety fixes as high priority and coordinate maintenance windows accordingly. Use staged rollouts and test images to validate that the vendor patch addressest introduce regressions.

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Final assessment and what to expect next​

CVE‑2025‑37776 is an upstream kernel fix addressing a use‑after‑free in ksmbd’s opener/lock break logic. Microsoft’s public inventory attestation that Azure Linux includes the implicated open‑source library is an important, actionable signal for Azure Linux customers. However, it does not preclude other Microsoft products from shipping the same upstream code; whether they do depends on artifact‑level build choices. Microsoft has committed to expand its VEX/CSAF attestations and update CVE records if more Microsoft products are found to include the component — but until those attestations exist, administrators should validate each Microsoft artifact in their estate.
Action checklist (short):

• Treat Azure Linux hosts as in‑scope and apply vendor patches promptly.
• Inventory Microsoft‑provided kernels you run (WSL2, Azure VM images, AKS nodes) and verify ksmbd presence using kernel config and module checks.
• If ksmbd is present and not required, disable or blacklist it until patched.
• Limit SMB exposure with network controls while you verify and remediate.
• Watch MSRC VEX/CSAF updates for new product attestations and updated CVE mappings.

CVE‑2025‑37776 is a reminder that kernel subsystems bundled into distribution and vendor kernels are precisely the kind of supply‑chain components that make machine‑readable attestations useful — but the attestation model is new and phased, so do not substitute it for artifact inspection and proactive verification in the meantime.

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Conclusion: Azure Linux is the only Microsoft product explicitly attested to include the vulnerable ksmbd code for CVE‑2025‑37776 at the time of Microsoft’s advisory, and administrators running Azure Linux should act immediately. However, the attestation is product‑scoped rather than universal — other Microsoft kernels or images could include the same upstream code depending on kernel build and configuration, so operators must verify each relevant artifact in their estate and apply patches or mitigations as required.

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Source: MSRC Security Update Guide - Microsoft Security Response Center