CVE-2025-39927: Ceph Client Race in Linux Kernel and Azure Linux Attestation

The Linux kernel CVE‑2025‑39927 — a Ceph client race that validates r_parent before applying state — is real, has been merged upstream, and Microsoft’s public advisory correctly notes that Azure Linux includes the implicated open‑source code and is therefore potentially affected, but that statement is a product‑scope attestation rather than a categorical declaration that no other Microsoft product can include the same code.

Background / Overview​

CVE‑2025‑39927 is an upstream Linux kernel fix addressing a race condition in the Ceph client: code that validates the cached parent directory inode (r_parent) before applying state returned by the Metadata Server (MDS) could be tricked by concurrent operations such as rename. The missing or insufficient validation could allow a reply to be applied against a stale parent inode, creating incorrect state transitions and reference accounting problems (notably mishandled CEPH_CAP_PIN bookkeeping). The vulnerability was recorded and published widely by major vulnerability feeds and distribution trackers. The upstream remediation is a defensive change that adds explicit validation of the cached parent inode and adjusts CEPH_CAP_PIN handling so pins are moved correctly when r_parent is updated; the fix has been applied in the kernel trees and distributed advisories list the CVE and the patched commits. Microsoft’s MSRC entry for the CVE contains the sentence many operators noticed: “Azure Linux includes this open‑source library and is therefore potentially affected.” That wording is factual and actionable for Azure Linux customers, but its meaning is precise and limited: it reports the completed inventory result for the Azure Linux product family and indicates Microsoft has published machine‑readable CSAF/VEX attestations for that family; it does not prove the component cannot appear in other Microsoft artifacts. Microsoft documented its phased VEX rollout in an MSRC blog post and began publishing these attestations in October 2025.

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What the CVE actually is (technical summary)​

The vulnerable surface​

• Component: Ceph client code in the Linux kernel.
• Fault: race condition where req->r_parent (cached parent directory inode) can become stale between request issuance and reply processing.
• Practical effect: upstream replies could be applied to the wrong directory inode; CEPH_CAP_PIN accounting could be left inconsistent (pins not moved), causing reference leaks or incorrect state transitions.
• Attack model: local/operation‑timing race — fault leads to potential correctness and availability problems; it is not documented as an immediate remote code‑execution vector in public advisories.

The upstream fix​

• Adds validation ensuring the cached parent inode matches directory info in MDS replies before applying state.
• Adjusts CEPH_CAP_PIN reference handling when r_parent is updated so accounting remains balanced.
• Patches merged into kernel stable trees; distributions and vendors map the kernel commits into package updates or backports.

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Why Microsoft’s wording matters — product attestation vs. exclusivity​

Microsoft’s statement that “Azure Linux includes the open‑source library and is therefore potentially affected” accomplishes two things in a single sentence:

• It provides an authoritative inventory attestation for the named product (Azure Linux), which is precisely what automated triage tools and SOC playbooks need to act immediately on those images. Microsoft began publishing CSAF/VEX attestations to provide machine‑readable mapping between CVEs and product artifacts; the VEX rollout started with Azure Linux in October 2025.
• It does not mean Microsoft is claiming that Azure Linux is the only Microsoft product that includes the Ceph code or that other Microsoft artifacts can't contain the same vulnerable code. In plain operational terms, Microsoft attested what it inspected first; additional product inventories are still in progress and will be published as they complete.

Multiple independent analyses and vendor trackers echo the same reading: the MSRC attestation is product‑scoped — a confirmation that Azure Linux was inspected — and absence of an attestation for other Microsoft products is absence of evidence, not evidence of absence. Security teams should not assume other Microsoft images or kernels are unaffected simply because Microsoft's VEX/CSAF output currently names only Azure Linux.

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

Short answer: No — not necessarily. Azure Linux is the only Microsoft product Microsoft has publicly attested to include the affected Ceph client code so far, but other Microsoft artifacts that ship or run Linux kernels could equally carry the same upstream code depending on kernel version, build configuration, and packaging choices.
Key reasons why other Microsoft products might be carriers:

• Microsoft maintains multiple Linux artifacts and kernel builds (for example, linux‑azure kernels for Azure images, CBL‑Mariner derivatives used as base images, and the WSL2 kernel that Microsoft builds and distributes). Any of these could include the affected Ceph code if built from a kernel tree containing the vulnerable commit and with Ceph client support enabled.
• Microsoft distributes Marketplace images, curated container images, AKS node images, and other artifacts that may embed an upstream kernel or userland packages. Whether those artifacts are carriers depends on build provenance and the exact packages included.
• A VEX/CSAF attestation is per‑artifact and per‑product; Microsoft’s phased approach means inventorying is ongoing and attestations are added product‑by‑product. Microsoft wrote publicly about starting with Azure Linux for VEX and expanding to other products.

Therefore, any organization running Microsoft‑supplied Linux artifacts (WSL2 on Windows hosts, Azure Marketplace images, AKS node pools, or Azure VM images) should independently verify whether the artifact includes the vulnerable Ceph code rather than relying on the current VEX/CSAF listing alone.

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Cross‑checks: independent corroboration of the CVE and Microsoft’s attestation​

To avoid single‑source dependency, the Ceph kernel fix and the CVE description are confirmed across multiple independent trackers and distribution advisories:

• The NVD / public CVE record lists the same description for CVE‑2025‑39927 and marks the record for enrichment.
• Distribution advisories (Ubuntu, SUSE, Oracle Linux) and security feeds recorded the CVE and described the same corrective action in the kernel code.
• Vendor and scanner databases (Rapid7, Tenable, Aqua) likewise document the vulnerability and map it against vendor package advisories.

For Microsoft’s VEX/CSAF program and the timing claim that Microsoft began publishing machine‑readable VEX attestations in October 2025, Microsoft’s own MSRC blog and the MSRC guidance confirm the program start and the Azure Linux rollout. That confirms the context and the operational meaning of Microsoft’s attestation sentence.

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Practical guidance for operators and Azure customers​

The correct operational posture is assume Azure Linux attestation is authoritative for Azure Linux images and verify other Microsoft artifacts directly. The following checklist prioritizes actions you should take immediately and in the short term.

Immediate (within hours)​

• Inventory: Identify all hosts, VMs, containers, and images that run Ceph client code or include Ceph kernel modules (rados, ceph FS mounts, etc.. Treat Azure Linux instances listed in Microsoft’s VEX as in‑scope first.
• For Azure Linux images, apply vendor updates as Microsoft or the Azure Linux maintainers publish patched kernel packages; the VEX/CSAF attestation provides the authoritative product mapping for automation.
• If you operate radosgw or Ceph clients, restrict network exposure and harden fronting proxies until patches are applied. While this CVE is kernel/ceph client focused, reducing exposure to race‑triggering workloads is a pragmatic step.

Short term (1–7 days)​

• Verify kernel versions and commit hashes: confirm whether the kernel binary running on VMs or images includes the patched commit from the upstream kernel trees. Use distro changelogs, package metadata, or git commit references in vendor patches.
• Inspect Microsoft‑supplied artifacts you run (WSL2 kernel package, Azure marketplace images, AKS node images, custom Marketplace appliances). Because the VEX attestation currently lists Azure Linux, check these artifacts directly — do not rely on the absence of their names from VEX as proof of safety.
• For WSL2 users: check the WSL kernel version on hosts and update WSL kernels if Microsoft releases updates that include the Ceph fix. WSL2 ships a Microsoft‑built kernel that is updated independently of the Windows release cadence.

Medium term (1–4 weeks)​

• Apply vendor backports and upgrades: watch your distribution vendor or cloud provider advisories for backported fixes; validate the changelog includes the ceph r_parent commit or a clear reference to CVE‑2025‑39927.
• Add automated checks to your CI/CD and image build pipelines to detect the presence of CVE‑2025‑39927 (or the upstream commit) in any kernel or Ceph-related package used when producing images for production.
• Request or ingest Microsoft’s VEX/CSAF artifacts into vulnerability automation tooling to reduce false positives and to have authoritative product‑scoped attestations for Azure Linux. Microsoft’s VEX rollout was announced explicitly to make such automation reliable.

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Detect, mitigate, and harden: actionable controls​

• Detection:
• Search logs for unexpected Ceph MDS reply flows and look for inode state anomalies after operations such as rename.
• Monitor kernel oopses, WARN_ON messages, or Ceph client errors on systems with mounted Ceph filesystems.
• Use file integrity and process monitoring to spot unusual state churn around Ceph client daemons.
• Immediate mitigations:
• Isolate Ceph clients from untrusted workloads where feasible.
• Use strict orchestration restart policies and supervisor backoffs to prevent crash loops from masking root cause.
• Enforce kernel updates in a coordinated maintenance window once vendor patches are available.
• Long‑term hardening:
• Build image provenance and SBOM tracing into your supply chain so you can answer “which artifacts include this kernel commit?” quickly.
• Where possible, prefer Azure Marketplace images and vendor images that publish their SBOM/CSAF/VEX attestations; ingest and act on them in your patch automation.
• Consider runtime controls (e.g., seccomp, cgroups isolation) to reduce the blast radius of local concurrency attacks in multi‑tenant environments.

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Strengths and limitations of Microsoft’s approach (analysis)​

Strengths​

• Machine‑readable attestations (CSAF/VEX): publishing deterministic VEX files for Azure Linux gives defenders an automated, authoritative input to triage and remediate quickly. This reduces ambiguity and false positives for the product named. Microsoft’s VEX blog explains how this reduces operational friction and enables faster triage.
• Transparency commitment: publicly committing to expand VEX/CSAF inventory asserts a repeatable process for auditing and attestation across product families rather than ad‑hoc advisories. This is a material win for customers that consume machine‑readable security metadata.

Limitations / Risks​

• Phased coverage introduces timing gaps: starting with Azure Linux is pragmatic, but the phased approach creates windows where only some Microsoft artifacts are attested and others are unverified. Customers with heterogeneous Microsoft artifacts must perform independent verification to close that gap.
• Potential for misinterpretation: short MSRC wording can be read by non‑specialists as an exclusivity claim — which it is not. That misunderstanding risks under‑triaging exposure in other artifacts (WSL, AKS, Marketplace images) that may be carriers. Multiple independent explainers emphasize this point: the attestation is product‑scoped, not universal.
• Dependency on vendor backports: kernel fixes are merged upstream, but the burden of timely distribution falls on vendors and cloud providers. Backport timing can vary; operators must verify package changelogs and commit hashes to be certain a fix has been delivered.

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When a vendor says “potentially affected” — how to read that in practice​

• “Potentially affected” = the vendor inspected the artifact and found the component in the build; it is a signal to treat the artifact as in‑scope for remediation until it is marked fixed. For Azure Linux customers, Microsoft’s VEX provides that determinism.
• Not saying “the rest of our products are safe.” The absence of an attestation for a given product means Microsoft has not yet published an inventory result for that product, not that it never shipped the code. This nuance matters operationally for large enterprises relying on Microsoft artifacts in many forms (VM images, containers, WSL, AKS).

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Unverifiable or open items (caution)​

• At the time of publication there is no public authoritative list beyond Microsoft’s VEX entries showing which other Microsoft artifacts (if any) include the Ceph code paths implicated by CVE‑2025‑39927. Microsoft has pledged to update CVE/VEX when additional Microsoft products are discovered to ship the component; until it does, customers must verify their own artifacts. Treat claims implying company‑wide absence of the component as unverified until Microsoft’s VEX files or vendor changelogs say otherwise.
• Public exploit activity: there are no widely reported, weaponized, public exploits targeting CVE‑2025‑39927 at disclosure in upstream advisories and distro trackers; however, timing/race bugs can be sensitive in practice and should be treated as operational priority for in‑scope artifacts. Confirm exploit activity with your threat‑intel feeds and vendor advisories; absence of reported exploitation is not proof of safety.

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Clear steps to reduce risk — a quick checklist​

• Treat Microsoft’s VEX/CSAF attestation for Azure Linux as authoritative for Azure Linux images and apply its remediation guidance immediately.
• Inventory all Microsoft‑supplied Linux artifacts you run (WSL kernels, AKS node images, Marketplace images, custom Marketplace appliances) and confirm whether they include Ceph code or carry the patched kernel commit.
• Patch or replace any affected images/kernels with vendor‑backported fixes; validate changelogs include the kernel commit for CVE‑2025‑39927.
• Harden front‑line controls for Ceph endpoints, monitor Ceph and kernel logs for anomalies, and prepare rollback/runbook steps in case of post‑patch regressions.

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Conclusion​

Microsoft’s MSRC advisory correctly identifies Azure Linux as a product that includes the open‑source code implicated by CVE‑2025‑39927 and therefore should be treated as potentially affected. That attestation is useful and authoritative for Azure Linux customers because it’s machine‑readable and designed for automation. However, it is not a blanket guarantee that other Microsoft products do not contain the same Ceph client code: absence of a VEX/CSAF attestation for other Microsoft artifacts is not proof those artifacts are free of the vulnerable code. Security teams running Microsoft‑supplied Linux artifacts must therefore perform targeted inventory and verification for their specific artifacts (WSL kernels, AKS images, Marketplace images, etc., apply vendor patches, and ingest Microsoft’s VEX/CSAF outputs as soon as they are published to automate remediation for the artifacts Microsoft has already attested. For the CVE itself, the upstream kernel fix is straightforward and backports are being tracked by distributions; treat the issue as a correctness/race and availability risk, validate vendor backports by commit or changelog, and prioritize remediation according to exposure and blast radius.

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Source: MSRC Security Update Guide - Microsoft Security Response Center