Microsoft’s brief advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate — but it is a
product‑scoped attestation, not a categorical claim that Azure Linux is the only Microsoft product that could include the vulnerable code.
Background / Overview
CVE‑2025‑38067 is a Linux kernel robustness fix: the kernel’s rseq registration path could dereference a leftover rseq_cs pointer and cause a segfault on return to user space. The upstream change clears rseq_cs on registration when it is non‑zero to avoid the crash while retaining compatibility with older glibc behavior that reuses rseq areas. The vulnerability and fix are recorded in public vulnerability databases and distribution advisories (NVD, Debian, Ubuntu and others). Concurrently, Microsoft’s Security Response Center (MSRC) published a machine‑readable attestation as part of its phased Vulnerability Exploitability eXchange (VEX) rollout, stating that Azure Linux includes the implicated open‑source library and is therefore potentially affected. Microsoft also announced that it began publishing CSAF/VEX attestations starting in October 2025 and will update CVE mappings if additional Microsoft products are found to ship the same component. That combination — an upstream kernel patch plus a vendor product attestation for Azure Linux — is the context for the common question: is Azure Linux the only Microsoft product affected?
What CVE‑2025‑38067 actually is (technical summary)
- Component: Linux kernel rseq registration code.
- Fault: the kernel did not enforce the documented requirement that the rseq_cs field be zeroed by user space before registration. If rseq_cs contained a stale pointer that did not point to a valid struct rseq_cs, a segfault could occur when returning to user space.
- Upstream remedy: clear rseq_cs at registration when non‑zero (instead of failing registration), preserving compatibility with older glibc variants that reuse rseq memory areas. This both prevents the immediate segfault and avoids breaking older runtimes that would otherwise abort secondary threads on failed registration.
Independent distro trackers and vulnerability feeds (Debian, Ubuntu, Red Hat, Amazon Linux etc. have ingested the upstream fix and mapped it into their kernel packages; operators should treat the issue as a kernel robustness/availability problem that is remediated by applying the fixed kernel packages.
What Microsoft actually stated — reading the MSRC language carefully
Microsoft’s public advisory includes two linked statements:
- Azure Linux has been inspected and the implicated upstream component was found in the Azure Linux artifacts; Azure Linux is therefore potentially affected. This is a machine‑readable VEX/CSAF attestation targeted to that product family.
- Microsoft began publishing CSAF/VEX attestations in October 2025 and will update CVE entries if additional Microsoft products are later identified as carriers of the same code. This is a procedural commitment to expand inventory coverage over time.
Those two sentences together are deliberate. They tell customers what Microsoft has verified so far (Azure Linux), and they promise continued inventory work. They do
not mean Microsoft has scanned every possible Microsoft artifact and confirmed absence everywhere else. Absence of an attestation for another Microsoft product is “not yet attested,” not definitive evidence that the component cannot be present in other Microsoft images or binaries. Multiple independent analyses and vendor guidance have emphasized this distinction.
Short answer to the user’s question
- Is Azure Linux the only Microsoft product that includes the implicated open‑source library and is therefore potentially affected?
- Short, plain answer: No — not necessarily. Azure Linux is the only Microsoft product Microsoft has publicly attested to include this particular kernel code so far, but that attestation is product‑scoped and time‑boxed; other Microsoft artifacts that ship Linux kernels or embed kernel code (WSL kernels, linux‑azure kernels, Marketplace images, vendor appliances, container images, etc. could include the same code depending on build configuration and kernel version. Microsoft has committed to update the CVE/VEX record if additional products are identified.
Why that distinction matters (technical and operational nuance)
- Product vs artifact: A vendor’s VEX/CSAF attestation is typically a product‑level statement. Microsoft’s Azure Linux attestation applies to the specific Azure Linux artifacts that Microsoft examined and published in its VEX feed. That does not automatically extend to other Microsoft SKUs, which may be built with different kernel trees, config flags and packaging choices. Kernel features can be built as modules, compiled into vmlinuz, or omitted entirely — presence is determined at build time.
- Inventory phasing: Microsoft deliberately started its VEX rollout with Azure Linux (formerly CBL‑Mariner) as a practical first target and will expand coverage over time. During that phased rollout, many Microsoft artifacts remain “unattested” rather than explicitly “not affected.”
- Real‑world implication: For defenders and operators, an attestation that names a product is actionable evidence to patch those product images. But for any Microsoft image or component not yet present in the VEX feed, the operator should treat it as “unverified” and perform per‑artifact validation where appropriate.
Cross‑checking the claim: independent verification
To validate both the technical details of CVE‑2025‑38067 and Microsoft’s attestation semantics, consult at least two independent sources:
- Canonical vulnerability trackers: NVD and major distro trackers (Ubuntu, Debian) document the kernel fix and describe the defect. These sources verify the nature of the bug and the upstream commits that remedied it.
- Microsoft’s public transparency post: MSRC’s VEX blog explains the phased approach (October 2025 launch) and the product‑by‑product attestations starting with Azure Linux. That blog is Microsoft’s own explanation of the programmatic approach used to generate the Azure Linux VEX outputs.
Putting those together: the CVE is a kernel rseq fix documented upstream and in distro advisories, and Microsoft’s statement is a product‑level inventory attestation for Azure Linux produced as part of a broader, phased VEX rollout.
Practical guidance for IT teams and security operators
Operators running Microsoft artifacts should not assume “no attestation equals not affected.” Instead, follow a short, prioritized operational plan:
- Immediate priority: If you run any Azure Linux images (Microsoft‑published Azure Linux), treat Microsoft’s VEX attestation as authoritative and apply the published kernel updates or distro packages that contain the upstream fix. Reboot where kernel updates are required.
- Inventory other Microsoft artifacts: Identify other Microsoft‑published images you run:
- WSL2 kernels or custom WSL distributions
- linux‑azure kernels supplied via Microsoft channels
- Azure Marketplace images and appliances
- Microsoft container base images (where kernels may not be present, but other runtime components might be)
For each artifact, verify kernel version, kernel configuration and module presence (search for rseq-related symbols or confirm the kernel package changelog references the upstream commit).
- Check SBOMs / VEX feed: Subscribe to Microsoft’s CSAF/VEX feed and your distro vendors’ advisories. Microsoft has published a CSAF provider metadata endpoint as part of the program and will update the VEX entries when additional products are identified. Automation here reduces manual effort.
- Per‑artifact validation techniques:
- Inspect kernel version: uname -a and compare to distro advisory fixed versions.
- Search the kernel config (if available in /proc/config.gz or the package) for the relevant CONFIG_ flags that control whether the rseq code is built into the kernel or as a module.
- For cloud images, inspect the kernel package changelog or use vendor advisory mapping (distributions list upstream commit IDs included in their packages).
- If you maintain custom kernels, ensure the stable backport containing the rseq fix is merged into your build.
- Short‑term mitigations if you cannot patch immediately:
- Restrict untrusted workloads’ ability to register rseq entries (if operationally feasible).
- Isolate multi‑tenant or CI runners that create large numbers of threads or run untrusted user code.
- Monitor for process crashes or dmesg kernel oops traces correlated with rseq registration paths.
Detection and forensic cues
This CVE is a kernel robustness/segfault condition; detection focuses on kernel and process telemetry:
- Kernel logs (dmesg, journalctl -k) containing traces or oopses tied to rseq or the thread registration path.
- Application crashes immediately after thread creation or registration operations (look for native process segfaults in core dumps).
- Reproducible crashes in environments that reuse rseq areas across thread creation (older glibc behavior). Reproduce in controlled lab environments to confirm the presence or absence of the fixed behavior.
Strengths of Microsoft’s approach — why the VEX attestation matters
- Machine‑readable attestation reduces noise for automation: a deterministic “Known Affected / Not Affected / Fixed / Under Investigation” entry for Azure Linux lets security teams triage alerts more accurately and avoid wasted effort. Microsoft’s October 2025 VEX announcement explains this intent.
- Actionable remediation: By attesting Azure Linux explicitly, Microsoft gives operators a clear, authoritative signal to apply updates for those images immediately. That is materially useful for large cloud operators who run many Azure Linux images.
Risks and limitations — what still leaves security teams exposed
- Phased coverage creates temporary “unknowns”: Until Microsoft completes inventory for all of its product families, other Microsoft artifacts remain unattested rather than explicitly “not affected.” Operators must therefore treat those artifacts as unverified and perform per‑artifact checks when risk warrants.
- Per‑artifact variability: Kernel features are toggled at build time. A single vendor may ship multiple kernels with distinct CONFIG_* flags; whether a particular image carries a vulnerable code path is an artifact‑level property. This complicates binary or package scanning heuristics that assume “vendor = safe.”
- Long‑tail and embedded images: Appliances, marketplace images, or older prebuilt artifacts can carry older kernels or static linkages that are slow to be inventoried or updated. These are the likely places a vulnerable component could persist outside the attested Azure Linux family.
Recommended checklist (quick reference)
- For Azure Linux images:
- 1. Apply Microsoft‑published Azure Linux kernel/distro updates immediately.
- 2. Reboot affected hosts and validate kernel changelogs reference the fix.
- For other Microsoft artifacts:
- 1. Map artifacts in use (WSL, linux‑azure, Marketplace images).
- 2. Verify kernel versions and module lists against distro advisories.
- 3. Query Microsoft’s CSAF/VEX feed for updates (the feed is being extended).
- For custom kernels:
- 1. Merge the upstream rseq backport from stable trees into your build and run a test matrix against older glibc behavior if you must preserve backward compatibility.
Closing analysis and final recommendations
Microsoft’s MSRC attestation that Azure Linux “includes this open‑source library and is therefore potentially affected” is an authoritative, machine‑readable declaration for the named product family. That attestation is meaningful and actionable for Azure Linux customers — it was precisely the purpose of Microsoft’s October 2025 VEX rollout. However, that wording is
not a technical proof that no other Microsoft product could contain the same vulnerable Linux kernel code. The presence of kernel subsystems, drivers or code paths is a build‑time artifact decision; Microsoft ships multiple kernel artifacts across its product portfolio (Azure images, WSL kernels, linux‑azure builds, marketplace appliances, etc. and those artifacts must be inventoried individually. Until Microsoft publishes additional VEX attestations for those products, operators should treat non‑Azure Linux Microsoft artifacts as “unverified” and perform per‑artifact checks where necessary.
Actionable takeaways:
- Prioritize patching Azure Linux images now — Microsoft’s attestation gives you that signal.
- Maintain an inventory of all Microsoft kernel artifacts you run and verify them individually.
- Subscribe to Microsoft’s CSAF/VEX outputs and your distro vendors’ advisories to capture any updates Microsoft publishes as its inventory work progresses.
This combined approach — trust Microsoft’s attestation where it exists, and verify elsewhere where it does not — balances the benefit of vendor transparency with the reality that large vendors roll out attestations in phases and that kernel presence is an artifact‑level fact.
Source: MSRC
Security Update Guide - Microsoft Security Response Center