Azure Linux CVE-2025-39829 Attestations Explained

Microsoft’s initial advisory for CVE-2025-39829 makes a narrow, but important, claim: Azure Linux is the Microsoft product Microsoft has identified so far as including the affected open‑source component (the kernel trace fgraph notifier code), and Microsoft will update its CVE/VEX attestations if additional Microsoft products are discovered to be affected.

Background / Overview​

CVE-2025-39829 is a Linux kernel issue fixed in upstream kernels in mid‑ to late‑2025. The defect arises in the kernel’s trace/fgraph code path: when writing to the function_profile_enabled interface, a notifier callback registration could be left in place if start_graph_tracing failed, producing a repeated warning — specifically, the kernel log message “notifier callback ftrace_suspend_notifier_call already registered” and an associated stack trace. The upstream patch simply added an unregister call on the exception path to avoid the duplicate registration. This is a correctness/warning fix rather than a memory‑corruption or code‑execution primitive, but it can surface as a reliability issue in test or production kernels. Multiple vendor trackers (Debian, Ubuntu, SUSE) and common vulnerability feeds list the CVE, classify it as moderate in impact and local in vector, and show which kernel branches received the fix. Those trackers also show which distribution kernel versions remained vulnerable until patched.

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What Microsoft said — and how to read it​

Microsoft’s public position follows the company’s recently expanded transparency program: Microsoft now publishes machine‑readable CSAF/VEX attestations that map upstream CVEs to Microsoft product artifacts, starting with Azure Linux. That program is designed to provide deterministic, automatable answers about which Microsoft SKUs ship a given open‑source component and whether those SKUs are Known Affected, Not Affected, Under Investigation, or Fixed. Microsoft’s VEX rollout is described in the MSRC blog announcing VEX for Azure Linux. When Microsoft’s update guide says Azure Linux “includes this open‑source library and is therefore potentially affected,” the phrase is best understood as a statement of current, validated scope — not an absolute exclusion of other products. Put plainly:

• Microsoft has validated and published a VEX/CSAF attestation for Azure Linux that maps this kernel component and its CVE status.
• Microsoft has not yet publicly mapped other Microsoft products to this specific CVE; the company has pledged to update the CVE record / VEX artifacts if further Microsoft product impact is found.

This is a standard industry approach: vendor attestations document what the vendor has verified at disclosure time and are updated when additional internal product mappings are discovered. The critical operational implication is that the vendor’s attestation reduces uncertainty for Azure Linux customers, but it does not absolve operators of the responsibility to verify other Microsoft‑supplied kernels or images they run.

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Technical reality: why “is Azure Linux the only Microsoft product?” is not a yes/no question​

A few facts change how you should interpret Microsoft’s wording:

• The presence of a kernel feature or module (trace/fgraph, ntfs3, cfg80211, etc. is determined by how a particular kernel was configured and built, not by the upstream source alone. A kernel config flag like CONFIG_FTRACE or CONFIG_FUNCTION_TRACER will govern whether the fgraph code paths exist in a given kernel build. That makes product mapping a build-time provenance question rather than a binary yes/no about Microsoft as a whole.
• Microsoft publishes multiple kernel artifacts and images across its cloud and tooling portfolio: Azure Linux (azl3), linux‑azure / kernel‑azure packages, marketplace images, container/VM images, and the WSL2 kernel are distinct distribution surfaces. Any of those artifacts may or may not include the same kernel config and therefore may or may not include the affected code. A single Microsoft statement naming one product does not automatically cover all these artifacts.
• Practically, a vendor will often start VEX/CSAF publication with a single product family (Azure Linux) and expand coverage — Microsoft expressly followed this phased model in its VEX rollout. That is a process decision, not a technical guarantee that other products are safe.

Therefore, the correct operational reading of Microsoft’s statement is: Azure Linux is the only Microsoft product Microsoft has publicly validated and attested for this CVE so far — but other Microsoft‑supplied kernel artifacts may also include the affected code and must be verified individually.

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Cross‑checks and independent corroboration​

To avoid relying on a single vendor statement, security teams should cross‑verify using multiple independent sources:

• The NVD entry and upstream kernel commits provide the canonical technical description of the bug and the patch semantics (unregister notifier in the exception path). These sources confirm the defect is in trace/fgraph and was fixed upstream.
• Distribution security trackers (Debian, Ubuntu, SUSE) show which package versions were vulnerable and list fixed versions, providing a way to map kernel package versions to the upstream fix. Use these trackers to find vendor‑backported packages.
• Microsoft’s VEX/CSAF outputs and the MSRC transparency blog explain how Microsoft will publish machine‑readable attestations for Azure Linux and why those attestations should be trusted for that product. The MSRC blog also documents the phased approach and the meaning of the attestation categories.

Taken together, these sources validate three important facts: (1) the bug exists and was fixed upstream; (2) distributions and vendors are packaging fixes into kernels and publishing advisories; and (3) Microsoft’s Azure Linux attestation is a vendor‑level mapping, not a global safety guarantee for all Microsoft products.

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Practical guidance — how administrators should respond now​

Short version: inventory → verify → patch → validate. Microsoft’s attestation narrows the immediate blast radius for Azure Linux customers, but host-level verification is the only authoritative way to prove whether a given system is affected.

1. Inventory: quickly identify Microsoft images and kernels in your estate​

• Enumerate Azure VMs, scale sets, AKS node images, Marketplace images and other images that use Microsoft‑published kernels (look for linux‑azure, kernel‑azure, azl3 packaging). Microsoft’s VEX files for Azure Linux provide machine‑readable attestations for those images — use that to accelerate mapping for Azure Linux images.
• For Windows hosts, enumerate WSL2 installations and custom WSL kernels: WSL2 runs a Microsoft‑packaged kernel; if you or your users deployed custom WSL kernels, those are in scope for separate inspection.

2. Verify on hosts — quick, reliable checks​

Run these commands on candidate hosts to confirm whether the affected kernel code is present:

• Check whether trace/fgraph or relevant ftrace features exist in the running kernel config:
• zgrep -w CONFIG_FUNCTION_TRACER /proc/config.gz
• grep -w CONFIG_FUNCTION_TRACER /boot/config-$(uname -r)
• Inspect loaded modules and function tracer interfaces where relevant (for other CVEs this would vary, but the pattern is the same):
• lsmod | grep ftrace (or the module name)
• sudo modinfo <module> (if module is modularized)
• Confirm package-level provenance: on package-managed systems examine the kernel package changelog and vendor advisories:
• Debian/Ubuntu: apt changelog linux-image-$(uname -r) or consult distro security tracker pages.

These checks give deterministic answers about whether the specific kernel instance running on a host includes the vulnerable code. If the code is not present, the host is not in scope for this CVE; if it is present, treat the host as within scope and remediate accordingly.

3. Patch and mitigate​

• Locate vendor‑mapped fixes (using NVD and distro trackers) and apply the vendor kernel package that contains the upstream fix. Reboot into the patched kernel.
• If you cannot immediately patch, apply compensating controls: restrict who can write to function tracer interfaces, disable the interface where feasible, or move untrusted image processing to hosts that do not include the tracer code. These are short‑term mitigations only.

4. Validate and document​

• After patching, confirm the kernel changelog references the upstream fix or CVE (the distribution changelog will often include the upstream commit hash).
• Reboot and verify the kernel is running the patched version: uname -r and kernel logs should no longer show the WARN_ON signature or duplicate registration messages.

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Risk analysis — strengths, caveats, and residual exposure​

Strengths

• The upstream fix is surgical and small; that reduces regression risk and makes backporting to stable vendor kernels straightforward. Distribution trackers already show fixed versions across many branches.
• Microsoft’s VEX/CSAF program for Azure Linux materially reduces ambiguity for that product family: customers using Azure Linux can automate triage decisions and reduce false positives when the vendor marks an attestation as Not Affected / Known Affected / Fixed.

Caveats and residual risk

• Vendor attestations are only as accurate as the vendor’s product cataloguing and build‑time metadata. Microsoft’s current attestation scope is Azure Linux; other Microsoft kernels (linux‑azure artifacts, WSL kernels, third‑party marketplace images repackaged for Azure) may or may not ship the same component depending on build choices. Administrators must verify hosts individually rather than relying on a single statement.
• The technical class of this CVE is a local correctness/warning defect. While that reduces the immediate risk of remote code execution, reliability issues at kernel level can still cause service disruption in multi‑tenant or automated environments where an attacker can present crafted inputs (or mount crafted images). Treat the risk as availability/operational continuity rather than confidentiality or integrity breach.
• Image drift and custom builds create a long tail: Marketplace appliances, third‑party images, and self‑built kernels in CI pipelines can remain unpatched or ship different kernel configs. Those long tails require targeted discovery and communication with image publishers.

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How to interpret Microsoft’s promise to “update the CVE” if other products are affected​

Microsoft’s wording — that it “will update the CVE to reflect this” if impact to additional products is identified — is both helpful and procedural:

• Helpful because it commits Microsoft to expand the VEX/CSAF attestations and the CVE product mapping when new internal artifact mappings are discovered. That reduces future uncertainty.
• Procedural because such updates occur after Microsoft’s internal inventory and triage pipelines find additional affected artifacts. There can be a timing delay between discovery and public mapping; during that window, operators should not assume absence in other Microsoft products. Verify host‑by‑host.

In operational terms: use Microsoft’s attestation as authoritative for the product it covers (Azure Linux), but continue to inventory other Microsoft-supplied images and kernels you run and apply the verification steps above.

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Checklist for WindowsForum readers and system operators​

• Inventory your Azure images and check Microsoft’s Azure Linux VEX/CSAF JSON for this CVE to see if your image SKUs are listed as Known Affected or Fixed. Use automated ingestion of VEX files where possible.
• On any Linux host (Azure VM, on‑prem VM, or container host) run the verification commands below to confirm presence or absence:
• zgrep -w CONFIG_FUNCTION_TRACER /proc/config.gz || grep -w CONFIG_FUNCTION_TRACER /boot/config-$(uname -r)
• lsmod | grep ftrace
• sudo modinfo <module name> (if modularized)
• Check kernel package changelog: apt changelog linux-image-$(uname -r) / rpm -q --changelog kernel-$(uname -r)
  These checks will quickly tell you whether the affected code is present and whether the running kernel matches a vendor package that has been patched.
• For WSL2 users: check the WSL kernel config in the running instance (zcat /proc/config.gz | grep -w CONFIG_FUNCTION_TRACER) or inspect the WSL2 kernel repo if you use Microsoft’s published kernel. Custom WSL kernels require custom rebuilds to incorporate fixes.
• Prioritize remediation for: multi‑tenant hosts, image‑processing systems, CI/CD runners, virtualization hosts, and any system that mounts or processes third‑party images. These are highest blast radius locations.

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Conclusion​

Microsoft’s public statement that Azure Linux is the Microsoft product that includes the open‑source kernel component relevant to CVE-2025-39829 is accurate as a description of the current, validated scope of Microsoft’s VEX/CSAF attestations. That attestation is useful: it gives Azure Linux customers a clear, machine‑readable signal they can automate against. However, it is not a blanket guarantee that no other Microsoft product or image includes the same kernel code. The presence of the affected code is governed by per‑artifact kernel configuration and build choices, and Microsoft maintains multiple kernel artifacts across Azure, marketplace images, and developer tooling (for example, WSL). Operators must therefore perform host‑level verification (module checks, kernel config inspection, package mapping) and follow vendor/distro advisories to identify and remediate exposed systems. Cross‑checking the upstream patch, distribution trackers, and Microsoft’s VEX output gives the most reliable picture for prioritization and remediation. In short: Azure Linux is the only Microsoft product Microsoft has publicly attested as affected at this time — but it should not be the only place you look. Inventory, verify, patch, and validate across your estate.

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Source: MSRC Security Update Guide - Microsoft Security Response Center