CVE-2025-39677: Linux Net Sched Backlog Bug and Azure Linux Attestation

The Linux kernel CVE tracked as CVE‑2025‑39677 fixes a subtle but real backlog accounting bug in net/sched that affects several widely used queuing disciplines (hhf, fq, fq_codel, fq_pie). Microsoft’s MSRC statement that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate as a product‑scoped attestation — it documents Microsoft’s inventory for Azure Linux at the time of the advisory — but it is not a technical proof that no other Microsoft product or image can contain the same vulnerable kernel code. Azure Linux is the only Microsoft product publicly attested so far, and customers must treat that attestation as authoritative for Azure Linux while independently verifying any other Microsoft-supplied images, binaries, or appliances in their environments.

Background / Overview​

CVE‑2025‑39677 is a Linux kernel scheduling/networking bug fixed in the upstream kernel after being assigned by the Linux kernel CVE team. The defect arises in qdisc change handlers when adjusting a qdisc’s limit under a TBF (token bucket filter) parent. When the TBF parent runs out of tokens, certain qdiscs place skbs inside a gso_skb and use peek handlers (qdisc_peek_dequeued) that account for both qlen and backlog. In the specific qdisc_dequeue_internal path, only qlen was being accounted for when pulling from gso_skb; backlog accounting (qdisc_qstats_backlog_dec) could be missed. The result: qdisc statistics and internal backlog bookkeeping could be inconsistent and can lead to a TBF parent backlog underflow or other incorrect accounting. Practical reproductions were included in the public descriptions and distribution advisories. Why this matters operationally

• The bug primarily affects availability and operational correctness: queues may show 0 packets but retain non‑zero backlog bytes; underflows and incorrect tbf backlog adjustments have been observed when exercising limit changes.
• Affected qdiscs include hhf, fq, fq_codel, and fq_pie — all commonly used in modern Linux networking stacks and traffic-control setups.
• The vulnerability is fixed in kernel stable branches (commits backported) and carriers such as Ubuntu, SUSE and other distribution advisories have documented the issue and the remediation path.

Microsoft’s wording and what it actually means​

Microsoft’s public CVE update language — the text you quoted — performs two distinct functions in practice:

• It declares that Microsoft’s inventory work for the Azure Linux product family discovered the implicated component, and Azure Linux has been attested as potentially affected (a machine‑readable CSAF/VEX attestation).
• It commits to update the CVE/VEX mapping should additional Microsoft products be found to ship the same vulnerable code.

Reading that precisely: Microsoft has completed and published the inventory result for Azure Linux; it has not asserted that other Microsoft products were inspected and found clear. The company’s phrasing reflects a phased, product‑scoped attestation model that begins with Azure Linux and expands as internal inventories complete. Treat the Azure Linux VEX/CSAF entry as authoritative for Azure Linux, and treat the absence of attestations for other products as absence of inventory mapping, not as proof of absence of the vulnerable component elsewhere.

Technical reality: why “Azure Linux only?” is the wrong question​

Whether any Microsoft product contains a given upstream open‑source component is a per‑artifact property. Several concrete technical realities explain why a single product attestation does not mean exclusivity:

• Build‑time provenance matters: inclusion of kernel code is decided at kernel-config and build time (e.g., whether a module is built-in or compiled as a module). Different product images or kernel packages can ship different kernel configurations. A vulnerability in net/sched exists in the upstream source; whether it is present in a Microsoft artifact depends on that artifact’s exact kernel config and version.
• Multiple Microsoft distribution surfaces exist: Azure Linux images, linux-azure/kernel-azure packages, WSL kernels, Azure Marketplace images, curated container or VM images, telemetry/agent bundles — each of these is a distinct artifact and must be inventoried separately.
• Statically embedded or vendor‑statically linked binaries and appliances: kernel modules, embedded images, or partner Marketplace images that include specific kernel builds or binaries can carry the vulnerable code independent of the main Azure Linux package set.
• Third‑party marketplace and partner images: Azure hosts many images built by independent vendors; those images can ship older kernels or different build pipelines and are not automatically covered by Microsoft’s attestation for a Microsoft-curated product.

In short: Microsoft’s statement is a correct, useful transparency signal for Azure Linux, but it is not evidence that other Microsoft artifacts are unaffected. The right operational posture is confirm and inventory, not assume.

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Detailed technical summary of CVE‑2025‑39677​

What the bug does (concise)​

• A logic gap in qdisc_dequeue_internal fails to decrement backlog counters in certain gso_skb dequeue paths while correctly decrementing qlen, leaving the qdisc and parent (TBF) backlog counts inconsistent. That mismatch can cause the tbf parent's backlog to underflow or to hold stale positive backlog bytes after packet drops. The misaccounting shows in tc statistics and can be reproducibly observed using the demonstration steps many distribution advisories published.

Where it lives in the tree​

• Implementations touched: net/sched/ (qdisc code)
• Affected qdiscs: hhf, fq, fq_codel, fq_pie (and by extension related fq variations)
• Introduced in older commits (kernel history shows the design evolved long ago) and fixed in recent kernel versions; the Linux kernel CVE announcement points to specific fix commits and their inclusion in stable kernels. The kernel-team listing indicates fixes landed in recent 6.x trees and provides commit references for backporting.

Exploitability and impact profile​

• Attack vector: local / operational. This is not a direct unauthenticated remote code‑execution issue.
• Privileges required: a process or tenant that can manipulate traffic control on the host or cause the host to process traffic shaping changes (local user, container, or higher).
• Primary impact: availability and correctness — incorrect accounting could cause unexpected queue behavior, unpleasant edge-cases in traffic shaping, or metric mismatches that mask real conditions.
• Practical severity: medium in many distributions’ advisories; the largest risk is in multi‑tenant shared hosts, network appliances, or any system where qdisc behavior governs tenant isolation or traffic shaping and where stale backlog accounting can have cascading operational effects.

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Cross‑checks and independent confirmation​

Key claims in public advisories align across multiple authoritative sources:

• The Linux kernel CVE announcement and stable commit references explain the technical fix and list the affected files and qdiscs.
• NVD and major distributions (Ubuntu, SUSE) published CVE entries describing the same root cause, affected qdiscs, and the expected remediation (update kernel or apply upstream commits).

These independent confirmations make the technical facts robust: the vulnerability exists as described and is fixed upstream; distribution vendors will map fixes into their kernel packages on their release/backport schedules.

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Operational guidance — how to verify exposure (step‑by‑step)​

The only reliable way to determine whether a particular host or product artifact is affected is to inspect the actual artifact. The following steps prioritize high‑risk surfaces and are actionable for administrators.

1. Inventory high‑priority artifacts​

• List all Linux kernels and kernel-derived artifacts you run: Azure Linux images, Azure Marketplace VM images, guest kernels inside VMs, WSL kernels deployed on Windows hosts, container host kernels, and any appliance or device images that embed Linux kernels.
• Include third‑party and partner Marketplace images — they may use kernels that differ from Microsoft‑curated images.

2. Check kernel version and patch status​

• On a running Linux host: uname -a to capture kernel version.
• Confirm the vendor kernel package changelog or security advisory contains the upstream commit IDs mentioned in the kernel CVE announcement — search packaged kernel changelogs for the specific fix commit (the upstream stable commit hashes are referenced in the Linux kernel CVE announcement).
• If building from source, confirm your tree includes the upstream fix commit(s) (the commit hashes are in the kernel announcement and stable branch links).

3. Inspect qdisc behavior (repro test)​

• The published reproducible demonstration uses tc: add the TBF parent, attach an affected qdisc (e.g., fq), generate local traffic, then change the qdisc limit to 0 and observe qdisc statistics with tc -s -d qdisc show dev <dev>.
• If you observe 0 packets but positive backlog bytes after the change, the pre‑fix behavior is present; after installing the patch, the inconsistency should not reproduce. (Only perform such tests in controlled environments.

4. Search images and artifacts for kernel versions​

• For images (VM or container): inspect the kernel packages inside the image (package manager query), or examine the /boot/vmlinuz-* and module lists to identify whether the affected qdisc modules are present.
• For WSL or other Microsoft-provided kernels: check the specific WSL kernel build version Microsoft ships and consult Microsoft VEX/CSAF entries where Microsoft has attested its inventory. Microsoft’s attestation for Azure Linux applies specifically to that product family; other Microsoft artifacts should be checked individually.

5. For appliances and vendor kernels​

• Contact the vendor for confirmation if appliance kernels cannot be easily inspected. Vendors must state whether the upstream fix has been backported. The kernel team recommends installing vendor updates rather than cherry‑picking commits.

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Remediation and mitigation​

• Primary remediation: install vendor-supplied kernel updates that include the upstream fixes (preferred).
• If upgrading the kernel is not possible immediately, consider:
• Isolating affected hosts from untrusted tenants and reducing attack surface (disable untrusted local process capabilities, restrict container privileges).
• Avoid performing qdisc limit changes as an operational mitigation unless necessary; be cautious with tc changes on multi‑tenant hosts.
• Enable robust kernel crash logging (kdump) and central log collection to capture evidence if systems experience weird qdisc behavior or underflows.

For Azure Linux customers

• If you run Microsoft’s Azure Linux images, Microsoft has published a CSAF/VEX attestation stating Azure Linux includes the component and is therefore potentially affected; follow Microsoft’s update guidance for those images. That VEX attestation is authoritative for Azure Linux products. However, do not assume the attestations cover other Microsoft-provided artifacts — they may not.

For other Microsoft artifacts (WSL, Marketplace images, curated containers, agents)

• Do not rely on the Azure Linux attestation as a universal statement. Inventory the specific artifact and verify its kernel version or ask the vendor/owner of that artifact for a VEX/CSAF mapping or confirmation.
• If you run Microsoft-curated service images (AKS node images, Azure Marketplace images, agent images), consult the product-specific advisory or product support channel for confirmation and remediation timelines.

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Practical checklists for different teams​

For cloud operators (Azure / multi‑tenant hosts)​

• Inventory which VM images and guest images you run, including third‑party Marketplace images.
• Confirm kernel package versions and whether vendor updates contain the upstream fix commit.
• For multi‑tenant hosts, prioritize reboot windows to apply patched kernels where feasible.
• If immediate patching is impossible, isolate tenants and enforce minimal privileges for network/tc operations.

For devops & CI/CD teams​

• Scan build images and runner images for kernel packages and check their versions against the fixed kernel list.
• Rebuild and redeploy any custom VM images with patched kernels.
• Ensure container host kernels are patched; container images themselves generally do not contain kernel code but may contain userspace tools that interact with qdiscs.

For Windows teams that run Linux artifacts (WSL, nested VMs, tooling)​

• If using WSL with a Microsoft-provided kernel, check the WSL kernel version and Microsoft’s advisories.
• Inspect any Linux VMs or guest images running on Hyper-V/VMware for vulnerable kernel versions.

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Risk assessment and final analysis​

Strengths of the advisory ecosystem

• The kernel-team CVE announcement, NVD, and multiple distribution advisories converge on the technical details, the affected qdiscs, and the remediation route. This multi‑source corroboration reduces ambiguity about the vulnerability’s nature and the fix.
• Microsoft’s adoption of machine‑readable CSAF/VEX attestations (starting with Azure Linux) is a positive transparency step: it gives customers deterministic automation inputs for one important product family. Microsoft’s pledge to expand attestations when other internal products are identified is operationally useful and reduces time‑to‑answer for Azure Linux customers.

Limitations and residual risk

• Microsoft’s attestation covers Azure Linux only so far. Other Microsoft artifacts (WSL kernel, Marketplace images, custom appliances or static binaries) may include the vulnerable code; absence of an attestation for those SKUs is not proof of safety. Customers must perform artifact-level inventory.
• Vendor lag and the “long tail” problem: embedded devices, vendor kernels, and third‑party Marketplace images can remain exposed for long periods if vendors do not promptly backport fixes.
• Operational complexity: qdisc behavior and backlog accounting bugs can be subtle in production; reproducing the issue requires controlled testing. Some environments may detect symptoms only after an operational incident.

Unverifiable claim flag

• It is not possible to assert definitively — from public advisories alone — whether every Microsoft product, Marketplace image, or appliance worldwide is free of the vulnerable code. That claim would require Microsoft (or the artifact owner) to publish attestations for each product. Microsoft has committed to doing more attestation work, but until then the only definitive statements are the product-scoped attestations themselves. Where other Microsoft products are critical to your estate, request explicit attestations or inventory proofs from Microsoft support or the product team.

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Clear verdict: directly answering the user’s question​

• Short answer: No — Azure Linux is not necessarily the only Microsoft product that could include the affected net/sched code and therefore be potentially affected.
• Precise interpretation: Azure Linux is the only Microsoft product Microsoft has publicly attested as including the vulnerable component for CVE‑2025‑39677 at the time of the advisory. That attestation is authoritative for Azure Linux and useful for customers running those images. It is not a blanket denial that other Microsoft artifacts might contain the same vulnerable kernel code. Determining exposure in other Microsoft products requires artifact‑level inspection or additional Microsoft‑published attestations.

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Final recommendations (prioritized)​

• If you run Azure Linux images: apply Microsoft’s recommended updates for those images immediately and use Microsoft’s VEX/CSAF output as a machine-readable signal for automation.
• Inventory and verify every Microsoft-provided image and artifact you run (Marketplace, WSL kernel, curated containers). Do not assume product-level attestations generalize across Microsoft’s entire portfolio.
• Patch kernels to a vendor-supplied updated package that includes the upstream fix; prefer vendor updates over cherry‑picking commits. If you cannot patch quickly, isolate affected systems and limit local operations that modify qdiscs.
• For third‑party Marketplace images or appliances, obtain confirmation from the image vendor that their kernels include the upstream fix or that they have backported it.
• Embed these verification steps into your CVE triage playbooks: whenever a vendor publishes a product‑scoped VEX/CSAF attestation, treat that attestation as authoritative for the product named, and immediately follow the artifact‑level inventory approach for any other images you run.

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CVE‑2025‑39677 is an example of how a small accounting bug in the kernel’s networking qdisc code can have outsized operational effects, particularly in shared, multi‑tenant, or appliance contexts. Microsoft’s move to publish product‑scoped CSAF/VEX attestations — starting with Azure Linux — materially improves transparency for operators using those images, but it does not obviate the need for artifact‑level verification across the broader estate. Treat Microsoft’s statement as an authoritative signal for Azure Linux, and treat all other Microsoft artifacts as candidates for independent inventory until attestations explicitly cover them.

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Source: MSRC Security Update Guide - Microsoft Security Response Center