CVE-2024-27018: Azure Linux Carrier and Microsoft Kernel Risk

Microsoft’s brief public wording — “Azure Linux includes this open‑source library and is therefore potentially affected” — is accurate as a product‑level attestation, but it should not be read as a categorical guarantee that no other Microsoft product could include the same vulnerable Linux kernel code. In plain terms: Azure Linux is a confirmed carrier and should be treated as such immediately; however, any Microsoft product or image that ships a Linux kernel (or a kernel‑derived artifact) is a plausible carrier until inventories or machine‑readable VEX/CSAF attestations say otherwise.

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Background / Overview​

What is CVE‑2024‑27018?​

CVE‑2024‑27018 is a Linux kernel vulnerability in the netfilter/bridge integration code: specifically, a logic error in br_netfilter where packets delivered to tap devices while a bridge is in promiscuous mode can reach the conntrack input hook when they should not. That misrouting can cause kernel warning splats, confuse connection‑tracking state, and in some downstream cases produce availability or reliability issues. The NVD summarizes the defect and the upstream fix that resets conntrack handling for those promiscuous packets.
Security trackers and multiple distribution vendors describe the practical impact as a high‑severity networking bug that can produce repeated kernel warnings, conntrack table confusion, and a denial‑of‑service‑style availability impact in noisy multi‑tenant or bridged VM/container deployments. The CVSS v3.1 base score recorded by some downstream trackers is consistent with an important local/low‑privilege attack that affects confidentiality, integrity and availability in certain configurations.

Why this bug matters in cloud and virtualized environments​

The vulnerability lives at the intersection of three commonly used features in clouds and virtual platforms:

• Bridging: Linux bridges are used to connect VM or container interfaces to a host network segment.
• Promiscuous mode: Bridges or NICs placed in promiscuous mode accept all traffic so that tap devices or monitoring processes can inspect it.
• Conntrack / netfilter: Connection tracking is central to many firewall/NAT behaviors (iptables, nftables, NAT, MASQUERADE). Improper input hook behavior can lead to incorrect state or spurious error paths.

Where clouds, hypervisors, or container hosts use bridges with taps in promiscuous mode (a common pattern when attaching VMs, certain CNIs, or virtual network appliances), the bug can be exercised by any tenant or workload that can send crafted traffic across the bridge. In practice, this translates to a real risk for multi‑tenant hosts, managed container nodes, and virtual appliances unless kernels are patched or configurations changed.

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Microsoft’s public message and what it actually says​

The MSRC wording and Microsoft’s VEX/CSAF rollout​

Microsoft’s Security Response Center (MSRC) entries for third‑party CVEs often include a short “FAQ” line that summarizes Microsoft’s inventory findings for a given CVE. For CVE‑2024‑27018 (and other Linux kernel issues), Microsoft used the phrasing the user quoted: “Azure Linux includes this open‑source library and is therefore potentially affected by this vulnerability.” That wording is a product‑scoped attestation: Microsoft has completed an inventory for the Azure Linux distribution and found the implicated upstream code in that product family. Microsoft also committed to publishing machine‑readable VEX/CSAF attestations beginning in October 2025 and to update CVE entries if other Microsoft products are later found to ship the component.
Two important operational takeaways flow from that wording:

• The attestation is authoritative for Azure Linux — if you run Azure Linux images supplied by Microsoft, treat that product as a confirmed scope and follow Microsoft’s remediation guidance.
• The attestation is not proof of exclusivity — it is not a technical guarantee that other Microsoft products, kernels, or images do not include the same upstream component. It is a statement of inventory completion for the product Microsoft has inspected so far.

Why Microsoft starts with Azure Linux​

Microsoft’s public transparency program deliberately started with the Azure Linux distribution (formerly CBL‑Mariner) because it is Microsoft’s published Linux distribution and therefore the most practical initial target for accurate SBOM/VEX mapping. Microsoft’s blog about the VEX rollout explains that the company planned a phased approach — publish deterministic attestations where inventory is complete, then expand coverage as inventories for other products finish. This approach helps automate triage for customers while Microsoft scales its internal inventory work.

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Is Azure Linux the only Microsoft product that includes the affected code?​

Short answer: No — not necessarily.
Long answer: Azure Linux is the only Microsoft product that Microsoft has publicly attested to include the implicated open‑source kernel component for CVE‑2024‑27018 at the time of the advisory. That is an authoritative statement for Azure Linux customers. But Microsoft ships and maintains multiple distinct Linux‑kernel artifacts — and the underlying vulnerability is in upstream kernel source code. Any Microsoft product or image that ships a Linux kernel build containing the vulnerable commit range and the relevant kernel configuration (i.e., netfilter / bridge / conntrack built in or available as modules) is a plausible carrier and should be treated as potentially affected until verified.

Microsoft products or artifacts you should explicitly consider​

• Azure Linux distribution (confirmed) — Microsoft has attested Azure Linux as including the implicated library; treat Azure Linux images as known, potentially affected artifacts.
• Windows Subsystem for Linux (WSL2) kernel images — Microsoft publishes and ships a WSL2 kernel and keeps a public repository of that kernel tree; that kernel can and often does include netfilter and related modules depending on configuration. If the WSL kernel running on your Windows host contains the vulnerable code, it could be impacted in scenarios where promiscuous bridging is used (for example, networking modes that expose host bridging). Administrators should verify the WSL kernel version and configuration.
• linux‑azure / Azure VM kernel packages and Marketplace images — Azure VM images sometimes run kernels that Microsoft builds or “tunes” for Azure. These kernels can contain upstream netfilter subsystems and therefore should be inventory‑checked.
• CBL‑Mariner derivatives and internal/managed appliance images — Microsoft uses CBL‑Mariner lineage in several internal and customer‑facing artifacts. Those artifacts are separate builds and may or may not include the same components; they must be checked individually.
• Third‑party Marketplace appliances or partner images that Microsoft distributes — some Marketplace items ship with vendor kernels or kernel modules that include upstream code; those artifacts are outside the Azure Linux attestation unless explicitly enumerated. Treat them as unverified until their SBOMs or VEX attestations indicate otherwise.

In short: Microsoft’s message identifies Azure Linux as a confirmed carrier but does not eliminate the practical possibility that other Microsoft-shipped kernels — especially those that are themselves Linux kernel builds — could carry the same defect.

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Technical verification: how to determine if a specific Microsoft artifact is affected​

Kernel vulnerabilities are an artifact‑level property. Whether an image is exposed depends on three objective things:

• The kernel version and upstream commit range (did the build include the vulnerable commit or its upstream commit range?).
• The kernel configuration (are the netfilter/bridge/conntrack subsystems compiled in or available as modules?).
• Packaging/backporting choices (was the upstream fix backported into the vendor’s kernel package?).

Below is a practical checklist for defenders that covers Azure Linux, WSL2, Azure VM images, and Marketplace appliances.

1. For Azure Linux images (VMs, container hosts)​

1.) Check the kernel package version that’s installed (uname -r and distribution package metadata).
2.) Compare installed kernel versions to the fixed kernel versions listed in vendor advisories (Microsoft’s VEX/CSAF for Azure Linux, NVD, distribution advisories).
3.) If an update is available from Microsoft for the Azure Linux kernel, apply it or follow Microsoft’s remediation guidance. Azure Linux customers should treat the MSRC attestation as actionable.

2. For WSL2 instances (developer/desktop hosts)​

1.) Determine the active WSL kernel: run uname -r inside WSL or check the Windows side via the shipped kernel binary.
2.) Inspect the WSL kernel configuration or module list: WSL kernels may ship with or without br_netfilter / conntrack modules. Attempt modinfo br_netfilter or lsmod; absence of modules does not prove safety—some kernels compile features directly rather than as modules.
3.) If you manage a fleet of Windows desktops, consider whether WSL is used in networking modes that could expose promiscuous bridging; if uncertain, prioritize verifying the kernel or removing unneeded promiscuous bridging modes until patched.

3. For Azure Marketplace images, linux‑azure kernels, and partner appliances​

1.) Request SBOMs or VEX/CSAF attestations from the vendor or publisher where available. Microsoft has promised to publish VEX/CSAF for more products as inventories finish; in the meantime, vendors or publishers should provide SBOMs on request.
2.) Inspect the shipped kernel binary or l uname -r and check module availability.
3.) If a vendor kernel lacks a public SBOM, treat the artifact as unverified and plan to patch or replace when possible.

4. Quick runtime checks administrators can run (examples)​

• uname -r — get the running kernel version.
• zgrep BR_INPUT_SKB_CB /proc/config.gz or grep for CONFIG_BRIDGE_NETFILTER in /boot/config-* — check if bridge/netfilter options are enabled.
• lsmod | grep br_netfilter && modinfo br_netfilter — see if the module exists and what version it reports.
• dmesg | grep -i br_netfilter | tail — look for warning splats of the sort described in the upstream bug.
• Use kernel package managers (rpm -q kernel, apt-cache policy linux-image-*) to compare installed package versions against vendor advisories.

If any of these checks suggests the vulnerable code is present and the kernel is at or before a recorded vulnerable version, treat the artifact as affected until you can confirm a fixed package or vendor attestation.

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Mitigations and recommended actions​

Whether you run Azure Linux, WSL2, Azure VMs, or Marketplace images, take a defense‑in‑depth approach.

• Patch promptly where Microsoft has published fixes: For Azure Linux, apply the kernel updates distributed by Microsoft. Microsoft’s VEX/CSAF attestation for Azure Linux is the canonical signal for that product.
• Inventory Microsoft artifacts in your estate: Identify where Microsoft provides kernels or kernel‑dependent images (WSL2 hosts, managed nodes, virtual appliances). Treat any artifact that ships a kernel as a candidate for verification.
• Request SBOMs or VEX attestations from vendors: If you run Marketplace images or third‑party appliances, ask the publisher for an SBOM or VEX/CSAF attestation. Microsoft has committed to publishing VEX for more products over time, but you may need direct vendor attestations for third‑party items.
• Harden network configuration: Where practical, avoid unnecessary promiscuous bridging on multi‑tenant hosts. Use isolated bridges, SR‑IOV, or VLANs that avoid shared L2 surfaces for untrusted tenants.
• Monitor kernel logs and conntrack behavior: Watch for the specific warning splats described in upstream notes and for abnormal conntrack table activity — these operational signals can indicate mis‑handling or attempted exploitation.
• Consider disabling br_netfilter if you do not need it: If your workflows do not rely on bridge‑level netfiltering, disabling or blacklisting br_netfilter can eliminate the attack surface. This is a configuration change and must be tested for functional impact.
• Apply risk‑based containment: For managed services, isolate workloads that can generate large volumes of bridged traffic or restrict tenant capabilities that could create malicious floods.

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Practical examples: WSL2 and why it matters to desktop fleets​

Microsoft maintains a public WSL2 kernel repository and ships a WSL kernel via Windows Update. That kernel historically has been updated with new featuresontrol) and can be built with many common netfilter options. Issues associated with iptables and kernel module availability for WSL kernels have been reported, and Microsoft occasionally rolls back or updates kernels when regressions appear — a reminder that WSL kernels are real, shipped Microsoft artifacts that can carry upstream network subsystems. If you run WSL2 at scaaptops), the WSL kernel version and configuration should be part of your inventory and patching plans.
This matters because a vulnerable WSL kernel on a developer workstation can be triggered only under specific local conditions (bridge + promiscuous tap setups), but those setups are not unheard of: developers running containers, testing virtual network appliances, or debugging overlay networks can create the exact environment that exercises the bug. For enterprise desktop fleet managers, confirm the WSL kernel version or temporarily restrict WSL kernel updates until you have confidence in the patch cadence.

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Assessing Microsoft’s transparency promise — strengths and limitations​

Microsoft’s move to publish CSAF and VEX attestations is an important step toward supply‑chain transparency. VEX files allow automated triage: a security platform can ingest a VEX attestation and mark specific products as Not Affected, Known Affected, Fixed, or Under Investigation. Microsoft’s blog and MSRC posts explain the phased rollout and the practical benefits fume machine‑readable attestations. That is a significant operational improvement for defenders.
However, there are important limitations to bear in mind:

• The initial VEX coverage is product‑scoped and iterative: early attestations cover Azure Linux first; other product families follow as inventories complete. Absence of an attestation for a product is not proof that the product is unaffected — it means the product has not yet been fully inventoried or attested.
• Kernel code inclusion is build‑time specific: the presence or absence of a subsystem like br_netfilter depends on CONFIG flags and packaging choices. Two Microsoft images with the same upstream base might differ in kernel configuration and therefore differ in exposure. That complexity is why artifact‑level verification (SBOM, VEX, or direct inspection) is needed.

Be cautious when interpreting the MSRC sentence: it is helpful and actionable for Azure Linux, but it is not a universal safety certificate for all Microsoft artifacts.

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When to escalate and what to ask Microsoft or your vendor​

If you operate production infrastructure and you rely on Microsoft‑supplied kernels (Azure VM images, Microsoft marketplace images, WSL kernels in developer environments), escalate the question internally and to Microsoft/publisher teams

• Provide the exact artifact identifier (image SKU, kernel version, Marketplace listing ID, or WSL kernel release) and ask whether the vendor has an SBOM/VEX entry mapping that artifact to the upstream component implicated by the CVE.
• If you lack the ability to patch quickly, ask for a short‑term mitigation recommendation specific to that artifact (e.g., a recommended kernel package, configuration to disable br_netfilter, or a containment pattern).
• For third‑party Marketplace appliances, insist on an SBOM or require the publisher to confirm their kernel configuration and whether the fix has been applied.

If a vendor responds that the artifact is “not yet attested” rather than “not affected,” treat that as an operational red flag: unverified artifacts require prioritized verification or temporary containment.

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Conclusion​

Microsoft’s public line that “Azure Linux includes this open‑source library and is therefore potentially affected” is a precise, machine‑readable inventory statement: it confirms Azure Linux as a known carrier and gives Azure Linux customers a clear remediation signal. But it is not an exclusivity statement. Any Microsoft product that ships a Linux kernel — notably WSL2 kernels, linux‑azure builds, Marketplace images, or other kernel‑derived artifacts — can plausibly include the same upstream netfilter/bridge/conntrack code depending on kernel version and configuration. Defenders should therefore:

• Treat Microsoft’s Azure Linux attestation as authoritative and act on it quickly.
• Inventory all Microsoft‑distributed kernels in their environment (WSL2, Azure images, Marketplace appliances) and verify whether the vulnerable component is present using SBOMs, VEX attestations, or direct runtime inspection.
• Patch where vendors provide fixes, and apply network hardening and containment measures where patching is delayed.

In short: Azure Linux is the confirmed and attested carrier for CVE‑2024‑27018 in Microsoft’s public mapping — but it is not, from a technical standpoint, the only Microsoft artifact that could be affected. Treat the MSRC attestation as the starting point for immediate remediation and treat other Microsoft‑supplied kernels as “unverified until proven otherwise.”

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Source: MSRC Security Update Guide - Microsoft Security Response Center