CVE-2024-35870: Azure Linux Attestation and Cross Product Exposure

Microsoft’s short advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate — but it is a product‑scoped attestation, not proof that no other Microsoft product could ever carry the same vulnerable code. view
CVE‑2024‑35870 is a Linux‑kernel vulnerability that patches a use‑after‑free (UAF) in the CIFS/SMB client code path — specifically the function smb2_reconnect_server() in the kernel’s SMB client implementation. The bug occurs when a reconnect worker accesses a session that another thread is concurrently tearing down, which can result in dereferencing freed session data and producing a kernel general protection fault or other memory corruption. The Linux kernel CVE announcement lists the affected files, the fixed commits and the stable releases that received the patch.
This article explains what Microsoft’s wording means in practice, why the statement about Azure Linux is deliberate and limited, what other Microsoft artifacts you should consider when assessing exposure, how to verify whether your Microsoft‑supplied images include the vulnerable kernel code, and recommended mitigation and operational steps for defenders.

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What Microsoft actually said — and why the exact wording matters​

Microsoft’s public MSRC entries (and the VEX/CSAF attestations the company is now publishing) often use concise, machine‑readable language to describe which product families include a given upstream open‑source component. The repeated phrase you quoted — that “Azure Linux includes this open‑source library and is therefore potentially affected” — is an authoritative inventory result for the Azure Linux product family. Microsoft has explained that it began publishing machine‑readable VEX/CSAF attestations to provide this exact kind of per‑product clarity.
Key points about that phrasing:

• Authoritative for Azure Linux: When Microsoft says Azure Linux includes the component, that statement is the company’sfor that product. Customers running Azure Linux should treat the attestation as a high‑confidence signal that the product ships the upstream code referenced by the CVE and therefore requires attention.
• Not a global denial of exposure: The wording is not a categorical guarantee that no other Microsoft product contains the same component. Microsoft has repeatedly documented that its VEX rollout is phased and product‑scoped; Azure Linux was an early product to receive attestations. Microsoft has committed to update the CVE/VEX mapping if additional Microsoft products are discovered to ship the implicated upstream code. That is a process guarantee — not a technical proof of exclusivity.
• Why Microsoft uses product‑scoped attestations: Large vendors ship many distinct artifacts (distribution images, custom kernels, embedded appliances, marketplace imagesment agents). Whether the upstream code is present in a particular artifact depends on kernel version, configuration flags, and packaging decisions — all artifact‑level properties that must be verified per product. The attestation model lets Microsoft provide deterministic, machine‑readable status per product as inventory checks are completed.

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The technical reality: how the vulnerability maps to artifacts​

Understanding whether any given Microsoft image or product is affected by CVE‑2024‑35870 requires answering three technical questions:

• Which kernel versions and commit ranges include the vulnerable code?
• The Linux kernel CVE announcement and stable‑tree commits identify the exact commits and the kernel versions where the fix was applied (for example, fixes landed in releases such as 6.6.29, 6.8.5, and a later 6.9 commit). Those commit IDs are documented in the kernel mailing/archive notices.
• Is the affected code compiled into the kernel or provided as a module in the particular build?
• Kernel builds are controlled by configuration flags (CONFIG_*). A distribution or product may enable the CIFS/SMB client code as a built‑in or as a loadable module, or it may omit it entirely. Presence in the source tree does not equal presence in a shipped binary — you must inspect the kernel build configuration or module list for the artifact. Several downstream advisories (Debian, Ubuntu, Oracle, Amazon) list specific package versions that include the fix, showing how distributors track and backport the change into their kernels.
• Has the vendor backported the fix into older supported kernel branches used by that product?
• Distributors often backport security fixes into their supported kernel branches. The Linux CVE announcement lists the stable commit links and indicates which stable versions incorporated the fix; downstream vendors then map those commits into their release trees. If Microsoft publishes an attestation for Azure Linux, that means Azure Linux images were inventoried and found to include the vulnerable code (or the relevant upstream component) prior to remediation or as “potentially affected.”

Because of these three artifact‑specific variables, whether a given Microsoft product or image is exposed is not a single yes/no global property — it’s an artifact inspection problem.

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Short, operational answer to the user’s question​

• Is Azure Linux the only Microsoft product that includes the open‑source library and is therefore potentially affected by CVE‑2024‑35870?
• Short answer: No — not necessarily. Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the upstream component maphat attestation is product‑scoped and phased; absence of attestation for other Microsoft SKUs is not proof those SKUs do not contain the code. Microsoft has committed to update the CVE/VEX mapping when additional Microsoft products are identified as carriers.
• What you should take away: treat the Azure Linux attestation as authoritative for Azure Linux, and treat other Microsoft‑supplied artifacts as “not yet attested” (i.e., unverified) until Microsoft issues an attestation for them or you verify the artifact yourself.

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What to check across your Microsoft footprint — practical, prioritized steps​

If you operate Microsoft‑supplied images, appliances, or clients, follow these prioritized checks to determine exposure and obtain verifiable evidence.

• Inventory your Microsoft artifacts (ordered by risk):
• Azure Linux images (VM images, scale set images, Azure Marketplace images that use Azure Linux).
• Any custom kernels Microsoft ships (e.g., cloud‑optimized kernels used in managed services).
• WSL2 / Windows Subsystem for Linux kernels bundled with Windows or provided by Microsoft.
• Azure Marketplace images and publisher images that might include a Linux kernel.
• Management agents, appliance images, IoT/edge images distributed by Microsoft that embed Linux kernels.
• For each artifact, collect:
• Kernel version and precise kernel package release identifier (e.g., linux-image-<ver>-<distro‑packaging>).
• Kernel config (the .config used to build the kernel) or the list of modules shipped.
• The presence/absence of CIFS/SMB client support (CONFIG_CIFS, CONFIG_CIFS_DEBUG, and whether CIFS is built as module cifs.ko or compiled in).
• Any vendor backport notes or advisory mapping (SBOM or VEX/CSAF entry).
• How to verify quickly (technical checks):
• Boot the image (or inspect the image contents) and run: uname -a; lsmod | grep cifs; zgrep CONFIG_CIFS /proc/config.gz (or inspect /boot/config-<version>) to see whether CIFS/SMB client code is present. If CIFS is absent, the specific SMB client UAF cannot be triggered on that kernel. If CIFS is present, note the kernel version and consult vendor advisories for patch status. (This is an artifact‑level verification step; do it for each image type you use.)
• For WSL2 kernels, check the kernel version that WSL is using (wsl --status or wsl uname -a) and whether Microsoft’s WSL kernel images include CIFS. Historical practice shows WSL kernels are often trimmed, but verification is required per build.
• Consult vendor attestations and SBOM/VEX:
• If Microsoft publishes a VEX/CSAF attestation for a product, consume that machine‑readable file as an authoritative inventory for that product. For Azure Linux, Microsoft’s attestation is the canonical signal. For other Microsoft products, wait for the vendor attestation or perform the artifact checks above.
• Patch prioritization:
• If your artifact contains a vulnerable kernel version or the CIFS client is compiled in, apply the vendor kernel update that includes the commit(s) listed in the Linux CVE announcement (or update to a fixed stable kernel release) as soon as operationally feasible. Linux distributors and downstream advisories (Debian, Ubuntu, Oracle, Amazon Linux, eckage versions and guidance — use those as your remediation target.

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Why you shouldn’t assume “Microsoft = safe” or “only Azure Linux affected”​

A few operational realities make the Azure‑only assumption dangerous:

• Microsoft ships many Linux artifacts: distribution images (Azure Linux), cloud‑tuned kernels (linux‑azure or cloud kernels), custom images for Marketplace publishers, appliances, and WSL2 kernels. Each artifact may use different kernel versions and configuration sets. Microsoft’s attestation for Azure Linux is a single product family inventory item, not a retroactive global sweep across all SKUs.
• Vendors backport fixes selectively: a patch may be backported into some stable trees and not others; a product that uses an older branch may or may not include the fix depending on the vendor’s backport policy and patching schedule. The Linux kernel CVE announcement identifies the commit IDs and which stable lines received the changes; downstream vendors then decide how and when to expose the fix in their package repositories.
• Build-time decisions matter: whether a vulnerability is exploitable on a given host is also a function of configuration. If CIFS support is not present in the shipped kernel, the SMB client UAF cannot be triggered — but that absence must be proven, not assumed. Inspect the binary or booted kernel to confirm.

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Cross‑referenced facts (what we verified and where)​

• The vulnerability is in the Linux kernel CIFS/SMB client (function smb2_reconnect_server()), described as a use‑after‑free. This is recorded in the Linux CVE announcement and NVD.
• Kernel commits fixing the issue are referenced in the official kernel announcements; the mailing‑list post and stable commit links show the exact commits used to remediate the defect. These are the primary technical artifacts for patching.
• Microsoft’s public attestation model: MSRC and the company blog explain a phased VEX/CSAF rollout and show the canonical wording that Azure Linux “includes this open‑source library and is therefore potentially affected”. Microsoft has promised to expand attestations to additional products as inventory work continues. This is Microsoft’s stated process and the practical reason why only Azure Linux may be mentioned in the initial CVE om]
• Downstream distributors have issued advisories and fixed package versions (Debian/Ubuntu, Amazon Linux ALAS references, Oracle). Those advisories list which package versions or kernel releases include the fix and are the operational remediation sources for customers who use those distributions.

If any single claim in these streams cannot be independently verified for your exact artifact (for example, which kernel version your WSL2 instance uses), perform the artifact checks described earlier. Where vendor attestations are missing, local inspection is the only reliable path to evidence.

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Practical remediation advice for administrators and security teams​

• Immediate action (within 24–72 hours):
• Identify any Azure Linux images in your environment and apply Microsoft’s recommended updates for Azure Linux kernels as provided in the attestation or security advisory. Treat Microsoft’s Azure Linux attestation as the canonical signal for those images.
• For non‑Azure Linux artifacts from Microsoft (WSL kernels, Marketplace images, appliances), begin artifact verification: collect kernel version, module list and kernel config as described above.
• Tactical measures while you patch:
• If CIFS mounts are not required, consider disabling CIFS client usage or restricting who can mount CIFS shares.
• For container‑heavy environments, restrict privileged containers and avoid giving untrusted containers the ability to mount host filesystems.
• Monitor dmesg and kernel logs for CIFS/SMB client errors (e.g., messages referencing smb2_reconnect_server, CIFS: VFS, or session teardown errors) — these can indicate attempted exploitation or triggering of the bug.
• Long‑term operational hygiene:
• Build an artifact‑level inventory and SBOM process for any software images you run. Machine‑readable VEX/CSAF attestations are useful where vendors publish them; otherwise, automate artifact inspection (kernel version, config, module presence).
  -CSAF feeds where available to reduce false positives and automate targeted remediation for product families (Microsoft’s VEX rollout is precisely aimed at solving this problem).

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Risk assessment and realistic exploitability​

CVE‑2024‑35870 is a kernel‑level use‑after‑free; its primary impact modes are kernel crashes and potential memory corruption. The Common Vulnerability Scoring and vendor advisories indicate a medium‑severity profile (downstream CVSS vectors vary), but kernel UAFs can sometimes be chained to achieve privilege escalation in hostile environments. As with any kernel bug, exploitability depends on local access and the ability for an attacker to reach the CIFS client code path under the right conditions (for example, specific mount options, session states, or worker scheduling). In short: treat it seriously on hosts that mount untrusted CIFS shares or that expose local user access paths that could exercise CIist.gov]

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Final assessment — clear, evidence‑backed answer​

• Microsoft’s statement that “Azure Linux includes this open‑source library and is therefore potentially affected” is an authoritative, product‑scoped attestation for Azure Linux and should be acted on by customers who run Azure Linux images.
• That statement is not a technical guarantee that no other Microsoft product contains the vulnerable open‑source code. Microsoft’s VEX/CSAF rollout is phased; Azure Linux was an early attestation target and additional Microsoft products may be inventoried and published later. Treat other Microsoft artifacts that have not yet been attested as unverified rather than proven safe. Microsoft has publicly committed to update CVE/VEX mappings if additional products are found to ship the implicated component.
• Operationally, the safest course is to:
• Apply vendor patches for any artifact that includes the CIFS/SMB client and is running a kernel in the vulnerable commit range.
• For Microsoft artifacts not yet attested, perform artifact‑level verification (kernel version, config, modules) and rely on Microsoft’s VEX/CSAF attestations as they become available.

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Quick checklist (copyable)​

• Confirm whether you run Azure Linux images in any environment. If yes — treat Microsoft’s attestation as authoritative and patch immediately.
• For all Linux images (including WSL2 or Marketplace images from Microsoft): collect kernel version, inspect /boot/config-<ver> or /proc/config.gz, and verify presence of cifs module. If present, check vendor advisories for fixed package versions.
• Subscribe to Microsoft VEX/CSAF feeds and your downstream distribution security advisories to automate future attestation‑driven triage.

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CVE‑2024‑35870 is a straightforward kernel UAF in the CIFS client that vendors have fixed upstream and in stable trees; Microsoft’s Azure Linux attestation is a precise, useful inventory signal for that product but not an exclusivity statement for all Microsoft artifacts. Defenders should combine vendor attestations with artifact inspection and prioritized patching to reduce risk.

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Source: MSRC Security Update Guide - Microsoft Security Response Center