CVE-2025-38236: Azure Linux Exposure and AF_UNIX Use-After-Free Patch Guide

The Linux kernel received a targeted fix for a use‑after‑free bug in the AF_UNIX socket handling code — tracked as CVE‑2025‑38236 — and Microsoft’s public advisory confirms that Azure Linux (the Azure Linux Distribution) is a known carrier of the upstream component that contained the bug and is therefore potentially affected. Microsoft also states that it began publishing machine‑readable CSAF/VEX attestations as part of a phased rollout (starting with Azure Linux in October 2025) and will update CVE mappings if additional Microsoft products are later found to ship the same upstream code. This article explains what CVE‑2025‑38236 is, why the Microsoft wording matters, how widely the vulnerability could realistically appear across Linux artifacts, and what operators should do now to establish definitive exposure and remediate risk.

Background / Overview​

CVE‑2025‑38236 is a kernel‑level defect in the AF_UNIX (Unix domain socket) implementation that can lead to a use‑after‑free condition when out‑of‑band (OOB) data handling leaves consecutive “consumed” socket buffers (skbs) in the receive queue. The bug was reported by a security researcher and led to corrective commits in the kernel source.
At a technical level, the problem arises because when a process reads OOB data from a Unix domain socket, the kernel may keep the skb that originally held that OOB byte as a marker for the OOB boundary. Under a particular sequence of sends and receives that intermix MSG_OOB and normal reads, the receive queue can end up with multiple leading skbs that report a consumed length of zero. Subsequent logic in unix_stream_read_generic and related helpers assumes skb lengths are non‑zero in some code paths; that assumption breaks and the kernel may dereference freed memory (observed as a KASAN slab‑use‑after‑free), or produce incorrect results for ioctl(SIOCATMARK).
The kernel team fixed the problem by ensuring the AF_UNIX receive path does not leave consecutive consumed OOB skbs on the receive queue; the patch frees an earlier consumed OOB skb when a new OOB skb is received and the previous skb is already consumed, avoiding the inconsistent state that triggers the use‑after‑free.

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What the bug actually does (plain language)​

• AF_UNIX sockets can carry a single “out‑of‑band” byte that marks urgent data boundaries. The kernel keeps a record of those boundaries using skbs.
• If a user repeatedly reads OOB bytes in a certain pattern, the kernel may leave placeholder skbs whose payload length becomes zero (because the OOB byte was read) but which still remain on the queue as boundaries.
• The receive logic has paths that iterate/skips over skbs and expects non‑zero lengths. When it encounters zero‑length skbs in certain sequences, it can read or free memory incorrectly and later access freed data — a classic use‑after‑free.
• Practically speaking, this is a local kernel bug: an attacker needs local code execution (or to run processes that can open AF_UNIX sockets with the right send/recv pattern) to exercise it. The typical real‑world impact is kernel instability, crashes, or local privilege escalation only if further exploitable chaining exists; it is not a remote, unauthenticated network worm‑type bug.

Why this matters operationally​

A kernel use‑after‑free is serious because:

• It can destabilize a host or cause data corruption.
• In the worst case, with additional bugs and processor features exploited, it may be chained into local privilege escalation.
• Kernel defects affect any product that ships a Linux kernel build which contains the vulnerable code path (i.e., the AF_UNIX implementation at the affected code range).

Consequently, identifying which deployed images, VMs, appliances, or kernels contain the vulnerable code is the first priority for defenders.

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Microsoft’s advisory: what did it actually say and mean?​

Microsoft’s public advisory for the CVE states that Azure Linux is a Microsoft product that includes the implicated open‑source component and is therefore potentially affected. The advisory also explains Microsoft has begun publishing machine‑readable CSAF/VEX attestations (a phased rollout started in October 2025) and will update the CVE/VEX entry if additional Microsoft products are found to ship the same upstream component.
Key operational interpretation:

• That phrasing is a product‑scoped attestation: Microsoft has completed inventory for Azure Linux and confirms the upstream code was present in those published artifacts.
• The statement is not a blanket claim that no other Microsoft product contains the code. Rather, it identifies what Microsoft has verified so far.
• Microsoft commits to updating its machine‑readable attestation set if its mapping work finds additional Microsoft SKUs that carry the same code.

This distinction is crucial for defenders: treat Microsoft’s attestation as authoritative for Azure Linux, but do not assume other Microsoft images or kernel artifacts are safe merely because they are not named in the VEX/CSAF feed yet.

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Are other Microsoft products likely to carry the same AF_UNIX code?​

Short answer: Yes, possibly — but whether they actually do depends on which kernel builds Microsoft ships for each product and the kernel configuration used when building those images.
Microsoft (and other vendors) produce multiple, distinct Linux artifacts:

• The Azure Linux Distribution (the artifact Microsoft has attested).
• linux‑azure kernels that may be packaged for certain Azure VM families or Marketplace images.
• Windows Subsystem for Linux (WSL) kernel builds (Microsoft distributes a WSL2 kernel binary for Windows).
• Official VM images, curated Marketplace images, container base images, and appliance images that embed Linux kernels.
• AKS/agent node images or third‑party images available in Azure Marketplace.

Each of these artifacts is a separately assembled kernel build. A kernel source tree may contain the same net/unix/af_unix.c file, but whether that file’s code is present in a given kernel image depends on:

• The kernel version and the upstream commit range included or backported.
• Build‑time CONFIG_* options (drivers and subsystems can be compiled in, built as modules, or omitted).
• Whether the vendor cherry‑picked or backported the specific fix before shipping.

Because of this variability, the presence of the vulnerable AF_UNIX code in one Microsoft product (Azure Linux) does not prove or disprove the presence of the same vulnerability in other Microsoft products without artifact inspection or an explicit VEX/CSAF attestation covering them.

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Evidence and verification: what to check and how​

To determine if a particular image or host is affected, combine vendor attestations with artifact inspection. Use the following checklist to reach a definitive answer:

• Confirm Microsoft’s attestation status for the CVE for the product in question
• For Azure Linux: Microsoft has published a VEX/CSAF attestation that names Azure Linux as a carrier of the upstream component and therefore potentially affected.
• For other Microsoft products: check the vendor VEX/CSAF feed for product‑scoped attestations. Absence of an attestation = “not yet verified,” not “not affected.”
• Identify the kernel version and build on the host
• Query uname -r or the package manager to get exact kernel package names and versions.
• Map the kernel version to upstream commit ranges
• Many CVE advisories list the upstream fixes or patch commits that resolve the issue. Compare those commit IDs (or their presence/absence) against the upstream commit range present in your kernel build.
• If your kernel tree contains the fix commits or the vendor’s security changelog documents a backport, you’re patched.
• Inspect the kernel build/configuration
• Confirm whether the AF_UNIX code path was compiled in or compiled as a module; kernel configuration flags influence code inclusion.
• If the AF_UNIX code path exists and your kernel is in the vulnerable commit range, treat the host as exposed until vendor updates are applied.
• If you maintain fleets of images, automate the checks
• Ingest vendor VEX/CSAF metadata automatically for Azure Linux and other vendors.
• Create a small inventory script to enumerate kernel package versions and cross‑reference known‑vulnerable version ranges.

Caveat: Exact detection sometimes requires access to kernel build metadata or the uncompressed vmlinux or module sources; trivial checks of kernel version strings can be insufficient when vendors backport patches or apply corrective commits without changing the visible version string.

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Recommended immediate actions (prioritized)​

• Patch Azure Linux artifacts immediately
• Because Microsoft explicitly attested that Azure Linux includes the upstream component and is potentially affected, customers running Azure Linux images should apply the published kernel updates from Microsoft / the Azure Linux distro as soon as practical.
• Inventory all Microsoft‑provided Linux artifacts you run
• Treat any Windows images that ship a WSL kernel, Azure VM images, Marketplace appliances, AKS node images, and any Microsoft‑distributed Linux kernels as items to verify.
• For each artifact, verify kernel version, kernel config, and whether the upstream fix is present.
• Ingest Microsoft VEX/CSAF artifacts and automate triage
• Microsoft’s rollout is machine‑readable; consuming CSAF/VEX programmatically speeds triage and reduces false positives. Use VEX to automate marking Azure Linux SKUs as “Known Affected” and to watch for future attestation updates.
• Apply upstream or distribution patches where available
• If you run vendor kernels from mainstream distros (Debian, Ubuntu, Red Hat, Oracle, Amazon), follow those distributors’ advisories — they will generally release patched kernel packages or backports.
• If you manage custom images, apply the kernel patch or upgrade to a kernel release that contains the upstream fixes.
• Monitor for signs of exploitation or instability
• Watch kernel logs for KASAN slab‑use‑after‑free reports, kernel oops, or crashes tied to AF_UNIX code paths.
• Increase audit and kernel logging temporarily on high‑value hosts until the patch is in place.

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Detection tips and practical commands​

Use the following pragmatic steps to establish presence/absence of the vulnerable code on a host (adapted to your distro):

• Determine the running kernel:
• uname -r
• Check package manager for kernel package version (apt, rpm, zypper, etc.
• Compare against vendor advisories:
• Look for kernel package updates or security advisories from your distribution or provider. If they list CVE‑2025‑38236 and a patched package, apply it.
• Inspect kernel modules and configuration:
• If you have the kernel build or source used to produce the image, search the kernel tree for net/unix/af_unix.c and examine commit logs for the known fix commits.
• If you only have binaries, consult the vendor’s changelog or the image’s SBOM if available.
• Fleet automation:
• Collect kernel package names and versions from all images and instances.
• Cross‑reference that inventory with known vulnerable version ranges or vendor advisories.
• Flag mismatches for manual verification.

Note: Backports complicate automated checks based on version strings; when in doubt, request the vendor’s attestation or build logs.

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Strengths and weaknesses of Microsoft’s VEX/CSAF approach​

Strengths:

• Machine‑readable VEX/CSAF attestations materially speed triage and reduce noise. When a vendor can say “Product X includes component Y and is Known Affected/Fixed,” automation can mark only those SKU families for remediation, avoiding over‑triage.
• Microsoft’s phased rollout (starting with Azure Linux) is pragmatic: it allows an initial set of product attestations to be validated, preventing errors that could propagate at scale.
• The commitment to update VEX/CSAF records if further products are found to ship the component is an explicit positive change in transparency.

Risks and limitations:

• Product‑scoped attestations are not an exhaustive statement about every vendor artifact. Absence of an attestation does not equal absence of the vulnerable code.
• Large vendors ship many kernel variants and images. Completing an exhaustive inventory across all SKUs is time consuming and, during a phased rollout, some artifacts may remain unverified for weeks.
• Automated consumers may misinterpret “not listed” as “not affected.” Security teams must treat “not yet attested” as a verification gap and not as a clearance.
• Backports and vendor patching practices can hide vulnerability presence from simplistic version checks, requiring careful source/commit verification.

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Risk scenarios and practical impact analysis​

• High‑impact scenario (localized): An attacker with local access runs code that triggers the AF_UNIX OOB sequence to induce a use‑after‑free and then chains another memory corruption gadget to elevate privileges or bypass controls. This requires additional exploitability steps beyond the initial crash; real‑world exploitation is non‑trivial but the risk cannot be dismissed on critical hosts.
• Medium‑impact scenario (diagnostics and stability): The bug causes KASAN reports, kernel oopses, or crashes on hosts that use AF_UNIX heavily (for example, some systemd or interprocess communication heavy workloads). This results in instability and reduced availability.
• Low‑impact scenario (no local attacker capability): Systems that do not permit untrusted local code execution remain at lower immediate risk. However, multi‑tenant environments where guests or containers share host resources must treat the hypervisor and host kernels as high priority to patch.

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Guidance for different audiences​

• For Azure Linux customers:
• Treat Microsoft’s VEX/CSAF attestation as authoritative. Apply vendor‑provided kernel updates as soon as they are released.
• Ensure image lifecycle policies pick up updated kernels and redeploy node pools / images in a controlled maintenance window.
• For teams running WSL or Windows with a shipped WSL kernel:
• Do not assume WSL kernels are unaffected. Check the WSL kernel repository/build metadata or Microsoft’s VEX/CSAF feed for the product attestation, and update the WSL kernel if Microsoft publishes a fix.
• For operators of Azure Marketplace images or AKS clusters:
• Inventory node images and check whether they are built from Azure Linux kernels or other kernels. If they are Microsoft‑packaged kernels, request or consult the vendor’s VEX/CSAF attestation.
• For security teams building detection:
• Ingest Microsoft’s VEX/CSAF feed for automation.
• Instrument kernel log monitoring for KASAN and AF_UNIX‑related oops signatures.

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When a vendor says “we will update the CVE if additional products are identified” — why that matters​

That wording commits the vendor to an ongoing inventory exercise. For defenders, that should be interpreted as:

• Microsoft has performed the mapping for Azure Linux and published the result; treat that as the immediate operational input.
• Microsoft will continue to expand its attestation coverage over time; subscribe to the vendor’s CSAF/VEX feed and watch for updates.
• Until attestations exist for other Microsoft products, maintain a default posture of “unverified” for those artifacts and audit them proactively if they are in your critical path.

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Final assessment and practical checklist (quick reference)​

• Confirmed: The AF_UNIX bug (CVE‑2025‑38236) is a kernel use‑after‑free that can be triggered by a specific sequence of OOB socket operations.
• Confirmed: Microsoft attested that Azure Linux is a carrier of the upstream component and therefore potentially affected; Microsoft began publishing VEX/CSAF attestations in October 2025 and will update the record if it discovers other Microsoft products that ship the same component.
• Not proven: That statement does not prove that no other Microsoft product contains the code — absence of attestation equals “not yet verified.”
• Immediate actions:
• Patch Azure Linux images immediately using vendor updates.
• Inventory all Microsoft‑provided Linux artifacts and verify kernel versions and build metadata.
• Ingest Microsoft VEX/CSAF and automate triage for Azure Linux SKUs.
• Monitor kernel logs for KASAN/oops evidence and raise detection thresholds for local exploitation attempts.
• Request attestations or build metadata from Microsoft for any Microsoft kernel artifact you depend on but is not yet present in the VEX feed.

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CVE‑2025‑38236 is a kernel‑level correctness bug that should be taken seriously where local code execution is possible or where host stability is critical. Microsoft’s move to publish machine‑readable VEX/CSAF attestations — beginning with Azure Linux — is a practical improvement in transparency that meaningfully helps customers reduce noisy, manual triage. At the same time, the phased nature of the rollout means defenders must combine vendor attestations with proactive artifact inspection to reach definitive conclusions about exposure for other Microsoft products. Apply patches where Microsoft or your distro provides them, automate VEX ingestion, and maintain a small but effective inventory program to close the verification gap while vendors complete their attestations.

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Source: MSRC Security Update Guide - Microsoft Security Response Center