CVE-2025-37817 Kernel Double Free in mcb Chameleon: Azure Linux Risk

Microsoft’s one-line mapping of CVE-2025-37817 to Azure Linux is accurate as far as it goes — Azure Linux has been confirmed to include the vulnerable kernel code — but it is not a technical guarantee that no other Microsoft product ships the same vulnerable component, nor does it change the broader facts about the vulnerability: this is a double‑free bug in the Linux kernel’s mcb subsystem (chameleon_parse_gdd), and any product shipping an affected kernel version or an unpatched backport of the same code is potentially at risk.

Background: what is CVE-2025-37817 and why it matters​

CVE-2025-37817 is a memory-management bug fixed upstream in the Linux kernel’s mcb subsystem. The flaw occurs in the function chameleon_parse_gdd(): when mcb_device_register() fails, execution can follow an error path that frees the same device object twice, producing a classic double free (CWE-415). Double frees in kernel space are serious because they can lead to kernel memory corruption, crashes (denial of service), or — in some cases — escalation to arbitrary code execution in kernel context depending on allocator behavior and the layout of kernel objects.
From a practical security perspective, this class of bug is high‑risk for privileged environments: the Linux kernel runs at ring 0 and any memory-corruption primitive there can be abused by a local attacker (or by an attacker who can trigger kernel code via some device interface) to disrupt or subvert system integrity. Multiple upstream fixes were applied across kernel stable branches; distribution vendors subsequently issued updates for specific kernel packages.

Overview of the technical fix and affected code paths​

• The root cause is an error‑handling path that attempts to free a device structure twice. Specifically, when mcb_device_register() fails, it already drops a reference to the device via put_device(); control then falls into an error cleanup label that frees the same object again.
• The upstream remediation is simple and correct at the code level: do not attempt the second free after register() fails; return early instead.
• The fix is present in the kernel stable repository as part of several backported patches across affected branches; distributions applied these patches to their kernel packages in subsequent updates.

This is a textbook double‑free fix: straightforward in intent, but important because double frees in kernel memory allocators may be exploitable depending on allocator state, timing, and surrounding code.

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Which products are affected?​

Kernel versions and distribution scope​

The vulnerability is a Linux kernel issue, therefore the primary determinant of exposure is the kernel version and vendor backports, not a product brand name alone. Upstream analysis and vendor advisories show the bug impacted a broad swath of kernel versions used in many mainstream distributions and vendor kernels.
Significant points about affected ranges:

• Vendor guidance identifies multiple kernel version ranges where the vulnerable commit is present; distributions that shipped kernels in those ranges or backported the code were treated as in‑scope.
• Patches were applied across stable branches and vendor kernels; distribution advisories list fixed kernel package versions to install.

In short, any OS image or appliance that includes an affected kernel tree — whether that kernel is distributed directly by an open‑source distro, baked into a cloud image, or packaged into a customer appliance — can be impacted until a patched kernel is installed.

Microsoft’s statement: what it actually says​

Microsoft’s Security Response Center (MSRC) mapped the upstream component (the vulnerable kernel code) to the Azure Linux distribution and published that mapping as part of their machine‑readable VEX/CSAF attestations. The wording used across Microsoft CVE pages is consistent: they say “Azure Linux includes this open‑source library and is therefore potentially affected by this vulnerability” and note that they publish CSAF/VEX attestations and will update the CVE if they identify impact to additional products.
What that wording does — and does not — mean:

• It does mean Microsoft has inspected the Azure Linux distribution (the images and packages they publish internally) and has confirmed the presence of the upstream component that maps to the CVE. For Azure Linux customers, this is an authoritative, actionable statement: treat Azure Linux images as in‑scope and apply fixes when available.
• It does not mean Microsoft has asserted that Azure Linux is the only Microsoft product that may include the vulnerable code. Microsoft explicitly reserves the right to update the mapping if other products are later found to include the same upstream component.
• It does not substitute for per‑product inventory checks. The presence or absence of the vulnerable code in other Microsoft deliverables (for example, custom kernel images used by hosted Azure services, Windows Subsystem for Linux (WSL) kernel builds Microsoft publishes, or other Linux artifacts Microsoft distributes) depends on kernel version, backports, and build provenance.

Put plainly: Microsoft confirmed Azure Linux is a hit; they have not declared exclusive ownership of the problem domain. That conservatively worded public mapping is useful, but it should not be read as a guarantee that other Microsoft artifacts are unaffected.

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Why the wording matters for customers and incident responders​

There are two separate but related facts that matter to defenders:

• Confirmed inventory: When a vendor attests a product contains a specific upstream component, customers running that product can rely on the vendor’s inventory and prioritize patching for that product immediately.
• Potential exposure outside the attested product: The same upstream component can appear in other artifacts, possibly outside the vendor’s initial attestation scope. This is common when the same kernel driver or subsystem is included in multiple kernels or when vendors reuse upstream code in different builds.

Customers must therefore do two things:

• Treat the vendor attestation as authoritative for the attested product (apply updates for Azure Linux images promptly).
• Independently check other systems (WSL2 kernels, custom kernels deployed in cloud VMs, appliance kernels, container host kernels) for the vulnerable kernel version or for the presence of the specific device driver code, irrespective of the vendor’s public attestation.

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How to determine whether your system is vulnerable​

Below are practical detection steps aimed at system administrators and security teams. These are general, distribution‑agnostic checks that will show whether a given host is running a kernel that could be impacted.

Quick checks (what to run now)​

• Check kernel release:
• Run: uname -r
• If the kernel version falls within ranges known to carry the vulnerable code or is an unpatched vendor kernel, the host is a candidate for remediation.
• Identify distribution and kernel package:
• For Debian/Ubuntu: dpkg -l | grep linux-image
• For RHEL/CentOS/Oracle: rpm -qa | grep kernel
• For SUSE: rpm -qa | grep kernel-default
• Consult your vendor advisories and apply vendor-recommended kernel update packages. Distributions published fixed kernel package versions after the upstream patch was merged.
• For WSL2 and custom kernel builds:
• If you or your vendor build kernels from source, inspect the source tree for the mcb subsystem or compare your commits to the upstream patch set.
• If you use the vendor’s distributed WSL2 kernel image, check the published kernel version included with that distribution build.

Deeper verification (if you need proof)​

• Search the kernel source tree present on the host for the chameleon code paths (drivers or subsystems named mcb/chameleon). If you maintain a source checkout, compare commit hashes to the upstream patches.
• Where vendor tools exist (image manifests, SBOMs, or CSAF/VEX attestations), use them. Microsoft began publishing CSAF/VEX attestations for Azure Linux; other vendors likewise may publish machine‑readable attestations or SBOMs for their images.

Important note: the vulnerability is in kernel code, not a userland library. Container images that depend on a host kernel are only vulnerable if the host kernel includes the bug; static container runtimes that include their own kernels are only relevant if those kernels include the code.

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Exploitability, observed risk, and real‑world impact​

• Technical severity: double frees in kernel space are high‑impact issues because they open the door to memory corruption. In practice, exploitability depends on many factors — allocator behavior, surrounding kernel state, and the presence of additional primitives that an attacker can use to shape memory.
• Attack vector: local. Most public analyses and vendor advisories characterize this as a vulnerability that requires local access (an unprivileged local attacker) or a way to interact with the driver interface that triggers chameleon_parse_gdd().
• Known exploitation: as of the time of the upstream fixes and vendor advisories, there was no broadly reported proof‑of‑concept widely weaponized in the wild. That said, the presence of a simple memory‑corruption primitive in the kernel is non‑trivial, and defenders should not assume low real‑world exploitability indefinitely.

Operational impact for organizations:

• Hosts running vulnerable kernels could be crashed (denial of service) or, under favorable conditions for an attacker, could escalate to kernel compromise.
• Multi‑tenant environments such as cloud VM hosts or shared developer machines represent higher risk because they provide more potential vectors for local privilege abuse.

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Mitigation and remediation: prioritized checklist​

• Apply vendor kernel updates immediately
• The most reliable remediation: install the patched kernel package from your distribution or vendor and reboot into the updated kernel.
• For Azure Linux customers: apply the Azure Linux update packages Microsoft provides for the distribution images and redeploy or reboot instances as directed.
• Use livepatch services where available
• Where reboot windows are constrained, consider vendor livepatch solutions that apply safe kernel fixes without full reboots. Confirm the livepatch covers the specific mcb/chameleon fix set.
• Restrict local untrusted access
• Minimize the number of local accounts with permission to interact with device interfaces that might trigger the vulnerable code.
• Harden access controls for multi‑user systems and developer workstations.
• Reduce attack surface
• If the mcb subsystem or the specific device interface is not required, consider blacklisting or disabling the driver in the kernel configuration or via module blacklisting until you can update.
• Be cautious: removing kernel modules may impact dependent subsystems; test in staging.
• Audit WSL2 and custom kernels
• If you rely on vendor‑provided WSL2 kernels or maintain custom kernel builds, verify the kernel version and apply patches as appropriate.
• Monitor for indicators
• Watch for anomalous kernel crashes, OOPS, or messages in dmesg that reference the mcb/chameleon code paths; such indicators can signal attempted or successful abuse.

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For Azure Linux customers: what the MSRC attestation means for you​

Microsoft’s attestation that Azure Linux includes the upstream library means the company has done the inventory work for its Azure Linux images and has declared them in‑scope for remediation. If you run Azure Linux images in your environment:

• Prioritize patching those images as MSRC indicates — treat the attestation as definitive for Azure Linux.
• If you use managed Azure services that abstract the OS layer (for example, certain platform services), consult the specific service documentation or Microsoft advisories to understand how and when the vendor will remediate managed platform kernels.

Remember: the attestation makes the case for immediate action on Azure Linux, but it does not obviate the need to check other kernel artifacts you control.

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Why this is not just an “Azure Linux” question: broader Microsoft artifacts to consider​

Microsoft distributes many artifacts beyond Azure Linux images. A few possibilities administrators should consider:

• WSL2 kernel builds: Microsoft publishes and maintains WSL2 kernel sources and binaries. If those kernels include the affected code and are distributed to customers, they could be in scope. Whether a particular WSL2 kernel contains the vulnerable commit depends on kernel version and whether the patch had already been applied in that branch.
• Azure VM kernel images: Some Azure VM images use vendor kernels or Microsoft-provided kernel images tailored for the cloud. These could include the vulnerable code if they are built from affected kernel branches.
• Appliance and IoT images: Any Microsoft‑supplied images that embed Linux kernels should be checked.
• Third‑party products from Microsoft partners: Partners may ship Linux kernels in appliances or devices that Microsoft distributes; their exposure varies.

Because of this diversity, the conservative operational posture is to assume any Microsoft‑supplied Linux kernel image you run should be checked against the vendor advisories and patched if needed.

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Practical policy and supply‑chain recommendations for enterprises​

• Maintain a canonical SBOM and image‑inventory for all Linux kernels in use. Knowing which kernel versions are deployed is the first and most powerful defense.
• Integrate vendor CSAF/VEX or SBOM feeds into your vulnerability management workflow. Machine‑readable attestations make it much easier to map CVEs to deployed assets.
• Treat vendor attestations as one authoritative input, not the only one. Combine vendor attestations with your local inventory and build provenance to produce a full coverage map.
• Implement a fast patch‑and‑reboot cadence for kernel updates in non‑mission‑critical environments to reduce exposure windows.
• Where downtime is constrained, test and deploy livepatch mechanisms and ensure you have a tested rollback path.

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What we still don’t know and what to watch for​

• Microsoft’s public mapping currently confirms Azure Linux; Microsoft will update attestations if more products are found to include the component. Until then, any statement asserting exclusivity is unsupported.
• Exploit code: as of the initial fixes there was no public proof‑of‑concept widely circulated. Watch threat intel feeds and vendor advisories for any signs of exploit attempts or proof‑of‑concept releases.
• Vendor coverage: vendors have applied backports differently; some may have issued kernel package updates in LTS channels with different package names. Check your distribution advisory and errata stream to ensure you install the correct patched package.

If you cannot find a vendor advisory for a product you run, contact the vendor or, when appropriate, the upstream maintainers to request confirmation.

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Bottom line — what to do right now​

• If you run Azure Linux, follow Microsoft’s guidance: install the Azure Linux kernel updates they provide and reboot or redeploy per their instructions.
• For all Linux hosts in your environment, check kernel versions now (uname -r) and consult vendor advisories for patched package names. Apply patches and reboot according to your change windows.
• For WSL2 and custom kernels, verify the kernel version and source tree; apply patches or obtain updated kernel binaries from your vendor or the upstream project.
• Implement the mitigation steps above if immediate patching is not possible (module blacklisting, access restrictions, livepatch where supported).
• Integrate CSAF/VEX or SBOM feeds into your vulnerability management workflow to make future attestations actionable at scale.

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Final assessment: transparency is good, but inventory is essential​

Microsoft’s public attestation that Azure Linux includes the open‑source component tied to CVE-2025-37817 is a helpful and actionable step that reduces ambiguity for Azure Linux customers. However, the attestation does not logically imply exclusivity. The vulnerability is fundamentally about kernel code present across many vendor kernels; whether a given Microsoft product is affected depends on the kernel version and whether that product’s build contains the vulnerable commit.
Defenders should treat vendor attestations as authoritative for the attested product, but they must also rely on rigorous local inventory and SBOM practices to identify exposure across other artifacts. For CVE-2025-37817, the correct operational response is unambiguous: verify your kernels, apply vendor patches, use livepatches where needed, and harden access to reduce the risk of local exploitation while you remediate.

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Source: MSRC Security Update Guide - Microsoft Security Response Center