Microsoft’s brief MSRC entry for CVE-2025-37792 — “Bluetooth: btrtl: Prevent potential NULL dereference” — is accurate for the product it names: Azure Linux has been identified as a carrier of the upstream Bluetooth code that required a fix. That attestation, however, is a product‑scoped inventory statement, not a guarantee that no other Microsoft artifact contains the same vulnerable code. In short: Azure Linux is the only Microsoft product Microsoft has publicly attested so far, but it is not technically proven to be the only Microsoft product that could include the vulnerable btrtl driver.
What the MSRC wording does not do is prov‑ve a scope: they confirm what the vendor has inventory‑checked and mapped. They do not, by themselves, prove that every other product or internal image (WSL kernels, Marketplace images, Azure‑published VM images, container host kernels, firmware‑bundled device images, etc.) lacks the same lines of upstream code. Numerous past CVE mappings from MSRC follow the same pattern — Azure Linux is often the first Microsoft product attested because Microsoft started the VEX rollout with that distro family; other Microsoft artifacts may be added if/when they are verified. Treat Microsoft’s attestation as authoritative for Azure Linux images, and as a prompt to verify other Microsoft images rather than assume they’re safe.
Why “possibly”? Because the btrtl driver is an upstream Linux kernel component — whether a Microsoft product includes it is an artifact‑level property. Microsoft ships or maintains a number of Linux‑based kernels and images beyond Azure Linux (for example: the WSL2 kernel builds that Microsoft publishes, Azure Marketplace/VM images, AKS node images, older CBL‑Mariner lineage artifacts, and custom kernels used in some managed services). Any of those artifacts could contain the vulnerable code if the kernel version and configuration used to build the image included drivers/bluetooth/btrtl and if distribution or backporting choices left the vulnerable commit in place. The presence/absence depends on three concrete factors:
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
What is CVE‑2025‑37792?
CVE‑2025‑37792 is a Linux‑kernel vulnerability in the Bluetooth Realtek helper driver btrtl. The bug lives in btrtl_initialize(): when the helper that loads firmware/config files (rtl_load_file()) returns a zero‑length file the code does not correctly set the error return; an error‑pointer vs NULL mixup can lead to a NULL pointer dereference in the kernel. Static analysis (Smatch) flagged the suspicious ERR_PTR usage that pointed maintainers to the fix. The practical impact of this class of bug is availability — a local operation that triggers the condition can cause a kernel oops or crash (Denial‑of‑Service).Where the vulnerable code lives
- The btrtl driver is the kernel module named btrtl (module file btrtl.ko) and is built from drivers/bluetooth/btrtl.c in the upstream kernel tree. It provides Realtek Bluetooth support and is widely present in Linux kernels that enable Realtek Bluetooth drivers. Typical module metadata shows firmware file names under rtl_bt/ and the module name “btrtl.”
- Upstream fixes and follow‑on patches were merged into the stable kernel trees; distributors backport fixes into distribution kernel packages. You’ll find multiple distro advisories listing the CVE and referencing the stable kernel commits that correct the ERR_PTR / zero‑length file handling.
Why Microsoft’s MSRC wording matters — and what it does and doesn’t say
Microsoft’s short FAQ response for the CVE — the exact wording you quoted — reflects a deliberate, operational choice: the company inspected the Azure Linux artifacts, found the upstream btrtl code in those images, and therefore marked Azure Linux as “potentially affected.” Microsoft also explained that it began publishing machine‑readable CSAF/VEX attestations in October 2025 and will update CVE records if other Microsoft products are later found to contain the same upstream library. That is a useful transparency step and gives Azure Linux customers a clear, automatable signal.What the MSRC wording does not do is prov‑ve a scope: they confirm what the vendor has inventory‑checked and mapped. They do not, by themselves, prove that every other product or internal image (WSL kernels, Marketplace images, Azure‑published VM images, container host kernels, firmware‑bundled device images, etc.) lacks the same lines of upstream code. Numerous past CVE mappings from MSRC follow the same pattern — Azure Linux is often the first Microsoft product attested because Microsoft started the VEX rollout with that distro family; other Microsoft artifacts may be added if/when they are verified. Treat Microsoft’s attestation as authoritative for Azure Linux images, and as a prompt to verify other Microsoft images rather than assume they’re safe.
Does any other Microsoft product likely include btrtl?
Short answer: Possibly. The long answer requires inventory.Why “possibly”? Because the btrtl driver is an upstream Linux kernel component — whether a Microsoft product includes it is an artifact‑level property. Microsoft ships or maintains a number of Linux‑based kernels and images beyond Azure Linux (for example: the WSL2 kernel builds that Microsoft publishes, Azure Marketplace/VM images, AKS node images, older CBL‑Mariner lineage artifacts, and custom kernels used in some managed services). Any of those artifacts could contain the vulnerable code if the kernel version and configuration used to build the image included drivers/bluetooth/btrtl and if distribution or backporting choices left the vulnerable commit in place. The presence/absence depends on three concrete factors:
- Kernel version / commit range used in the build.
- Kernel configuration (was CONFIG_BT_RTL enabled as a module or built‑in?).
- Whether the distribution or image vendor backported the upstream fix.
How to verify whether a given Microsoft artifact includes the vulnerable btrtl driver
Below are practical checks you — or your automation — can run against any given Linux image, VM, container host, or appliance:1) On a running Linux instance (VM / physical host / WSL custom kernel)
- Check for the module:
- Run:
modinfo btrtl— if the module exists you’ll see filename, firmware names and module metadata. If modinfo fails, the module may not be present. - Check loaded modules:
- Run:
lsmod | grep -i btrtlorlsmod | grep -i rtlto see whether the module is currently loaded. - Check the filesystem for the module:
- Run:
ls /lib/modules/$(uname -r)/kernel/drivers/bluetooth | grep btrtl - Check kernel config for build option:
- Run:
grep -i CONFIG_BT_RTL /boot/config-$(uname -r) || zcat /proc/config.gz | grep -i CONFIG_BT_RTL - If the config shows
=mor=ythe btrtl source was enabled in that kernel build.
2) For WSL kernels and shipped kernel images
- Microsoft’s WSL2 kernel source is public; you can inspect the tree or the kernel config used in a specific WSL release to see if btrtl is present or enabled. However, presence in the source tree doesn’t mean every WSL release enables the Bluetooth driver. If you run WSL and need Bluetooth, check your WSL kernel instance with the commands above.
3) For offline images (Azure Marketplace, VM images, custom appliance images)
- Mount the image and inspect
/lib/modules/<version>/kernel/drivers/bluetooth/for btrtl. Alternatively, unpack the image’s kernel and inspect the config for CONFIG_BT_RTL. - If you cannot run those steps locally, ask the vendor (e.g., the Microsoft product team) for a machine‑readable VEX/CSAF attestation. Microsoft has committed to publish those for Azure Linux first and expand to other products; watch MSRC VEX outputs and CVE entries for updates.
Immediate mitigations and remediation guidance
If you determine an image or host is building with btrtl and you cannot immediately apply an upstream/stable distro fix, consider these mitigations:- Patch promptly:
- Apply your vendor’s kernel update as soon as a fixed package is available. Distributors (Ubuntu, Debian, Red Hat, Oracle Linux and others) have already listed the CVE and published fixed kernel package versions or backports. Follow your distro’s guidance and lifecycle policy.
- Temporary functional mitigations:
- Unload and block the module until patched:
sudo modprobe -r btrtl(if not in use)- To prevent it loading on reboot: create
/etc/modprobe.d/blacklist-btrtl.confcontainingblacklist btrtland thensudo update-initramfs -u(or the distro equivalent). - Consequence: Bluetooth support for Realtek devices will be disabled; balance availability vs. security need.
- Stop Bluetooth userspace:
sudo systemctl stop bluetoothsudo systemctl disable bluetooth- Consequence: any services that depend on Bluetooth will be interrupted.
- Detection:
- Look for kernel oops messages in
dmesgor journal logs with Bluetooth driver traces (e.g., unexpected NULL dereference, “RTL: cfg_sz 0”, or oops stack traces that referencebtrtl). Those signals indicate attempted execution of the problematic code path. - Inventory & automation (recommended):
- Add checks to your image‑validation pipeline to:
- Inspect
/boot/config-*or kernel package metadata for CONFIG_BT_RTL. - Grep built kernel modules for
btrtl. - Request VEX/CSAF attestations from vendors and ingest them into your vulnerability management system. Microsoft’s VEX rollout will make this easier as more products are attested.
Operational impact and exploitation difficulty
- Attack complexity: low to moderate. The bug is local‑triggerable via the kernel driver’s firmware/config loading codepath. An attacker needs the ability to interact with the host’s Bluetooth subsystem (for example, by inserting or interacting with a Realtek Bluetooth adapter) to trigger the zero‑length file condition, or must be aver to attempt to load a malformed firmware/config file. This usually requires local or guest access.
- Impact: primarily availability (kernel oops / DoS). The CVSS and vendor advisories place the severity in a medium band because confidentiality and integrity are not directly impacted by the NULL dereference alone; an attacker may be able to crash a host or require a reboot. Distributor advisories assign similar priority and recommend kernel updates.
- Exploitation in the wild: at time of writing there are no public reports of active exploitation for CVE‑2025‑37792. Nonetheless, kernel DoS conditions are useful for denial‑of‑service and can be abused in multi‑tenant or embedded contexts where availability is critical. Vendors have issued patches and distributors have prepared kernel package updates.
Supply‑chain and policy implications
This CVE illustrates two important supply‑chain realities:- Product‑scoped attestations are valuable but limited. Microsoft’s decision to start VEX/CSAF publication with Azure Linux gives Azure customers a fast, machine‑readable signal about which Azure Linux artifacts include particular upstream components. However, the absence of an attestation for other Microsoft products is not definitive proof those other artifacts are unaffected — it generally means they have not yet been inventory‑checked and published. Customers running multiple Microsoft‑supplied kernels or images must verify each artifact.
- Upstream fixes propagate but build choices matter. The btrtl code is upstream kernel source; distributions and vendors choose which kernel versions, backports, and configurations to ship. That explains why a fix is available upstream but the presence of the fix in a particular product may lag. Your vulnerability management workflow must map CVEs to the actual binaries in your estate, not only to upstream advisories. Distribution trackers (Debian security‑tracker / OSV / vendor advisories) and the NVD provide useful references for which versions are fixed.
Practical checklist for administrators (actionable)
- Identify Microsoft images you run:
- Azure Linux images (attested - WSL kernels on developer machines.
- Azure Marketplace VM images or Managed Offerings that embed Linux kernels.
- AKS/AKS node images / custom node pools.
- For each image/host, run the verification steps in this article (modinfo / lsmod / check modules directory / inspect kernel config).
- If btrtl is present and you cannot apply a vendor kernel patch quickly:
- Blacklist/unload btrtl and stop Bluetooth services.
- Verify no critical service depends on Bluetooth.
- Schedule patching:
- Apply vendor/distro kernel updates as they become available and test before wide deployment.
- Rebuild and revalidate custom WSL kernels, AKS node images, or custom VM images after applying patches.
- Watch MSRC VEX/CSAF outputs:
- Microsoft has begun publishing machine‑readable attestations and will update CVE mappings if other Microsoft products are identified. In time, those attestations should reduce uncertainty for Microsoft‑published artifacts.
Critical analysis — strengths and gaps in Microsoft’s approach
- Strengths
- Microsoft’s early publication of VEX/CSAF attestations for Azure Linux is an operational win: it gives customers an automatable signal about which Microsoft‑published artifacked and how they are affected. That reduces noise and makes triage faster for Azure Linux users.
- Gaps / Risks
- The attestation is intentionally conservative and product‑scoped. If your organisation runs multiple Microsoft images (WSL kernels, Marketplace images, AKS node images, or vendor images provisioned by Microsoft), you cannot assume those artifacts are clear just because they’re not listed in the MSRC entry. Absence of evidence is not evidence of absence. The operational burden falls on customers to inventory their Microsoft‑supplied kernels or to wait for Microsoft to expand its VEX coverage.
- Machine‑readable VEX is new and Microsoft’s phased rollout means some customers will continue to face manual checks until attestations are extended across more products. That time lag creates windows of uncertainty for organisations with mixed Microsoft Linux artifacts.
Conclusion
CVE‑2025‑37792 is a real but medium‑severity kernel NULL‑pointer dereference in the Realtek Bluetooth helper (btrtl). Microsoft’s advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is correct and actionable for Azure Linux customers — it signals that Microsoft found the implicated upstream code in those images and will supply fixes or attestations. However, that single‑line attestation is not a universal exclusion of other Microsoft products: Azure Linux is the only Microsoft product publicly attested so far, but other Microsoft‑distributed kernels or images could still include the vulnerable driver until they are independently inventoried and attested. Administrators should therefore inventory their kernels and images, apply patched kernel packages from their distro/vendor, and consider temporary mitigations (blacklisting btrtl, disabling Bluetooth services) where immediate patching is not feasible. Microsoft’s VEX/CSAF rollout will reduce this uncertainty over time — but for now, responsible operators must verify, not assume.Source: MSRC Security Update Guide - Microsoft Security Response Center
