CVE-2025-37857: Azure Linux Attestation and SCSI St Driver Patch

The Linux kernel fix tracked as CVE‑2025‑37857 — described upstream as “scsi: st: Fix array overflow in st_setup()” — is a real, targeted patch that removes an array overflow by sizing a local buffer from the incoming parms length rather than a hardcoded value. Microsoft’s public advisory for the CVE explicitly states that Azure Linux includes the affected open‑source component and is therefore potentially affected, but that statement is a product‑level attestation: it confirms Azure Linux as a known carrier so far, not that Azure Linux is the only Microsoft product that could possibly contain the vulnerable code. This article explains what the vulnerability is, why Microsoft’s wording matters, how to interpret the attestation, where the same kernel code can hide across Microsoft artifacts, and practical remediation and mitigation steps for defenders and administrators. (msrc.microsoft.com)

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Background / Overview​

The upstream description of CVE‑2025‑37857 is short and precise: a buffer/array sizing error in the SCSI tape driver (the st driver) inside st_setup() was fixed by changing a statically sized array to follow the parms length supplied by userland or the kernel calling path. This class of bug can cause an out‑of‑bounds write or overflow in kernel memory when crafted inputs exceed the fixed buffer length, which in turn can crash the kernel or — in some exploit scenarios — be escalated to code execution depending on surrounding memory layout and mitigations in place. Multiple independent vulnerability trackers and distribution advisories document the fix and assign a medium severity/CVSS 3.x 5.5 score with an availability‑focused impact.
Linux distribution security trackers (Ubuntu, Oracle Linux, Amazon Linux, SUSE and others) list CVE‑2025‑37857 against their kernel packages and describe the same upstream change; several maintainers classify the impact similarly (local attack vector, low privileges required, availability impact), and they provide platform‑specific advisory and patch information for kernel packages. Those vendor advisories are the canonical place to find how and when the fix landed for a particular kernel stream.
Upstream commits for this fix are referenced by third‑party trackers and security feeds (for example, Snyk and other code trackers list git.kernel.org commit references for the stable kernel branches that carry the patch). When combined, the upstream patch record + distribution advisories give a clear view of the technical root cause and the timeline for remediation across kernels.

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What Microsoft actually said — and what that wording means​

Microsoft’s Security Response Center (MSRC) entry for this Linux kernel CVE includes the FAQ line many administrators have noticed: “Is Azure Linux the only Microsoft product that includes this open‑source library and is therefore potentially affected by this vulnerability? One of the main benefits to our customers who choose to use the Azure Linux distro is the commitment to keep it up to date ... If impact to additional products is identified, we will update the CVE to reflect this.” That wording has been used consistently across multiple Linux CVE entries on MSRC as Microsoft rolled out machine‑readable CSAF/VEX attestations. (msrc.microsoft.com)
Important nuance: the MSRC line is a product‑scoped attestation about their inventory. It does two things:

• Confirms Azure Linux (the Azure‑branded distribution derived from CBL‑Mariner workstreams) has been inspected and contains the upstream component implicated by this CVE.
• Signals that Microsoft will expand its published product mapping (via CSAF/VEX) if additional Microsoft products are later identified as carriers of the same upstream code.

It does NOT say “no other Microsoft product contains the code” or “only Azure Linux is affected.” Treating the attestation as an exclusivity guarantee is a misreading. That distindefenders who must decide whether to treat other Microsoft SKUs or artifacts as in‑scope. Multiple analyses and community posts have emphasized this point: Azure Linux is the one Microsoft product MSRC has validated and published for this CVE so far, and the attestation is authoritative for that product, but it intentionally does not assert global exclusivity across all Microsoft artifacts.

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Why Azure Linux being named is useful — and why it’s limited​

Naming Azure Linux in the MSRC advisory is operationally useful. If you run Azure Linux images or kernels in your fleet, you now have an explicit Microsoft signal that those images were found to include the impacted upstream component and therefore should be patched according to the listed guidance. Microsoft’s CSAF/VEX rollout — beginning with Azure Linux — is intended to provide machine‑readable, definitive product‑level status for third‑party CVEs so customers can automate risk triage. That is a meaningful step forward in supply‑chain transparency.
However, real world software supply chains are messy. Upstream Linux kernel subsystems are reused across multiple kernel variants and images. Microsoft ships a variety of Linux artifacts and kernels that may be derived from common upstream code:

• Azure Linux kernels used for Cloud VMs (the attested product).
• Azure‑tuned or vendor‑adapted kernel builds used for specific Azure services or Marketplace images.
• Kernel builds embedded in products like , appliance images sold through the Azure Marketplace, or custom VM images published by other teams.
• Third‑party or vendor‑provided kernels that Microsoft ingests or repackages for specialized services.

Because the same upstream kernel source (or backported fixes) can be present in more than one Microsoft product, the absence of an attestation for other Microsoft products should be read as “not yet attested,” not “not affected.” Administrators should therefore treat un‑attested Microsoft artifacts as potentially affected until explicit inventory proves otherwise. This operational posture aligns with best practice for vulnerability triage in complex vendor ecosystems.

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Technical scope: where the vulnerable code can appear​

Understanding what “includes this open‑source component” means in practice is the key to scope decisions. The scsi/st code is part of the Linux kernel tree; any Microsoft image or product that ships a Linux kernel which:

• includes the SCSI tape driver (st) in the configuration (built‑in or as a module), and
• uses a kernel commit or branch that contains the pre‑patch st_setup() implementation,

may include the vulnerable code.
Concretely, that covers:

• distro kernels (Azure Linux kernel builds, CBL‑Mariner‑derived kernels).
• custom Azure VM images that include kernel packages derived from those builds.
• any Microsoft‑built container host images, embedded appliances, or IoT images that reuse the same kernel sources.
• potential third‑party or partner images that Microsoft distributes or uses internally without a separate VEX attestation.

Because Microsoft is gradually rolling VEX for Azure Linux first, other product inventories must be verified independently until Microsoft publishes explicit attestations for those products. This is a generic risk model for any vendor that publishes per‑product VEX/CSAF information: the first attested products are a good starting point, not a closure of the investigative task.

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Evidence from independent sources​

Two independent, authoritative sources document the vulnerability and the upstream fix:

• The U.S. NVD (NIST) entry for CVE‑2025‑37857 records the description and CVSS vector and is the canonical CVE aggregator. It lists the issue as an upstream kernel fix in scsi: st.
• Multiple distribution advisories (Oracle Linux, Amazon Linux/ALAS, Ubuntu, SUSE) list the CVE and provide distro‑specific remediation timelines and package names. Those advisories corroborate that the fix landed in stable kernel trees and that distributions applied fixes across kernel variants.

Third‑party security feeds and vulnerability aggregators (OpenCVE, Snyk, OSV) reference upstream git commits for the fix and index the CVE across ecosystems, which helps defenders spot how different distributions map the change into their package names and kernel versions. Where direct access to upstream git.kernel.org commits is restricted, these aggregators still provide usable cross‑references to the stable commit IDs that carry the change.
Where Microsoft’s MSRC has publicly published a VEX attestation (Azure Linux), that attestation is a separate, authoritative data point about Microsoft’s internal inventory for that product; it complements the upstream and distro evidence rather than contradicting it. (msrc.microsoft.com)

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Risk analysis: real world exploitability and impact​

From the public metadata:

• Attack vector: local. The attacker needs local access or a local user context that can trigger st_setup() with crafted parameters. This reduces remote exploitation likelihood unless the system already exposes attacker‑controlled userland on the target host.
• Privileges required: low. A low‑privileged local user may be sufficient to invoke the path, depending on how the st device is created and permissions for tape devices are configured.
• Impact focus: availability (kernel panic/oops) — CVSS vectors emphasize availability impact. Some kernel OOB writes can be turned into code execution in theory, but modern kernels include mitigation hardening that makes reliable escalation more difficult; that said, local denial‑of‑service is the most credible outcome.

Organizational impact depends on whether systems expose local users (multi‑tenant hosts, developer workstations, CI runners, shared services) and whether the st driver is present and reachable by untrusted users. For cloud VMs and images that ship the st kernel driver, the immediate priority is to apply the vendor kernel update and reboot. For environments that disallow local untrusted users, the urgency is lower but still non‑zero: patching the kernel remains the correct long‑term fix.

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Practical remediation and mitigation guidance​

Below are prioritized, pragmatic steps for defenders. These steps balance a fast, temporary mitigation posture with the recommended long‑term fix (apply vendor patch and reboot).
1.) Identify affected assets

• Inventory kernel versions and kernel package provenance across your estate (Azure Linux images, Marketplace images, custom images, appliances). Use OS package tools (rpm/dpkg) and kernel-version mapping to detect which hosts run kernel builds older than the vendor’s patched kernel. Refer to your distribution advisory for exact patched package names.

2.) Patch and reboot (recommended)

• For Azure Linux: apply the kernel update Microsoft or the Azure Linux advisories list, then reboot to activate the fixed kernel. Microsoft’s VEX/CSAF attestations for Azure Linux will point you to fixed package versions in machine‑readable form. For other distros, apply the distribution's kernel security update and reboot. This is the only complete remediation.

3.) Temporary mitigation: blacklist the st kernel module if your systems do not require SCSI tape support

• To prevent the st module from loading on systems that do not need the tape driver, create a blacklist file such as:
• /etc/modprobe.d/blacklist-st.conf containing:
• blacklist st
• Update your initramfs/initrd as needed (eg. sudo update-initramfs -u on Debian/Ubuntu or sudo dracut -f on RHEL/CentOS) and reboot to ensure the module is not auto‑loaded.
• Note: blacklisting removes tape support entirely — confirm you do not require the st driver for backup hardware or tape libraries before applying. Blacklisting is a temporary mitigation only; the kernel patch remains the recommended long‑term fix.

4.) Restrict local access and privileges

• Where possible, limit who can open or write to block or character device nodes associated with tape devices (/dev/st[I], /dev/nst[/I]). Use udev rules and filesystem ACLs to restrict access to privileged users only.

5.) Compensating detection and monitoring

• Instrument kernel logs and auditd to detect anomalous st_setup() calls or suspicious use of tape device ioctl paths if such monitoring is feasible in your environment. Although kernel ioctl telemetry is noisy, correlation with anomalous user processes may produce actionable alerts.

6.) Validate third‑party and vendor images

• If you consume third‑party Azure Marketplace images or vendor appliances, ask the vendor for a SBOM/VEX/CSAF mapping or verify the kernel version inside the image before deployment. Absence of an explicit Microsoft attestation for those images is not assurance they are unaffected; you must verify each image’s kernel provenance.

7.) Track MSRC VEX and vendor advisories

• Subscribe to MSRC CSAF/VEX feeds and your distro vendor advisories so you can detect when Microsoft expands its attested product scope beyond Azure Linux. Microsoft has publicly committed to update CVE/VEX records as impact to additional products is identified.

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Operational examples and what to look for in inventories​

• Azure Linux: If you run Azure‑provided images that Microsoft labels as Azure Linux (azl3 kernel builds, CBL‑Mariner derivatives), treat them as in‑scope immediately. MSRC’s VEX/CSAF files (and the MSRC advisory pages) list the specific kernel product identifiers Microsoft inspected for each CVE. Administrators should map those kernel package names to their own versions.
• Ubuntu/linux‑azure kernels: Ubuntu publishes linux-azure kernel builds for VMs running in Azure. Those kernels are often downstream packaging of Azure kernels and therefore deserve close attention; Ubuntu’s security notices indicate which linux‑azure builds received fixes for relatedast. If you operate Ubuntu images in Azure, check both the Ubuntu kernel advisory and the underlying Azure kernel lineage.
• Custom Marketplace images and appliances: Don’t assume attestation coverage. If a Microsoft advisory names Azure Linux, but you use a Marketplace image built by an ISV, request the ISV’s SBOM or patch status. Microsoft’s single attestation for Azure Linux does not automatically inherit to all Marketplace or partner images. This is a frequent cause of supply‑chain blind spots.

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Critical assessment — strengths and risks of Microsoft’s approach​

Strengths

• Concrete product attestation: Microsoft’s publication of CSAF/VEX data for Azure Linux gives defenders a machine‑readable, actionable statement about a specific product. That makes automated triage possible and reduces ambiguity for Azure Linux customers.
• Commitment to expand: Microsoft explicitly says it will update CVE entries and publish more VEX mappings as it identifies additional Microsoft products that ship the affected code. This phased approach reduces mistakes that could occur if Microsoft attempted a large, error‑prone blanket mapping across hundreds of artifacts. (msrc.microsoft.com)

Risks and limitations

• Attestation is not exclusivity: The messaging can be misread by operators as “only Azure Linux is affected,” which is dangerous. The safer operational assumption is that any Microsoft artifact shipping comparable kernel sources and configuration could be in scope until attested otherwise. Several community posts and advisory analyses have flagged this exact misinterpretation risk.
• Coverage lag: Microsoft’s initial VEX rollout focuses on Azure Linux; expanding to other Microsoft product families will take time. During the rollout, there is a window where other Microsoft‑shipped kernels may be un‑attested and thus invisible to customers relying purely on MSRC attestation. That window is where supply‑chain blind spots occur.
• Dependency opacity in composite images: Microsoft and other vendors frequently integrate upstream code into composite images and appliances. Unless vendors (or Microsoft) publish SBOMs and VEX statements for all artifacts, customers must perform independent inventories to confirm whether a given image contains vulnerable code.

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Final recommendations (concise checklist)​

• Treat the MSRC Azure Linux attestation as authoritative for Azure Linux images, and apply the listed kernel updates for those images immediately. (msrc.microsoft.com)
• Do not presume other Microsoft products are unaffected simply because they are not yet attested. Inventory your fleet and verify kernel provenance for any Microsoft‑provided or Microsoft‑derived images.
• If you do not require the st tape driver, consider blacklisting it as a short‑term mitigation while you plan, test, and deploy the vendor kernel update. (Blacklisting must be validated against your backup/restore requirements.)
• Subscribe to MSRC CSAF/VEX feeds and your distribution vendor advisories to receive machine‑readable attestations and fixes as soon as they are published.
• For Azure Marketplace images or third‑party appliances: require an SBOM or VEX statement from the publisher before deployment; otherwise, assume the image may carry unpatched upstream kernel code.

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Conclusion​

CVE‑2025‑37857 is a concrete Linux kernel fix for the SCSI tape driver that addresses an array sizing bug in st_setup(). Microsoft’s advisory naming Azure Linux is a useful, authoritative attestation for that product — it tells customers running Azure Linux images that they should treat those images as in‑scope and apply vendor patches. However, that attestation is intentionally product‑scoped and not a categorical statement that no other Microsoft product contains the same vulnerable kernel component. Defenders should treat other Microsoft artifacts as “not yet attested” until VEX/CSAF publications or independent inventory prove otherwise, prioritize patching and rebooting for attested images, and use targeted mitigations (such as blacklisting the st module) where appropriate while fixes are staged and tested. For the most reliable protection, combine vendor VEX attestations with your own SBOM‑based inventories and a disciplined patch‑and‑reboot program.

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Source: MSRC Security Update Guide - Microsoft Security Response Center