A recently published Linux-kernel vulnerability, tracked as CVE-2025-22108, patches an out-of-range handling bug in the Broadcom NetXtreme driver (bnxt_en) that can corrupt transmit descriptors and lead to packet transmission timeouts; Microsoft’s advisory currently lists Azure Linux as the only Microsoft product that bundles the affected kernel code, and the company says it will update the CVE entry if other Microsoft products are found to be impacted.
The vulnerability is in the bnxt_en network driver used for Broadcom NetXtreme family NICs in the upstream Linux kernel. The root cause is an incorrect treatment of the bd_cnt field in the TX buffer descriptor (TX BD). The field is encoded in five bits; the hardware uses a 5-bit value where the value representing 32 descriptors is encoded as zero. The upstream fix masks the field properly so that a value of 32 becomes the canonical wrapped value (zero) and does not leave an out‑of‑range value in the descriptor.
Two related context items make this bug material to real-world systems:
Operational takeaway:
Where a precise commit ID or a line-by-line code diff is needed for engineering validation, check the kernel stable branch releases for the series that correspond to your running kernel and review the bnxt_en driver changes under drivers/net/ethernet/broadcom. Note that the practical route for most administrators is to rely on vendor-supplied kernel packages rather than attempting to cherry-pick patches into production kernels.
(If you cannot verify the patch in your environment, treat the claim that Azure Linux is the only affected Microsoft product as current as of the advisory publication and follow vendor guidance while watching for CVE updates.
Action items for IT teams:
CVE-2025-22108 is a reminder that even a five‑bit field, improperly handled, can ripple into operational problems. The fix is straightforward and available through vendor kernels — the practical challenge is ensuring timely, coordinated patching across distributed fleets so that transient networking outages do not become outages for customers or services.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
The vulnerability is in the bnxt_en network driver used for Broadcom NetXtreme family NICs in the upstream Linux kernel. The root cause is an incorrect treatment of the bd_cnt field in the TX buffer descriptor (TX BD). The field is encoded in five bits; the hardware uses a 5-bit value where the value representing 32 descriptors is encoded as zero. The upstream fix masks the field properly so that a value of 32 becomes the canonical wrapped value (zero) and does not leave an out‑of‑range value in the descriptor.Two related context items make this bug material to real-world systems:
- The kernel build-time configuration option CONFIG_MAX_SKB_FRAGS controls the maximum number of fragments per socket buffer (skb). Distributions or custom kernel builds that increase this value can produce more fragments per packet than the NIC expects.
- When the driver writes an unmasked/out‑of‑range bd_cnt into the hardware descriptor, the NIC may interpret the descriptor incorrectly and the transmit ring can be corrupted, which in practice may cause TX timeouts and availability problems for networking on affected hosts.
What the vulnerability actually does (technical summary)
How bnxt_en uses TX BDs
The bnxt_en driver prepares hardware transmit descriptors (TX BDs) that include a small bitfield labeled bd_cnt which denotes the number of buffer descriptors composing a transmitted packet. That field is 5 bits wide in the hardware descriptor layout. Because of the encoding rules used by the hardware, the value that represents 32 descriptors is encoded as 0 in the field (a common hardware encoding trick).The bug
When the kernel or driver code computes the number of fragments for a transmit operation, it must ensure that the count written into the BD is masked to fit the 5-bit field and follow the hardware encoding semantics (i.e., 32 → 0). In some kernel builds where CONFIG_MAX_SKB_FRAGS is increased or when a sk_buff actually contains many fragments, the computed bd count can exceed the permitted range and be written without proper masking. That leaves the descriptor with an out-of-range value that the NIC will misinterpret, leading to descriptor corruption and eventually a transmit timeout.Practical impact
This is not a remote code-execution bug. The bug is a local driver-to-hardware encoding problem that primarily manifests as Availability impact: network transmission failing on affected interfaces, which can look like network interruption, packet drops, or NIC timeouts. The most realistic and likely impact is a local denial-of-service (DoS) against networking on the host or virtual machine using the Broadcom NIC and an affected kernel/driver.Who is affected
- Any Linux system running a kernel that contains the vulnerable bnxt_en code path and that uses Broadcom NetXtreme NICs is potentially affected. This includes physical servers and cloud VMs where the guest or host kernel includes the bnxt_en driver.
- Major Linux distributions quickly issued fixes and backports. Cloud vendor images that carry the affected kernel versions were updated by their maintainers.
- Microsoft’s public vulnerability entry specifically names Azure Linux images as a Microsoft product that ships the affected open-source code, and Microsoft states it will update the CVE if additional Microsoft products are found to include the vulnerable library or code path.
- The vulnerability is in a Linux kernel driver; Windows operating systems do not use the Linux kernel, so Windows clients and Windows Server installations are not affected by this specific Linux driver bug.
- Microsoft’s cloud and service portfolio includes many Linux-based components (for example, Azure Marketplace VM images, Azure Kubernetes Service node images, and “Microsoft-maintained” Linux images). The Microsoft advisory’s current public stance lists Azure Linux as the Microsoft-maintained image explicitly including the vulnerable kernel code; however, other Azure images or managed services that surface Linux kernels can also be affected depending on the kernel versions they run. Microsoft has committed to update their advisory should they discover additional products that include the vulnerable code.
Vendor patch status and distribution responses (summary)
Multiple vendors have identified, backported, and shipped fixes. The pattern across vendors was:- The upstream kernel tree received a patch that properly masks the bd_cnt field and vendors merged that change into their stable and distribution-specific kernels.
- Ubuntu produced kernel package updates targeting the linux-azure image family (Azure-optimized kernels) and other image SKUs.
- Amazon Linux 2023 (kernel6.12 kernel stream) produced ALAS advisories and published fixed kernel packages in June 2025.
- SUSE and other enterprise distributions included the fix in their kernel security updates and provided advisory notes to customers.
- Some vendor trackers initially listed the issue as resolved upstream but noted distribution packaging or backport timelines. A small number of downstream distribution pages indicated later backports or additional patches to also validate fragment count limits.
Microsoft’s position (what they said, and what it means)
Microsoft’s security update guide and advisory entry explains that Microsoft maintains the Azure Linux image family and commits to keeping Microsoft-maintained Linux artifacts current with upstream fixes. On the CVE listing, Microsoft indicates that Azure Linux is the Microsoft product that includes the affected open-source kernel code and therefore is the Microsoft product currently identified as potentially affected. They explicitly state that if additional Microsoft products are found to include the code, they will update the CVE and publish an appropriate response.Operational takeaway:
- If you run Azure Linux images maintained by Microsoft, treat the MSRC advisory as authoritative for Microsoft-managed images and follow their remediation guidance.
- If you run other Linux images on Azure (for example, vendor-supplied Ubuntu, Red Hat, SUSE, or custom images), you must rely on the distro vendor’s advisories and apply the vendor-provided kernel updates for your image. Microsoft’s CVE notice does not automatically imply other Azure-hosted images are safe or affected; the kernel version in the image determines exposure.
Technical verification and provenance
The technical description above and the vendor timelines are consistent across multiple independent sources: upstream kernel commit logs (stable branch merges), major distribution security advisories (Ubuntu USNs), cloud vendor advisories (Amazon Linux ALAS), and public vulnerability databases (NVD/OSV). The upstream fix was merged into the kernel stable trees and propagated into distribution kernels; several vendors indicate the change was included in 6.14-stable or backports into their maintained kernel streams.Where a precise commit ID or a line-by-line code diff is needed for engineering validation, check the kernel stable branch releases for the series that correspond to your running kernel and review the bnxt_en driver changes under drivers/net/ethernet/broadcom. Note that the practical route for most administrators is to rely on vendor-supplied kernel packages rather than attempting to cherry-pick patches into production kernels.
(If you cannot verify the patch in your environment, treat the claim that Azure Linux is the only affected Microsoft product as current as of the advisory publication and follow vendor guidance while watching for CVE updates.
Risk assessment and exploitability
- Exploitability: This is not a remotely exploitable arbitrary code execution vulnerability. It is a local driver-hardware mismatch that can be triggered by a sk_buff with a large fragment count. Typical remote attackers with only network access cannot directly craft packets to exploit a kernel driver in a guest without some form of local access; however, in multi-tenant or multi-process environments local vectors (container escape, unprivileged local processes that can send specially crafted traffic through kernel APIs, or user-space programs using zero-copy or XDP pathways that can produce many fragments) could in theory trigger the condition.
- Privileges required: Low to moderate, depending on the trigger path. Some bug triggers can arise from ordinary packet processing in special circumstances, while other triggers require specific workloads that generate many fragments or use kernel features like scatter-gather send paths.
- Primary impact: Availability — TX timeouts, degraded networking performance, packet loss, and the need to reload the driver or reboot to recover in some cases.
- Scope for cloud providers: Because the bug affects the kernel-level driver, cloud tenants running guest kernels that include the vulnerable driver are exposed. For hypervisors, the host kernel is not typically using bnxt_en unless the host hardware has Broadcom NICs and the host kernel includes the driver; that would expose host-level networking. Cloud providers generally patch both host and guest images as required.
Detection: How to check whether you are affected
Perform the following checks on each Linux host or VM to determine exposure:- Check kernel and driver presence
- View the running kernel:
uname -sr - Check whether the bnxt_en module is loaded:
lsmod | grep bnxt_en - Show module details:
modinfo bnxt_en— this shows the driver version and build info. - Identify Broadcom NICs
- List PCI devices:
lspci -nn | grep -i broadcom - Use
ethtool -i <ifname>to check the driver used by an interface. - Search logs for symptoms
- Kernel messages for transmit timeouts or driver errors:
dmesg | grep -i -E 'bnxt|tx timeout|TX timeout' - System journal:
journalctl -k | grep -i 'bnxt|tx timeout' - Inspect skb frag counts in suspicious workloads
- For applications that intentionally use scatter-gather I/O, check their send semantics. Look for workload patterns that produce unusually large numbers of fragments or use XDP/AF_XDP or zero-copy userspace interfaces.
- Check kernel configuration sources (if you build kernels)
- Confirm whether the running kernel was built with a large
CONFIG_MAX_SKB_FRAGSby examining/boot/config-$(uname -r)or the kernel build configuration file. IfCONFIG_MAX_SKB_FRAGSwas raised in a custom build, that increases the likelihood of seeing the condition in corner cases.
Immediate remediation and mitigation steps
Follow these prioritized steps for affected systems:- Patch promptly: Apply the vendor-supplied kernel update that includes the bnxt_en fix. Vendors have released updated kernel packages for affected kernel series; follow your distribution's security advisory and update accordingly. For Azure Linux customers, apply Microsoft’s Azure Linux updates as provided.
- Reboot or reload the module: After installing the updated kernel or driver package, reboot the host or, where supported and safe, unload and reload the bnxt_en module. Be aware that unloading a NIC driver in use will interrupt traffic.
- Mitigate temporarily if immediate patching is impossible:
- Move critical services off affected hosts (live migration for VMs) or fail over to redundant networking paths.
- For containerized workloads, move high-networking-load containers to nodes that are patched or use different NICs.
- Avoid triggering workloads: Identify workloads that heavily fragment packets (e.g., non-optimal MTU or unusual I/O patterns) and consider temporary tuning or reconfiguration to reduce fragmentation until you can patch.
- Monitor: After patching, monitor
dmesg, ethtool stats, and interface health for recurring timeouts.
Operational recommendations for Azure customers
- If you run Microsoft-maintained Azure Linux images, follow the Microsoft advisory and the Azure portal/automation update channels. Microsoft’s advisory currently lists Azure Linux as the Microsoft product that bundles the affected kernel; Microsoft has pledged to update the CVE entry if it discovers other Microsoft products that include the vulnerable code.
- If you run vendor images (Ubuntu, Red Hat, SUSE, etc. in Azure, treat the distribution vendor’s security notices as authoritative for that image. For Ubuntu images optimized for Azure, the distribution’s Azure-specific kernel updates were published and should be applied to
linux-azurepackages. - For managed services (AKS, managed images), consult the service-specific notices: some managed services perform node OS updates as part of maintenance windows; others require manual node pool updates. Confirm whether your managed service provider or image owner has applied the upstream fix.
- If you operate high-availability or multi-node deployments, schedule kernel updates in rolling fashion to avoid simultaneous network interruption. Leverage VM scale sets and orchestrated maintenance to reduce customer impact.
Why this matters for enterprise IT teams
- Network drivers are a common area where hardware-specific assumptions meet kernel logic. Small bit-field mistakes in descriptor encoding can lead to hardware misinterpretation and availability problems. For large data centers and cloud providers where high-performance NICs and scatter/gather I/O are common, these edge cases can surface with production workloads.
- Patching kernel-level networking drivers requires coordination: reboots, driver reloads, and validation. Avoid surprise reboots by scheduling kernel updates during maintenance windows and by automating validation tests for network stability after patching.
- The vulnerability underscores the importance of a layered vulnerability-management approach: track upstream CVEs, vendor advisories, and cloud-provider notices; test vendor-patched images; and automate kernel-image lifecycle management across environments.
Practical checklist (what admins should do now)
- Audit all Linux hosts and VMs for Broadcom NetXtreme hardware and bnxt_en driver presence.
- Check the running kernel version and vendor advisory for your distribution.
- Apply vendor-supplied kernel/driver updates immediately where available.
- For Azure Linux customers, follow Microsoft’s advisory and apply the provided updates for the Azure-optimized kernels.
- For managed node pools or Kubernetes clusters, drain and update nodes in a rolling pattern.
- Monitor for TX timeouts in system logs and network interface counters pre- and post-update.
- If you must delay patching, implement temporary network redundancy and reduce fragment-heavy workloads.
Strengths of the vendor response — and remaining risks
- Strengths
- Upstream reaction was rapid: the kernel stable tree received a patch to mask the BD field and subsequent vendor backports followed quickly.
- Major distributions and cloud providers published advisories and released updates in short order.
- Microsoft has publicly stated Azure Linux images were considered and will update their CVE entry if further Microsoft products are affected—this is a sign of vendor transparency and an acceptance of the need for an authoritative configuration/impact statement.
- Remaining risks and caveats
- A kernel bug affecting a hardware driver can require reboots or service migrations, and, for some platforms, there may be a patch lag between upstream and the particular enterprise or cloud kernel branch you use.
- Vendor packaging differences mean that even if upstream fixes exist, your deployed kernel ABI or enterprise branch might still be waiting for backports — which makes practical remediation dependent on vendor timelines.
- Microsoft’s current advisory naming only Azure Linux as an affected Microsoft product is correct for Microsoft-managed Azure Linux images as of the advisory, but does not guarantee other Microsoft-managed Linux images or Azure services are unaffected — checking image kernel versions remains essential.
- There is a small chance that an unusual workload path could trigger the bug in environments where the driver exists but typical workloads never hit the frag-count threshold; such rare conditions complicate test coverage and rely on operational telemetry to detect.
Final analysis and recommendations
CVE-2025-22108 is a pragmatic, availability-focused kernel driver bug that was responsibly patched upstream and propagated to distribution kernels. It is not a remote code-execution vector, but because it impacts the NIC transmit descriptor encoding it can cause meaningful network outages on affected hosts.Action items for IT teams:
- Treat this as a high-priority kernel update for systems using Broadcom NetXtreme hardware or Azure Linux images.
- Apply vendor kernel updates promptly and use rolling updates for production clusters.
- For cloud customers, verify the kernel version running in your images and node pools — do not assume that because a cloud provider manages nodes they are already patched unless you have an advisory or automated status confirming it.
- Implement monitoring for TX timeout symptoms and include network-driver related tests in post-patch validation steps.
CVE-2025-22108 is a reminder that even a five‑bit field, improperly handled, can ripple into operational problems. The fix is straightforward and available through vendor kernels — the practical challenge is ensuring timely, coordinated patching across distributed fleets so that transient networking outages do not become outages for customers or services.
Source: MSRC Security Update Guide - Microsoft Security Response Center
