A recently disclosed Linux-kernel vulnerability in the netfilter nftables subsystem can cause a kernel crash when a rule references certain stateful objects from the OUTPUT hook; maintainers fixed the defect by adding proper validation for objref and objrefmap expressions so that referencing a synproxy stateful object from OUTPUT no longer triggers infinite recursion and a stack-guard crash.
This matters because kernel code appears in many different images and builds. Even if Microsoft or another vendor has attested a specific image lineage (for instance, an Azure Linux image), other artifacts from the same vendor might still carry the vulnerable upstream code depending on version and build configuration. Administrators must therefore inventory their environment and reconcile vendor attestations with actual kernel builds present in their estate rather than relying solely on a single vendor’s published product scope.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
What changed and why it matters
The defect is located in the nftables objref handling code: when rules used an objref or objrefmap expression that pointed to a synproxy state object and that reference was reached from the OUTPUT hook, the kernel could run into infinite recursive calls that ultimately hit the kernel’s stack guard page and crash the system. The public descriptions accompanying the fix include a multi-frame call trace showing the recursion through route lookup, synproxy evaluation, nftables chain execution, and back into the networking output path — the exact chain that produced the stack exhaustion. Upstream developers addressed the root cause by implementing explicit validate functions for objref and objrefmap expressions. These validation routines ensure that object references are checked for hook- and context-compatibility before they are used in evaluation paths, preventing the pathological path that caused recursion and the subsequent crash. The change is recorded in the kernel stable trees and distribution advisories.Netfilter, nftables and the role of objref/objrefmap
- Netfilter / nftables is the kernel subsystem that implements packet filtering, classification and stateful firewall rules for modern Linux kernels.
- objref and objrefmap are expression types in nftables that allow rules to reference stateful objects (for example, conntrack, synproxy, or user-defined object types) rather than only literal values.
- Synproxy is a stateful helper used to protect servers from SYN flood attacks: it synthesizes SYN/ACK responses, validates client behavior, and only forwards real connection state to the stack when the handshake completes.
Technical anatomy — how the crash happened
The crash trace and the recursive loop
Public advisories reproduce the stack trace seen during the failure, which is instructive because it makes the attack surface and the failure mode concrete. The trace fragments reported by NVD and distribution trackers show the kernel walking routing/fib code and calling synproxy evaluation routines inside nftables, which then re-entered the packet output processing paths. That re-entry produced a recursion that exhausted stack space and hit the stack guard (a kernel hardening mechanism that prevents stack overflows), producing a fatal oops/crash. In human terms: an nft rule that referenced a synproxy object in a path that eventually invoked OUTPUT-style processing could cause the kernel to evaluate synproxy code that itself initiated an output path evaluation — the same path was re-entered repeatedly until the kernel stack overflowed.Why validation stops this
Before the patch, the code allowed certain objref/objrefmap references to be used in contexts where they could cause a chain-follow or a jump that is unsafe for that hook. The new validation routines explicitly check:- the object type (currently, the fix focused on NFT_OBJECT_SYNPROXY),
- whether a referenced object can be legally dereferenced in the current hook/context,
- and whether following a jump to a chain using that object is permitted.
Affected systems and practical exposure
Who is at risk
Any Linux kernel build that:- includes the nftables/netfilter code paths in question, and
- compiled with the synproxy object support and the relevant objref/objrefmap expression handling, and
- has not received the upstream stable fix or a vendor backport,
Attack vector and privileges required
- Vector: local/operational. The immediate trigger is a crafted nftables rule (or a rule installation operation) that references a synproxy object in an OUTPUT hook context. This means an attacker typically needs the ability to submit or install nft rules or manipulate objects on the host.
- Privileges: low-to-medium. The practical privilege needed depends on how a system exposes nft operations. On many systems, rule insertion requires administrative access; in multi-tenant or misconfigured environments with delegated management, an attacker with restricted but sufficient control over rules could trigger the crash. There is no authoritative public evidence showing a remotely triggerable, unauthenticated exploit that crashes arbitrary hosts over the network without local rule installation capabilities.
Severity and scoring
Different vendors and trackers have assigned varying severity assessments. Amazon’s ALAS tracker, for example, classifies the issue as Important and lists a CVSS v3 base score of 7.0 for their packaging mapping (their vector includes AV:L/AC:H/PR:L/UI:N with high impact flags for confidentiality, integrity and availability in some mappings). NVD’s public entry was initially marked “Awaiting Analysis” at the time of upstream publication, which is a normal part of NVD enrichment. Administrators should rely on their distro/vendor advisories for patched package versions and severity context for their specific kernel builds.What was changed upstream (and where to find the fix)
The upstream patch and stable merges
Upstream maintainers implemented the objref and objrefmap validate functions and merged the changes into the kernel stable trees. Public vulnerability trackers and distribution advisories point to the stable commit IDs (the stable commit references are linked in aggregators and distro trackers), and vendors have begun or will begin mapping these commits into packaged kernel updates and backports for maintained kernel branches. The fix is small and focused: add validation checks and return an error for invalid uses, rather than performing a risky in-place behavioral change.Where to look for patched kernels
- Check your distribution’s security tracker (Debian, Ubuntu, SUSE, Red Hat, Amazon Linux, etc. for entries referencing CVE‑2025‑40206 and the corresponding fixed package versions.
- For workloads running vendor/embedded kernels (network appliances, vendor images), consult the vendor advisory or support channel — upstream commits don’t automatically reach all OEM images at the same time.
- If you build kernels in-house, merge the upstream stable commit(s) flagged by public trackers and rebuild/test for your configuration before deployment.
Detection, triage and mitigation guidance
Detection and hunting signals
- Kernel logs (dmesg, journalctl -k) containing stack-guard or stack overflow messages, especially traces that include functions like nft_do_chain, nft_synproxy_do_eval, synproxy_send_client_synack_ipv6, or similar nexthop/route lookup frames.
- Reproducible kernel oopses observed during nft rule installs or when applying rules that reference synproxy objects.
- Unexpected “Operation not supported” errors when attempting to install previously working rules (this could indicate a kernel rejecting an invalid objref use after validation changes; verify package versions).
When triaging crashes, capture vmcore or use kdump where possible; kernel oops output is evidence for mapping the crash to the vulnerability.
Immediate mitigations (if you cannot patch immediately)
- Restrict who can install or modify nftables rules. Lock down management-plane access and netlink endpoints to trusted administrators only.
- Where possible, avoid referencing synproxy objects from OUTPUT hook rules; simplify rule sets to use only well-audited, common constructs until patched kernels are available.
- In multi-tenant environments, enforce stricter role separation and privilege controls so tenants cannot insert arbitrary nft rules or objects.
- Monitor kernel logs and set alerts for the stack-guard messages and the specific call-trace signatures reported in advisories.
These mitigations are compensating controls — they reduce exposure while waiting for vendor-supplied kernel updates.
Patch and validation checklist (recommended rollout)
- Inventory: list hosts running kernels that include nftables (query uname -r and check the kernel config for nftables/synproxy support).
- Map: match installed packages to vendor advisories for CVE‑2025‑40206 and identify fixed package versions or backport notes.
- Stage: deploy patched kernels in a pilot group; exercise representative network workloads, especially nftables rules that reference stateful objects.
- Validate: confirm the kernel no longer crashes when the previously repro’d rule is applied, and verify dmesg for related warnings.
- Rollout: deploy to production in waves and monitoring for kernel stability regressions.
- Preserve evidence: if you observed crashes, keep vmcores and logs for comparison during post-patch validation.
Vendor mapping and rollout nuance — Microsoft example
Microsoft’s Security Response Center and other major vendors have started publishing machine-readable attestations and product mappings for upstream CVEs. When a vendor (for example, Microsoft) states a particular Microsoft product includes an upstream component and is “potentially affected,” that is an inventory attestation for that product — not an exclusivity statement about every product the vendor ships. In other words, one should treat a vendor attestation as authoritative for the named product but still validate other vendor artifacts independently (for example, WSL kernels, CBL‑Mariner builds, marketplace images or custom kernels). This distinction has been emphasized in operational guidance because vendors often roll out attestations and product-by-product mappings in phases.This matters because kernel code appears in many different images and builds. Even if Microsoft or another vendor has attested a specific image lineage (for instance, an Azure Linux image), other artifacts from the same vendor might still carry the vulnerable upstream code depending on version and build configuration. Administrators must therefore inventory their environment and reconcile vendor attestations with actual kernel builds present in their estate rather than relying solely on a single vendor’s published product scope.
Critical analysis — strengths of the fix and lingering risks
Strengths and positive aspects
- The upstream change is surgical and conceptually simple: add explicit validation routines for objref/objrefmap expressions and reject invalid uses. That approach reduces the chance of unintentional behavioral regression because it avoids changing synproxy semantics or reordering complex networking logic.
- Fix landed in the stable kernel trees and has been referenced by distribution trackers — a well-trodden path for kernel fixes that lets distributions backport the change into supported branches. This makes it straightforward for operators to obtain fixes via standard package channels.
Risks and operational caveats
- Backport variance: Different vendors and distributions backport upstream commits on different schedules and with different strategies. The presence or absence of a numeric kernel version alone is not authoritative; operators must check package changelogs or advisory metadata to confirm the fix is present. This is a recurring source of confusion in kernel CVEs.
- Multi‑artifact exposure: Cloud images, appliance images, WSL kernels, and vendor-supplied images are separate artifacts; an advisory for one product does not guarantee all related images are patched. Administrators running mixed environments must treat each image/artifact independently.
- No widespread PoC (yet): At disclosure, there were no authoritative public proofs-of-concept showing mass exploitation or a reliably weaponized remote chain. That reduces immediate panic but does not eliminate long-term risk: kernel crashes are high-value primitives for attackers who already have a local foothold, and denial-of-service can be highly impactful in multi‑tenant cloud environments. Maintain conservative posture and prioritize patching for at-risk hosts.
Practical examples — what admins reported and what to watch for
- Symptom: attempt to create or use a synproxy object in an OUTPUT hook rule results in immediate kernel oops and host reboot or system hang. Kernel backtrace includes nft_synproxy_do_eval and nft_do_chain frames. If you see these symptoms, it maps strongly to the vulnerable code path described in CVE records.
- Symptom: installing an nft rule that previously worked now returns “Operation not supported” from nft (this may indicate the kernel’s validation rejects the objref/objrefmap as incompatible; confirm package versions because you may be running a patched kernel that intentionally disallows the previously invalid construct).
- Recommended quick triage commands:
- Capture dmesg immediately after the event: sudo dmesg | tail -n 200
- Search kernel logs for nft/sysproxy traces: sudo journalctl -k --no-pager | egrep -i 'nft|synproxy|nft_synproxy|nft_do_chain'
- Confirm kernel package metadata and changelog: dpkg -l | grep linux-image (Debian/Ubuntu) or rpm -q --changelog kernel-default (SUSE/Red Hat flavors).
Recommended timeline and prioritization
- Immediate (0–48 hours)
- Inventory hosts with nftables/synproxy support and identify whether they are multi‑tenant, cloud-facing, or otherwise exposed.
- If you host multi‑tenant or cloud images, prioritize these systems for patching.
- Short term (48–96 hours)
- Acquire vendor or distribution updates that reference CVE‑2025‑40206 or the upstream stable commits and stage them in a small pilot.
- Implement administrative controls to limit who can insert nft rules where possible.
- Medium term (1–3 weeks)
- Roll out patched kernels across production in waves, monitoring for regressions.
- Update documentation and automation (CMDB, patching systems) to reflect remediation status.
- Long term (ongoing)
- Add checks to automated inventory for kernel configs and upstream commit coverage so similar issues are detected quickly.
- Review rule-creation workflows and consider policy or tooling changes that limit dangerous or complex stateful object uses in rule chains.
Conclusion
CVE‑2025‑40206 is a targeted kernel robustness bug in the netfilter/nftables objref handling that can cause a kernel crash when a synproxy state object is referenced from the OUTPUT hook. The upstream fix is pragmatic and defensive: implement validation for objref and objrefmap expressions and reject invalid, context-incompatible uses rather than attempting risky behavioral changes. Administrators should treat the issue as a high-priority patching item for systems that run nftables and synproxy, especially multi‑tenant cloud hosts and network appliances. Confirm your actual kernel packages include the upstream stable commit or vendor backport before marking systems remediated, and use the compensating controls suggested above while you stage updates.Source: MSRC Security Update Guide - Microsoft Security Response Center
