A subtle lifetime-management bug in the Linux kernel’s module subsystem — tracked as CVE-2025-37995 — can lead to the kernel dereferencing an uninitialized completion pointer when code calls kobject_put() on certain internal module kobjects, creating a locally-triggered denial‑of‑service risk that distribution vendors have begun to patch.
Background / Overview
The vulnerability was assigned and published in late May 2025 and is described by upstream sources as a correctness issue in the kernel’s module kobject handling. Specifically, code that creates an internal kobject with the module_ktype can, on error paths, call kobject_put() against that object; the module-specific release path (module_kobject_release()) then attempts to use a completion pointer that was never initialized in this create‑on‑error scenario. The defensive fix is small and targeted: only call complete() when the completion pointer is actually present, so that kobject_put() is safe for module‑type kobjects.
Multiple major Linux distributors and vulnerability trackers catalog this as a real-but-narrow kernel defect and have assigned it moderate-to-high impact ratings based on how the bug can affect availability. Vendors including Ubuntu, Amazon Linux, SUSE, Oracle Linux and Debian have public advisory entries noting the flaw and the availability of patched kernel packages or updates.
What the bug is — a developer’s read of the codepath
To understand the issue we need to walk through three small pieces of kernel logic: the function that creates or looks up module-related kobjects, the module type’s release callback, and kobject_put()’s role in balancing references.
- lookup_or_create_module_kobject(): This helper creates an internal kobject for module bookkeeping and uses module_ktype so that the module‑specific release logic is attached to that kobject. On normal success this kobject participates in the module unload synchronization flow.
- kobject_put(): The generic kobject reference drop; when refcount reaches zero it invokes the ktype’s release method (module_kobject_release() in this case).
- module_kobject_release(): The module-specific release routine performs cleanup and — under normal conditions — uses a completion object to synchronize a full module teardown with the rest of the module system.
The root cause here is a mismatch between the assumptions in module_kobject_release() and the way lookup_or_create_module_kobject() constructs an internal kobject on an
error path. When the kobject was created purely as an ephemeral, internal object because an operation failed, its completion pointer is never initialized the same way it would be for a normal module lifecycle. If kobject_put() is invoked on that ephemeral object, module_kobject_release() can reference the uninitialized completion and call complete() on a garbage pointer. The practical result is an immediate kernel crash or undefined behavior — most notably a host kernel panic or hang — which maps to an availability impact for affected systems.
This is not a remotely exploitable userland bug by default: exploitation requires local kernel code paths that create or manipulate module kobjects, and the attacker must be able to trigger the specific create-on-error sequence that leaves the completion pointer uninitialized. That said, local unprivileged or low-privilege users sometimes have ways to provoke kernel module flows (via drivers, device interfaces, or crafted load/unload sequences), so distributions treat this as a risk worth patching.
Why this matters to system administrators and cloud operators
Kernel bugs that dereference uninitialized pointers are dangerous because they frequently lead to deterministic host crashes or kernel oopses, which in production environments translate to service instability and downtime. The CVE metadata and vendor advisories uniformly characterize the primary impact as an availability issue (denial‑of‑service). Several vendors and advisories have assigned moderate-to-high severity scores; for example, Amazon Linux’s advisory lists a CVSS v3 score of 7.0 for their builds, while SUSE and other trackers place the base scores in the mid‑5 range depending on the attack vector assumptions used. The variance reflects differences in how vendors interpret exploitability and preconditions (local vs. privileged access, required complexity).
For multi‑tenant hosts, hypervisors, container hosts, and systems that load many third‑party modules (for special hardware, kernel‑side tooling, or ephemeral drivers), a kernel panic can ripple into widespread service disruption. The fact that the bug lives in the module/kobject handling code — infrastructure used by many drivers — means it can be triggered indirectly by subsystems that create module kobjects as part of error handling. That makes the vulnerability higher‑impact in environments that dynamically load or probe drivers at runtime.
Upstream fix and code-level summary
Upstream kernel maintainers addressed the issue with a small, surgical change to module_kobject_release() (and/or the helper responsible for creating the kobject) so that the release path verifies whether the completion object is present before invoking complete(). The code change does not alter the normal module unload synchronization semantics for fully initialized module kobjects; it only prevents the erroneous complete() call for ephemeral kobjects created on the error path. Public vulnerability writeups and vendor patch notes point at the stable kernel patches and reference commits that were backported into distribution kernels. Security tooling vendors also reproduce those commit references in their advisory metadata.
In plain terms: the fix is about tightening the teardown logic so a release callback behaves correctly even when the kobject it’s called for was created in an atypical, error-oriented context. That’s a classic kernel hygiene fix — small in code but large in operational importance because it prevents a crash in an otherwise frequently-run path.
Realistic exploitability and threat model
- Attack vector: Local. The bug is not a network‑accessible remote code execution flaw by itself. It requires the attacker to be able to execute code or issue operations on the host that touch the module kobject create/put sequence.
- Privileges required: Low to medium. Several trackers list low privileges required (PR:L) in their vector strings because non‑root actions can in some contexts trigger module interactions; other vendors conservatively require higher privileges in their severity calculations. This split explains why CVSS and vendor severity vary.
- Impact: Availability-first (kernel panic, host hang, denial‑of‑service). There’s no evidence that confidentiality or integrity are directly compromised by this bug; the primary risk is a crash or persistent instability. Vendors characterize the impact as denial‑of‑service rather than remote code execution.
- Complexity: Low in the sense that once an attacker can reach the exact kernel path the crash is deterministic; higher in the sense of obtaining the necessary local foothold or crafting the input sequences to reliably hit the create-on-error path.
Taken together, the vulnerability is best treated as a stability/availability bug with a non‑negligible operational impact in production environments — not a secret, remote RCE that can be trivially weaponized from the Internet.
Which distributions and kernels are affected (and where to get patches)
Because CVE-2025-37995 is an upstream kernel defect,
multiple distribution kernels were affected until the appropriate stable patches were backported. Vendors that have published advisories or references include (non‑exhaustive list):
- Amazon Linux / ALAS — published kernel advisories and patched kernels for Amazon Linux 2 and Amazon Linux 2023 families. These advisories include CVSS scoring information used for internal prioritization.
- Ubuntu — published a CVE entry and enumerated the affected packages and release status; Ubuntu’s security pages reflect the upstream description and track fixes in their kernel packages.
- Debian — the Debian security tracker includes the CVE entry and links to distribution updates and errata.
- SUSE and OpenSUSE — published the CVE listing with a moderate severity and backport status for their supported kernels.
- Oracle Linux — added the CVE to its kernel CVE repository and published errata for UEK/RPM-based kernels.
- Red Hat derivatives and other enterprise distributions — security scanners and advisories (Snyk, Rapid7, etc.) reference the upstream fixes and vendor errata; availability of a fixed package depends on each vendor’s backport policy.
Administrators should consult their vendor’s kernel update channels and errata timelines for the specific kernels they run. The correct remediation is the vendor-supplied kernel package that includes the upstream stable patch; generic backports from upstream stable trees are typically incorporated into distribution kernel releases and into cloud provider images where applicable.
Practical mitigation and remediation guidance
If you manage Linux systems, follow these prioritized steps to reduce risk and restore safety:
- Identify affected hosts:
- Check kernel versions and distribution kernel package versions against your vendor’s advisory. Distribution security trackers list which package versions are patched.
- Apply vendor kernel updates:
- The authoritative remedy is to install the kernel update or backport provided by your distribution vendor (Ubuntu, Debian, SUSE, Oracle, Amazon, etc.). Apply the package updates and reboot at the next maintenance window.
- If you cannot patch immediately:
- Limit local access and untrusted local workloads that might trigger module operations (restrict shell/exec access, hardened container runtimes).
- Avoid loading untrusted kernel modules or running third‑party drivers until patched kernels are installed.
- For cloud environments:
- Use vendor-provided, patched images where available; for managed node pools (for example, in IaaS or container host fleets), ensure nodes are replaced or rebooted after the kernel packages are applied.
- Validation:
- After updating and rebooting, verify kernel package versions and confirm absence of the crash by exercising the code paths that previously triggered module kobject creation (if possible in a test environment).
These steps align with the standard operational playbook for kernel-level vulnerability remediation: identify, patch, reboot, and validate. Because this CVE’s fix is small and non-invasive to normal module semantics, vendor kernels that include the upstream stable patch should behave identically for normal module loads and unloads — the patch only tightens an error‑path assumption.
Why small kernel fixes still matter
CVE-2025-37995 is a textbook example of the asymmetric importance of small changes in kernel code: a single missing initialization or an unchecked pointer in a release path can produce wide-ranging availability impacts that affect thousands of hosts. Kernel code is simultaneously performance-sensitive, concurrently executed, and entrusted with core resource lifecycles; therefore
error-handling code — code that runs when things go wrong — must be as defensively correct as the happy-path logic it serves. This CVE demonstrates how ephemeral objects created during error handling can subvert assumptions in teardown callbacks and how a minimal defensive check prevents catastrophic side effects.
For ops teams, the lesson is operational: do not deprioritize kernel updates labeled “small” or “cosmetic.” Stability-focused fixes often carry high operational value because they close deterministic crash triggers that attackers (or even benign but malformed workloads) can hit to produce outages.
Limitations and remaining uncertainties
- Public advisories describe the bug and the upstream fix succinctly, but they typically do not publish a full exploit-poof-of-concept. There is, to date, no broad public evidence of widespread exploitation of this specific CVE in the wild; vendors treat it as a patch-for-stability and low-likelihood remote threat. That said, absence of public exploitation reports is not a guarantee of non‑exploitation.
- Severity ratings differ between vendors (mid‑5 vs. 7.0 CVSS v3, for example) because scoring depends on assumptions about attacker access and expected impact in a given product context. Administrators should prefer vendor guidance for scheduling patches rather than relying solely on CVSS numbers.
- The upstream code commit references are available in kernel stable branches and were backported by distributors; if you need the exact commit IDs for auditing, vendor advisories and several security databases list the commits that were incorporated into stable kernels. Review your distribution’s errata notes for exact mapping between upstream commits and vendor package versions.
Final assessment and recommendations
CVE-2025-37995 is a narrowly scoped but operationally important Linux kernel defect: it is an uninitialized‑pointer access in the module kobject release path that can produce deterministic kernel crashes when specific error‑path object lifetimes are exercised. Because kernel panics and host hangs are high-impact in production, this class of bug must be treated as a priority for patching by any organization that operates production Linux hosts, especially those that run dynamic module loads, third‑party drivers, or multi‑tenant workloads.
Action checklist (concise):
- Audit your fleet for affected kernels and plan immediate patch windows for critical hosts.
- Apply vendor-supplied kernel packages that include the upstream fix and reboot hosts promptly.
- Where rapid patching is impossible, harden local access and avoid loading non-essential kernel modules.
- Track vendor advisories for your specific distribution and kernel series to confirm backport coverage and package versions.
In short: the technical fix is small and well-scoped, but the operational consequences of leaving the vulnerable kernels unpatched can be large. Prioritize distribution kernel updates, validate the changes in test environments, and treat this CVE as a stability-first vulnerability that warrants timely remediation.
Conclusion: Apply the vendor kernel updates, reboot at the next safe maintenance window, and verify that your host images and automated provisioning pipelines consume the patched kernel artifacts — this removes the crash trigger and restores the expected defensive behavior for module kobject release logic.
Source: MSRC
Security Update Guide - Microsoft Security Response Center