The Linux kernel team has assigned CVE-2025-68724 to a recently patched integer‑overflow bug in the asymmetric_keys subsystem — a defensive fix that uses explicit overflow checks (check_add_overflow/size_add/struct_size) in asymmetric_key_generate_id to prevent a potential buffer overflow when the kernel copies data taken from untrusted X.509 fields such as ASN.1 INTEGER serial numbers or long issuer/name fields.
The Linux kernel’s asymmetric key support is the in‑kernel mechanism that stores public‑key material used by features like module signing, integrity attestation and other crypto services. Asymmetric keys are created from data blobs (X.509 certificates, raw public keys, etc., parsed by in‑kernel data‑parsers, and indexed by generated identifiers. The parser and id generation paths must therefore handle arbitrary certificate input supplied by userspace or by build-time artifacts. On December 24, 2025 the vulnerability track for CVE-2025-68724 was published: it documents an arithmetic overflow when computing the allocation size for an asymmetric_key_id, caused by adding multiple variable blob lengths (certificate fields) and the fixed size of the identifier structure. The upstream remedy adds explicit overflow guards and uses struct_size/size_add helpers to compute and validate the intended allocation before any memcpy occurs, returning ERR_PTR(-EOVERFLOW) on detected overflow instead of proceeding.
The vulnerability underscores two enduring principles for systems security: validate sizes before allocation, and treat any kernel path that consumes externally supplied certificate material as high‑value attack surface. The record from upstream commits and multiple independent vulnerability trackers confirms the fix and the scope — administrators should treat this as a timely, actionable patch and prioritize updates where asymmetric key handling is reachable.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
The Linux kernel’s asymmetric key support is the in‑kernel mechanism that stores public‑key material used by features like module signing, integrity attestation and other crypto services. Asymmetric keys are created from data blobs (X.509 certificates, raw public keys, etc., parsed by in‑kernel data‑parsers, and indexed by generated identifiers. The parser and id generation paths must therefore handle arbitrary certificate input supplied by userspace or by build-time artifacts. On December 24, 2025 the vulnerability track for CVE-2025-68724 was published: it documents an arithmetic overflow when computing the allocation size for an asymmetric_key_id, caused by adding multiple variable blob lengths (certificate fields) and the fixed size of the identifier structure. The upstream remedy adds explicit overflow guards and uses struct_size/size_add helpers to compute and validate the intended allocation before any memcpy occurs, returning ERR_PTR(-EOVERFLOW) on detected overflow instead of proceeding. What the bug actually is
Technical anatomy
At a high level the bug is a classic signed/unsigned arithmetic and allocation error:- The kernel code sums several variable lengths (the sizes of binary certificate blobs parsed from X.509 fields) plus the static size of the asymmetric_key_id structure to compute the total bytes needed for an allocation.
- That addition sequence originally lacked robust overflow checks; if the sum wrapped (integer overflow) the allocator would receive a smaller-than-intended size and a subsequent memcpy could copy more bytes than were allocated, producing a buffer overflow and kernel memory corruption.
- The vulnerability is therefore a memory‑corruption risk rooted in unchecked arithmetic prior to an allocation + memcpy pair — precisely the kind of bug that defensive kernel code aims to eliminate by using check_add_overflow / size_add helpers and struct_size for allocation bounds.
Where it lives in the tree
The defect is in the crypto/asymmetric_keys code path, specifically in the routine that generates the asymmetric key identifier (asymmetric_key_generate_id). That function aggregates parsed parts of certificate blobs to build the key identifier blob; the added overflow guards prevent the arithmetic wrap while the use of struct_size and size_add ensure the kernel allocates exactly the right number of bytes. The patch series and downstream stable backports were circulated on the kernel lists and submitted to stable trees.Why this matters: attack surface and practical impact
What can be abused
The problematic inputs are X.509 certificate fields — ASN.1 INTEGER serial numbers, issuer/subject name fields, and other binary blobs — all of which can be arbitrarily long when an attacker controls the certificate material. If untrusted certificate blobs are passed into the kernel's asymmetric key instantiation path, an attacker can craft oversized fields to trigger the arithmetic wrap and subsequent unsafe copy.Realistic threat model
- The canonical exploitation model is to feed a vulnerable kernel path a carefully constructed X.509 blob that causes the sum of lengths to overflow the integer used for allocation.
- Whether an attacker can exploit this remotely depends on whether a reachable userspace component forwards untrusted certificates into the kernel’s asymmetric key parser. Typical examples include operations that add keys to kernel keyrings (keyctl padd asymmetric), module signing and module-loading flows that accept certificate blobs, or management agents that programmatically add certificates into kernel trust lists.
- Public trackers and early enrichment feeds do not list a confirmed remote exploit in the wild and classify the vector as requiring actor-controlled certificate input; that makes the issue primarily a local‑or‑supply‑chain risk unless a network path is present that injects kernel‑bound certificate blobs.
Likely impacts
- Primary impact: denial of service (kernel oops, panic) and memory corruption leading to system instability. Buffer overflows in kernel context can crash systems, corrupt other kernel structures, or cause undefined behavior.
- Secondary impact: conditional — kernel memory corruption has historically been used as a stepping stone in local privilege-escalation chains, but turning this specific overflow into reliable arbitrary code execution would require extra primitives (information leak, predictable layout, etc.. There is no public proof-of-concept today demonstrating RCE via this exact defect. Treat claims of immediate RCE as unverified until a technical write‑up or PoC appears.
What upstream changed (the patch)
The upstream patch series implemented two complementary defensive changes:- Use explicit overflow checks (check_add_overflow/size_add) when adding the blob lengths and the struct size; on overflow the function bails out with an error (ERR_PTR(-EOVERFLOW) rather than allocating or copying.
- Use struct_size/size helpers to compute allocation lengths in a way that documents intent and is less error prone than manual multiplication/addition.
- Add small code tidy-ups so the allocation path fails safely and returns error codes that callers can handle without invoking unsafe copies. The patches were circulated, reviewed and merged into stable trees; distribution trackers have begun to import the CVE and the stable commits.
Verification and corroboration
Multiple independent vulnerability indexes and distro trackers have recorded CVE-2025-68724 and describe the same technical fix and rationale:- NVD (NIST) published the CVE summary and points to the upstream remedy (overflow guard + return -EOVERFLOW).
- OSV and Debian tracker entries mirror the NVD summary and show the CVE mapped for downstream package tracking.
- Public kernel mailing list posts and stable‑patch announcements describe the patch subtleties and the use of size_add/struct_size to avoid arithmetic wrap and unsafe memcpy — these provide direct developer discussion and commit-level context.
Who should care and how to prioritize
High priority targets
- Systems that accept X.509 certificate blobs from untrusted sources and add them to kernel keyrings, module signing trust lists, or other kernel-integrated key stores.
- Gateways, appliance images and embedded devices where certificate parsing or key provisioning code runs with minimal supervision or where a user or remote actor can influence certificate contents fed into kernel paths.
- Build infrastructures and CI images that bake certificates into kernel images or populate built-in trusted keys during automated builds.
Lower priority targets (but still important)
- Desktops or servers that never add new asymmetric keys into kernel keyrings at runtime, or those that only process certificates in userspace with no kernel involvement, face a lower immediate risk — but they still must follow distro advisories because kernel images can be rebuilt or reconfigured over time.
Detection: how to know whether you’re affected
- Check whether your kernel was built with asymmetric key support: look for CONFIG_ASYMMETRIC_KEY_TYPE or the asymmetric_keys module. On many distributions this option is enabled because it underpins module-signing and integrity features. Use kernel config inspection or kernelconfig.io references to confirm.
- Look for asymmetric keys in the running keyrings: /proc/keys shows asymmetric key entries (the short tail of the key fingerprint and subtype are displayed). If you see asymmetric entries or the asymmetric_keys module loaded, your kernel has the relevant code paths active.
- Inspect distro security trackers and package changelogs: Debian, SUSE, and other maintainers have already added CVE records to their trackers; match your running package/kernel version to the distribution’s fixed package version to determine if an update is required.
- lsmod | grep asymmetric
- zgrep CONFIG_ASYMMETRIC_KEY_TYPE /proc/config.gz
- cat /proc/keys
Remediation and mitigation playbook
Definitive fix — apply upstream/distribution patches
- Install kernel packages from your distribution that include the upstream fix (look for stable backports that reference the asymmetric_keys patch or CVE-2025-68724). Reboot into the patched kernel. Distribution trackers (Debian, SUSE, etc. have entries mapping the CVE to package updates.
- For systems with vendor-supplied appliance kernels (embedded devices, OOM-edge gateways), contact the vendor for a firmware/kernel update and ask for confirmation that the asymmetric_key_generate_id fix was included in the build. Do not assume an absence of a distributor advisory means your product is unaffected — inventory is the only authoritative answer.
Short‑term compensations when patching is not immediately possible
- Restrict who can add asymmetric keys into kernel keyrings: audit and limit processes or accounts able to call keyctl padd asymmetric or other kernel key installation paths. On multi-tenant hosts, enforce least privilege and tighten user namespaces.
- Isolate certificate ingestion: run any service that auto-adds certificates to kernel keyrings inside a sandboxed container or a separate host, and rate-limit or gate the certificate ingestion pipeline so crafted inputs cannot reach the kernel unfiltered.
- Validate inputs in userspace before passing them to kernel interfaces: enforce maximum lengths on cert fields, reject suspiciously large certificates, or perform ASN.1 validation in userspace and only forward well-formed, sized artifacts. This is imperfect but can reduce attack surface until a patch is installed.
Long‑term hygiene
- Add unit/fuzz tests that exercise the asymmetric key parsing code with large and malformed certificate fields to ensure allocation math remains defensive across future changes.
- Prefer use of the kernel’s safe arithmetic helpers (size_add/check_add_overflow/struct_size) when computing allocation sizes; the upstream patch demonstrates this practice.
Risk analysis and caveats
- Public telemetry and vulnerability trackers list the CVE and the upstream fix; there is no widely published proof‑of‑concept exploit at this time. That reduces immediate panic but does not imply the bug is benign — overflow classes in kernel context are inherently high‑risk and warrant prompt attention.
- The effective exploitability depends heavily on local trust boundaries: if your deployment never transfers user‑controlled X.509 blobs into kernel key routines, your live exposure is small. Conversely, systems that automatically accept certificates into kernel trust stores (for module signing, remote management or automated provisioning) should treat this as urgent.
- Vendor advisories and distributor package versions are the operational truth for whether a host is fixed; kernel version numbers alone can be misleading because vendors sometimes backport selective fixes. Always match the vendor package changelog to the upstream commit rather than relying on raw kernel version heuristics.
The MSRC page and vendor mapping (note about the user-supplied link)
The Microsoft Security Response Center (MSRC) mapping page provided in the initial prompt either did not render or was unavailable in the environment used to check the advisory. Public CVE trackers and upstream kernel patch postings are the primary sources documenting CVE-2025-68724 at the time of writing; if you rely on Microsoft product attestations in addition to upstream kernel fixes, watch Microsoft’s update guide or vendor product pages for explicit product mappings or VEX/CSAF attestations that confirm whether specific Microsoft images include the affected component. If the MSRC page is missing or returns an error for this CVE, it means Microsoft has not (yet) published a product‑by‑product attestation for this specific kernel fix through that portal, not that the issue is not real — rely on upstream commits and distribution advisories for immediate technical guidance.Action checklist (operational summary)
- Inventory: Confirm whether your kernels are built with asymmetric key support (CONFIG_ASYMMETRIC_KEY_TYPE) and whether your hosts run asymmetric_keys module or have entries in /proc/keys.
- Patch: Install vendor/distribution kernel updates that list CVE-2025-68724 or include the upstream stable commit(s). Reboot hosts to activate the patched kernel.
- Mitigate: If patching is delayed, restrict key addition operations, isolate certificate ingestion, and validate certificate sizes in userspace before passing them to kernel interfaces.
- Monitor: Watch distribution security trackers (Debian, SUSE, upstream kernel lists) for backport notices and any emergent proof‑of‑concepts or exploitation indicators.
Conclusion
CVE-2025-68724 is a straightforward — but important — example of why careful arithmetic and safe‑allocation patterns are essential in kernel code that ingests untrusted binary formats. Upstream kernel maintainers have applied conservative, low‑risk fixes (overflow guards + structured allocation helpers) that remove the root cause. The practical remediation path is also straightforward: apply distribution kernel updates (or vendor firmware updates for embedded devices), restrict who can add asymmetric keys in the meantime, and add defensive validation in userspace ingestion pipelines.The vulnerability underscores two enduring principles for systems security: validate sizes before allocation, and treat any kernel path that consumes externally supplied certificate material as high‑value attack surface. The record from upstream commits and multiple independent vulnerability trackers confirms the fix and the scope — administrators should treat this as a timely, actionable patch and prioritize updates where asymmetric key handling is reachable.
Source: MSRC Security Update Guide - Microsoft Security Response Center
