# CVE-2025-21991: x86/microcode/AMD: Fix out-of-bounds on systems with CPU-less NUMA nodes > Out-of-bounds memory access in AMD microcode loading on CPU-less NUMA nodes can cause kernel crashes or memory corruption during privileged updates. - **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Local or same network only - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 2 Apr 2025 - **Updated:** 23 May 2026 - **Also known as:** ALSA-2025:10837, AZL-59785, AZL-59843, BELL-CVE-2025-21991, DEBIAN-CVE-2025-21991, GHSA-vp9x-33x6-jvvm, RHSA-2025:10837, RLSA-2025:10837, ROOT-OS-UBUNTU-2404-CVE-2025-21991, UBUNTU-CVE-2025-21991 ## What is CVE-2025-21991? CVE-2025-21991 is a high-severity memory corruption vulnerability in Linux kernel, published on 2 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim. ### Official description In the Linux kernel, the following vulnerability has been resolved: x86/microcode/AMD: Fix out-of-bounds on systems with CPU-less NUMA nodes Currently, load_microcode_amd() iterates over all NUMA nodes, retrieves their CPU masks and unconditionally accesses per-CPU data for the first CPU of each mask. According to Documentation/admin-guide/mm/numaperf.rst: "Some memory may share the same node as a CPU, and others are provided as memory only nodes." Therefore, some node CPU masks may be empty and wouldn't have a "first CPU". On a machine with far memory (and therefore CPU-less NUMA nodes): - cpumask_of_node(nid) is 0 - cpumask_first(0) is CONFIG_NR_CPUS - cpu_data(CONFIG_NR_CPUS) accesses the cpu_info per-CPU array at an index that is 1 out of bounds This does not have any security implications since flashing microcode is a privileged operation but I believe this has reliability implications by potentially corrupting memory while flashing a microcode update. When booting with CONFIG_UBSAN_BOUNDS=y on an AMD machine that flashes a microcode update. I get the following splat: UBSAN: array-index-out-of-bounds in arch/x86/kernel/cpu/microcode/amd.c:X:Y index 512 is out of range for type 'unsigned long[512]' [...] Call Trace: dump_stack __ubsan_handle_out_of_bounds load_microcode_amd request_microcode_amd reload_store kernfs_fop_write_iter vfs_write ksys_write do_syscall_64 entry_SYSCALL_64_after_hwframe Change the loop to go over only NUMA nodes which have CPUs before determining whether the first CPU on the respective node needs microcode update. [ bp: Massage commit message, fix typo. ] Weakness: CWE-129 (Improper Validation of Array Index) ## Which versions are affected by CVE-2025-21991? It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `kernel` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug-devel` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug-devel-matched` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug-modules` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug-modules-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-debug-modules-extra` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-devel` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-devel-matched` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-modules` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-modules-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-64k-modules-extra` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-abi-stablelists` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-cross-headers` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-devel` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-devel-matched` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-modules` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-modules-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-modules-extra` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-debug-uki-virt` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-devel` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-devel-matched` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-doc` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-headers` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-modules` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-modules-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-modules-extra` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt-64k` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt-64k-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt-64k-debug` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt-64k-debug-core` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt-64k-debug-devel` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | | `kernel-rt-64k-debug-modules` | AlmaLinux:9 | All versions | 5.14.0-570.26.1.el9_6 | Products: Linux kernel. ## How do I fix CVE-2025-21991? Apply the vendor's patch. ## Is CVE-2025-21991 being exploited? There is no known exploitation of CVE-2025-21991 in the wild and no public exploit code. ## Other vulnerabilities in Linux kernel - [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild) - [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild) - [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild) - [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild) - [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2025-21991 and fix it if it is affected. What it is: CVE-2025-21991 is a high-severity memory corruption vulnerability in Linux kernel, published on 2 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim. Exploitation: There is no known exploitation of CVE-2025-21991 in the wild and no public exploit code. Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2025-21991/ (Markdown: https://insights.sbomify.com/CVE-2025-21991/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21991 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21991 - OSV: https://osv.dev/vulnerability/CVE-2025-21991 --- From sbomify Insights: https://insights.sbomify.com/CVE-2025-21991/ Summaries are generated and may contain mistakes; the official description above is the record.