CVE-2026-31521

What it actually does

This flaw is a local availability vulnerability in the Linux kernel module loader. When the kernel processes symbols in a loadable ELF kernel module, it uses a symbol’s section index, st_shndx, to index into the module section header array without first confirming the value is in bounds. A malformed, corrupted, or unusual module using an out-of-range section index such as SHN_XINDEX/0xffff can cause an invalid memory access in simplify_symbols(), leading to a kernel page fault and panic. The known impact is denial of service through system crash; the provided CVSS vector does not indicate confidentiality or integrity impact.

Why it matters to the business

Successful exploitation can crash affected Linux systems, causing service outages, workload interruption, data processing failures, and possible reboot loops if the malformed module is loaded repeatedly during startup or automation. Because this affects the kernel, the operational impact can be high even though exploitation requires local access and a path to make the system process a crafted or corrupted kernel module. EPSS is very low and no KEV or ransomware exploitation is reported, so broad active exploitation is not currently indicated; however, systems that build, copy, package, or automatically load third-party kernel modules have higher practical exposure.

How an attack unfolds

An attacker first obtains local access to a Linux system or compromises a workflow that can place or load kernel modules. They then provide a crafted or corrupted ELF kernel module containing a symbol table entry with an out-of-bounds st_shndx value. When the module is loaded, for example through insmod, modprobe, boot-time module loading, driver installation, or automated package handling, the kernel module loader enters simplify_symbols(). Because the vulnerable code does not validate that st_shndx is within the section header array, it dereferences memory outside the valid section table. This can trigger a kernel page fault and panic, immediately crashing the host. A similar failure can occur unintentionally when legitimate module build or copy tooling emits module ELF metadata that uses extended section indexes incorrectly.

How to fix it

Apply Linux kernel updates from the relevant distribution or kernel vendor that include the module loader bounds check for st_shndx in simplify_symbols(). No exact fixed package versions were provided in the input, so remediation should be tracked through vendor advisories for the affected kernel branches in use. Where immediate patching is not possible, reduce exposure by allowing only trusted and signed kernel modules, enabling kernel lockdown or Secure Boot module-signing enforcement where supported, restricting CAP_SYS_MODULE and root-equivalent access, preventing untrusted users from writing to module directories, auditing DKMS and third-party driver pipelines, and disabling module loading after boot with kernel.modules_disabled=1 where operationally feasible. Remove any malformed or unexpected modules from boot-time load lists, initramfs images, DKMS outputs, and /lib/modules trees.

How to confirm the fix worked

Confirm remediation by verifying that the running kernel package includes the vendor fix for this issue, not just that an updated kernel is installed on disk. Reboot into the fixed kernel and confirm with uname -r and package manager metadata. Review vendor changelogs or source patches for bounds checking of symbol st_shndx before accessing module section headers in simplify_symbols(). Validate exposure reduction by confirming module-signing policy, lockdown state, and whether arbitrary module loading is disabled or restricted. For higher-assurance testing in a non-production environment only, attempt to load a malformed test module that previously triggered the panic and confirm the fixed kernel rejects the module cleanly without crashing.

Typical attack path

1. Gain local authenticated access or influence a local module build, packaging, installation, or loading workflow.
2. Create, corrupt, or introduce a kernel module ELF file with a symbol entry whose st_shndx value is outside the valid section header range, such as 0xffff.
3. Place the malformed module where it will be processed by kernel module loading tooling or directly invoke a module load path.
4. Trigger module loading through insmod, modprobe, driver installation, service startup, hardware-triggered autoloading, or boot automation.
5. The kernel processes the module symbol table in simplify_symbols() and performs an out-of-bounds section header access.
6. The invalid access causes a kernel page fault and panic, producing a denial of service until the system is rebooted and the offending load path is removed.

How to detect it

Look for evidence of kernel crashes or failed module loads immediately following module installation, DKMS rebuilds, driver updates, package upgrades, boot-time module loading, or invocations of insmod/modprobe. Relevant sources include system journal logs, /var/log/kern.log, /var/log/messages, /var/log/syslog, pstore/ramoops records, kdump/vmcore analysis, crash utility output, auditd exec logs for module-loading tools, package manager logs, DKMS logs, and EDR telemetry for local execution of insmod or modprobe. Key artifacts include kernel oops or panic traces referencing simplify_symbols(), page faults during module loading, RIP values in simplify_symbols, and a crash occurring directly after a .ko file was introduced or loaded. Also examine /lib/modules, /etc/modules-load.d/, /usr/lib/modules-load.d/, /etc/modprobe.d/, initramfs contents, and DKMS build directories for recently modified or unexpected .ko files. Since exploitation primarily results in a crash rather than persistence or command execution, detection should correlate module-load activity with sudden reboot, panic, or kdump generation.

• simplify_symbols
• simplify_symbols+0x
• RIP: 0010:simplify_symbols
• Kernel panic - not syncing: Fatal exception
• BUG: unable to handle page fault for address
• Fatal exception
• /sbin/insmod
• /sbin/modprobe
• insmod
• modprobe
• .ko
• /lib/modules/
• /etc/modules-load.d/
• /usr/lib/modules-load.d/
• /etc/modprobe.d/
• /var/lib/dkms/
• SHN_XINDEX
• SHN_HIRESERVE
• st_shndx
• 0xffff

What your security team should do

• Prioritize patching Linux systems that load third-party, DKMS, vendor, custom, or frequently rebuilt kernel modules.
• Inventory affected Linux kernel versions across servers, appliances, containers hosts, hypervisors, and embedded systems; ensure the running kernel, not only the installed package, is updated.
• Restrict kernel module loading to trusted administrators and signed modules; remove unnecessary CAP_SYS_MODULE privileges from users, services, and containers.
• Audit module build and packaging pipelines, especially those using objcopy/llvm-objcopy or producing custom modules, for malformed ELF section indexes.
• Search historical kernel logs and crash dumps for simplify_symbols() panics to identify possible prior exposure or accidental triggering.
• Remove suspicious or malformed .ko files and unexpected boot-time module load entries before rebooting vulnerable systems.
• Because EPSS is low, no public exploit maturity is reported, and the issue is not in KEV, treat this as availability-hardening priority unless the environment has high-risk local users or automated third-party module loading.