# CVE-2026-31521

> Vulnerability · severity: **MEDIUM** (CVSS 5.5).

## Description

In the Linux kernel, the following vulnerability has been resolved:

module: Fix kernel panic when a symbol st_shndx is out of bounds

The module loader doesn't check for bounds of the ELF section index in
simplify_symbols():

       for (i = 1; i < symsec->sh_size / sizeof(Elf_Sym); i++) {
		const char *name = info->strtab + sym[i].st_name;

		switch (sym[i].st_shndx) {
		case SHN_COMMON:

		[...]

		default:
			/* Divert to percpu allocation if a percpu var. */
			if (sym[i].st_shndx == info->index.pcpu)
				secbase = (unsigned long)mod_percpu(mod);
			else
  /** HERE --> **/		secbase = info->sechdrs[sym[i].st_shndx].sh_addr;
			sym[i].st_value += secbase;
			break;
		}
	}

A symbol with an out-of-bounds st_shndx value, for example 0xffff
(known as SHN_XINDEX or SHN_HIRESERVE), may cause a kernel panic:

  BUG: unable to handle page fault for address: ...
  RIP: 0010:simplify_symbols+0x2b2/0x480
  ...
  Kernel panic - not syncing: Fatal exception

This can happen when module ELF is legitimately using SHN_XINDEX or
when it is corrupted.

Add a bounds check in simplify_symbols() to validate that st_shndx is
within the valid range before using it.

This issue was discovered due to a bug in llvm-objcopy, see relevant
discussion for details [1].

[1] https://lore.kernel.org/linux-modules/20251224005752.201911-1-ihor.solodrai@linux.dev/

## Key facts

- **CVE ID:** CVE-2026-31521
- **Published:** 2026-04-22
- **CVSS severity:** MEDIUM
- **CVSS base score:** 5.5
- **CWE codes:** CWE-787

## Affected products

- `linux:linux_kernel`

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31521

## Citation

> AI Analytics. CVE-2026-31521. Retrieved 2026-09-01 from https://api.ai-analytics.org/cve/CVE-2026-31521. Derived from NIST NVD. Licensed CC0.

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