---
id: CVE-2026-13478
title: >-
  Out-of-bounds read in Zephyr ext2 block-bitmap validation from a crafted
  s_blocks_count
summary: >-
  The Zephyr ext2 filesystem driver validates the on-disk block bitmap in
  ext2_init_fs() (subsys/fs/ext2/ext2_impl.c) by passing fs_blocks =
  s_blocks_count - s_first_data_block to ext2_bitmap_count_set(). That helper
  (subsys/fs/ext2/ext2_b…
severity: medium
cvss: 5.5
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-125
vendor: zephyrproject
product: zephyr
affected:
  - zephyr >= 3.5.0 < 4.3.2
  - zephyr >= 4.4.0 < 4.4.2
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-08-25T19:39:34.544787Z'
published: '2026-08-25'
updated: '2026-11-04'
sourceUpdated: '2026-11-04T04:59:57.906Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-13478'
references:
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/commit/9c0f869da07009d9d4bca7a99e995f9b8cea7da2
    label: Fix commit
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-gj29-7f7m-4c29
    label: GHSA-gj29-7f7m-4c29
tags:
  - cve.org
epss: 0.00143
epssPercentile: 0.03662
ingestedAt: '2026-11-05T20:33:06.708Z'
---

## Overview

The Zephyr ext2 filesystem driver validates the on-disk block bitmap in ext2_init_fs() (subsys/fs/ext2/ext2_impl.c) by passing fs_blocks = s_blocks_count - s_first_data_block to ext2_bitmap_count_set(). That helper (subsys/fs/ext2/ext2_bitmap.c) treats its argument as a number of bits and reads one bitmap byte per eight bits, but the bitmap buffer (BGROUP_BLOCK_BITMAP) is a single fetched block of only fs->block_size bytes (capacity fs->block_size * 8 bits). s_blocks_count and s_first_data_block are taken verbatim from the superblock and were never bounded against this single-group capacity; ext2_verify_disk_superblock() checks the magic, revision, and block-size shift but not the block count.

A crafted ext2 image with an oversized s_blocks_count (up to ~4 billion, against a maximum 4096-byte block / 32768-bit bitmap) makes ext2_bitmap_count_set() scan roughly 512 MB of memory past the bitmap block — a large out-of-bounds read of the static block slab and adjacent memory.

The defect is reached during mount: ext2_init_fs() is invoked from ext2_mount() (subsys/fs/ext2/ext2_ops.c), the registered .mount operation. Any path that mounts an attacker-supplied ext2 image (removable media, a disk/flash partition, or a downloaded image) triggers it. The kernel-privileged parser operates on attacker-controlled data, so the bug is exploitable wherever untrusted ext2 media can be mounted.

Impact is an out-of-bounds read only: the resulting bit count is compared internally and the mount is rejected, so no attacker-controlled bytes are returned (not a useful information leak). The ~512 MB over-read will almost certainly cross an unmapped or MPU-protected boundary and fault, crashing the system — a denial of service triggered by mounting a single malformed image. The fix rejects any image whose fs_blocks exceeds fs->block_size * 8 before the scan.

## Affected

- `zephyr >= 3.5.0 < 4.3.2`
- `zephyr >= 4.4.0 < 4.4.2`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.