# CVE-2026-13478: Out-of-bounds read in Zephyr ext2 block-bitmap validation from a crafted s_blocks_count

> Attackers can crash Zephyr RTOS systems by mounting a crafted ext2 image, causing an out-of-bounds read and system fault.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** No
- **Published:** 25 Aug 2026
- **Updated:** 25 Aug 2026
- **Also known as:** GHSA-gj29-7f7m-4c29

## What is CVE-2026-13478?

CVE-2026-13478 is a medium-severity denial of service vulnerability in zephyrproject zephyr, published on 25 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-13478?


## How do I fix CVE-2026-13478?

No fix has been published for CVE-2026-13478 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-13478 being exploited?

There is no known exploitation of CVE-2026-13478 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-13478 and fix it if it is affected.

What it is: CVE-2026-13478 is a medium-severity denial of service vulnerability in zephyrproject zephyr, published on 25 Aug 2026. 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-2026-13478 in the wild and no public exploit code.

Affected product: zephyrproject zephyr. 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-2026-13478/ (Markdown: https://insights.sbomify.com/CVE-2026-13478/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-13478
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-13478
- OSV: https://osv.dev/vulnerability/CVE-2026-13478

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-13478/
Summaries are generated and may contain mistakes; the official description above is the record.