# CVE-2026-15923: Infinite loop denial of service in Zephyr SDIO byte-I/O from a card-supplied zero max_blk_size

> A malicious SDIO card can cause an infinite loop and permanent denial of service in Zephyr RTOS by reporting a zero block size.

- **Severity:** Medium 4.6 (`CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-4pvm-wrcp-jjf5

## What is CVE-2026-15923?

CVE-2026-15923 is a medium-severity denial of service vulnerability in zephyrproject zephyr, published on 14 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

The Zephyr SDIO subsystem function sdio_io_rw_extended_helper() in subsys/sd/sdio.c finishes transfers with a byte-I/O loop that uses size = MIN(remaining, func->cis.max_blk_size) as the per-iteration step. The value func->cis.max_blk_size is decoded directly from the SDIO card's CIS FUNCE tuple in sdio_decode_cis() and is not validated. When a card reports a maximum block size of zero, size is always 0, remaining never decreases, and the loop spins forever.

The loop is reached from the public SDIO client API used by drivers, including sdio_read_fifo(), sdio_write_fifo(), and the incrementing register read/write helpers, each of which enters the loop while holding the per-card mutex func->card->lock. A card advertising max_blk_size == 0 therefore hangs the calling thread permanently on its first non-block-aligned transfer and never releases the mutex, denying service to the SDIO peripheral (and any subsystem such as Wi-Fi that depends on it) until the device is reset.

The malicious value must come from the SDIO card itself, so the defect is exploitable where a removable SDIO/combo card slot lets an attacker insert a crafted or malfunctioning card (a physical attack vector); on boards with a soldered SDIO peripheral it is not attacker-influenceable. There is no memory-safety, confidentiality, or integrity impact — only a permanent availability loss. The fix returns -EIO when func->cis.max_blk_size is zero, before the loop is entered.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2026-15923?


## How do I fix CVE-2026-15923?

No fix has been published for CVE-2026-15923 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-15923 being exploited?

There is no known exploitation of CVE-2026-15923 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-15923 and fix it if it is affected.

What it is: CVE-2026-15923 is a medium-severity denial of service vulnerability in zephyrproject zephyr, published on 14 Sep 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-2026-15923 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-2026-15923/ (Markdown: https://insights.sbomify.com/CVE-2026-15923/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-15923
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-15923
- OSV: https://osv.dev/vulnerability/CVE-2026-15923

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-15923/
Summaries are generated and may contain mistakes; the official description above is the record.