# CVE-2026-107838: RIOT: nanocoap_fileserver ignores response initialization failure, leading to reachable assertion

> Remote attackers can cause a denial of service in RIOT OS devices by triggering an assertion failure in the nanocoap fileserver when token extensions are enabled.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/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:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 9 Oct 2026
- **Updated:** 13 Oct 2026
- **Also known as:** GHSA-39j3-3v73-5mj2

## What is CVE-2026-107838?

CVE-2026-107838 is a high-severity denial of service vulnerability in RIOT-OS RIOT, published on 9 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

RIOT is an open-source microcontroller operating system designed for Internet of Things devices and other embedded systems. From version 2023.07 through version 2026.07, nanocoap_fileserver callers in sys/net/application_layer/nanocoap/fileserver.c ignore a failure returned by _resp_init() when coap_build_reply() cannot fit a response header into the response buffer. A remote client can send a CoAP request with a sufficiently large extended token when nanocoap_token_ext is enabled, causing response initialization to fail while _get_file() or _get_directory() continues with stale response state. The path then reaches _calc_szx2() and its pdu->payload_len > reserve assertion, terminating the affected service or device task. No fixed release is available as of this review.

Weakness: CWE-617 (Reachable Assertion), CWE-252 (Unchecked Return Value)

## Which versions are affected by CVE-2026-107838?


## How do I fix CVE-2026-107838?

No fix has been published for CVE-2026-107838 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-107838 being exploited?

There is no known exploitation of CVE-2026-107838 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-107838 and fix it if it is affected.

What it is: CVE-2026-107838 is a high-severity denial of service vulnerability in RIOT-OS RIOT, published on 9 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-107838 in the wild and no public exploit code.

Affected product: RIOT-OS RIOT. 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-2026-107838/ (Markdown: https://insights.sbomify.com/CVE-2026-107838/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-107838
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-107838
- OSV: https://osv.dev/vulnerability/CVE-2026-107838

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-107838/
Summaries are generated and may contain mistakes; the official description above is the record.