CVE-2026-107836 RIOT: nanoCoAP Block2 client slice handling underflows on inconsistent server-controlled block size
A malicious CoAP server can trigger an integer underflow in RIOT OS nanoCoAP client, causing a crash and potential memory disclosure.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-107836?
CVE-2026-107836 is a high-severity vulnerability in RIOT-OS RIOT, published on 9 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
RIOT is an open-source microcontroller operating system designed for Internet of Things devices and other embedded systems. In 2026.07 and earlier, the nanoCoAP client function nanocoap_sock_get_slice() in sys/net/application_layer/nanocoap/sock.c accepts a Block2 response when _block_cb() sees the expected block number without also verifying that the server-controlled szx and derived offset match the requested block geometry. A malicious CoAP server can return the expected block number with a larger block size, causing the derived offset to exceed the client slice offset and making ctx->offset - offset underflow in _2buf_slice(). The resulting buffer-relative calculation can read before the payload buffer and crash the client, causing denial of service and potentially exposing adjacent memory. No fixed release is available as of this review.
Which versions are affected by CVE-2026-107836?
How do I fix CVE-2026-107836?
No fix has been published for CVE-2026-107836 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-107836 being exploited?
There is no known exploitation of CVE-2026-107836 in the wild and no public exploit code.
Sources
Also known as GHSA-x924-p5fq-26pc.