CVE-2026-107836
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().
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.29%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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.
- CVSS 4.0
- 7.1 HIGHCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
- EPSS
- 0.29% probability · 22th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-125, CWE-191
- Source
- security-advisories@github.com
EPSS moved from 0.3% on 10 October 2026 to 0.3% on 26 October 2026.
References
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.