CVE-2026-107836 in RIOT
Summary
by MITRE • 10/09/2026
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.
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Analysis
by VulDB Data Team • 10/09/2026
The nanoCoAP implementation within RIOT OS versions 2026.07 and earlier contains a critical logic flaw in its Block2 response handling mechanism that leads to integer underflow and subsequent out-of-bounds memory access. The vulnerability resides specifically in the nanocoap_sock_get_slice() function located in sys/net/application_layer/nanocoap/sock.c, which is responsible for processing CoAP block-wise transfer responses. When a client requests large resources via Block2 transfers, the server responds with chunks of data identified by block numbers and size indicators known as szx values. The security issue arises because the _block_cb() callback function validates that the received block number matches the expected sequence but fails to verify that the server-provided szx value is consistent with the originally requested block geometry. This omission allows a malicious CoAP server to manipulate the response parameters in a way that breaks the assumptions made by subsequent processing functions regarding buffer boundaries and offsets.
The technical exploitation of this flaw involves a deliberate mismatch between the expected and actual block size parameters sent by an attacker-controlled server. Upon receiving a Block2 response, the client calculates the offset for the current data slice based on the block number and the szx value provided in the packet header. Because the validation logic does not ensure that the received szx aligns with what was requested or is within valid bounds, an attacker can send a response containing the correct expected block number but with a significantly larger size indicator than anticipated. This discrepancy causes the derived offset calculation to become incorrect relative to the actual payload buffer structure. Specifically, when _2buf_slice() attempts to process this data, it performs arithmetic operations involving ctx->offset and the calculated offset that result in an integer underflow due to the unexpected magnitude of the values involved.
This integer underflow has severe operational consequences for the integrity and availability of the embedded device running RIOT OS. The buffer-relative calculation resulting from the underflow causes the system to read memory addresses preceding the start of the payload buffer, leading to out-of-bounds reads. In many cases, this immediate access violation triggers a crash or segmentation fault within the microcontroller environment, effectively causing a denial of service by halting network operations and potentially requiring a device reboot. Beyond simple availability impact, reading adjacent memory can expose sensitive data stored in neighboring stack frames or global variables to an attacker who controls the CoAP server endpoint. This information disclosure vector could reveal cryptographic keys, session tokens, or other confidential state information depending on what resides in the memory locations accessed by the erroneous pointer arithmetic.
From a classification perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound and CWE-787 Out-of-bounds Read, as the root cause is an unchecked integer calculation leading to invalid memory access patterns. In terms of attack tactics, it corresponds to MITRE ATT&CK techniques related to Denial of Service via resource exhaustion or crash induction, specifically leveraging network-based inputs to disrupt service availability on constrained IoT devices. The lack of proper input validation for server-controlled parameters like szx represents a fundamental failure in defensive coding practices common in embedded protocol stacks that assume benign behavior from remote endpoints.
Mitigation strategies must focus on rigorous validation of all CoAP block transfer parameters before they are used in arithmetic operations or buffer indexing. Developers should implement strict checks to ensure that the received szx value does not exceed predefined limits and is consistent with the initial request geometry. Additionally, incorporating bounds checking logic within _2buf_slice() would prevent underflow conditions by validating that calculated offsets remain within valid memory ranges before accessing buffers. Until a patched release of RIOT OS becomes available, network administrators should restrict CoAP traffic to trusted servers using firewall rules or mutual TLS authentication where possible, thereby preventing untrusted actors from injecting malicious block responses into the communication stream.