CVE-2026-107837: RIOT OS Buffer Overflow Vulnerability

CVE-2026-107837 Overview

CVE-2026-107837 is an out-of-bounds read vulnerability [CWE-125] in RIOT, an open-source microcontroller operating system for Internet of Things (IoT) and embedded systems. The flaw resides in the _receive() function in sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c. The code routes undersized packets into Selective Fragment Forwarding (SFF) fragment handling after only a minimal payload check, then interprets the packet as a sixlowpan_frag_t header without verifying the packet snip contains the required bytes. Versions 2026.07 and earlier are affected, and no fixed repository release is available as of this review.

Critical Impact
A remote attacker can send a malformed 6LoWPAN fragment that causes gnrc_sixlowpan_frag_recv() to read beyond the packet buffer, potentially disclosing memory and crashing the network stack on affected IoT devices.

Affected Products

RIOT OS versions 2026.07 and earlier
Deployments using the gnrc_sixlowpan network layer module
Deployments with MODULE_GNRC_SIXLOWPAN_FRAG enabled

Discovery Timeline

2026-10-09 - CVE-2026-107837 published to NVD
2026-10-16 - Last updated in NVD database

Technical Details for CVE-2026-107837

Vulnerability Analysis

The vulnerability exists in the 6LoWPAN receive path of RIOT's Generic Network Stack (GNRC). When _receive() processes an incoming packet, it dispatches fragmented traffic to gnrc_sixlowpan_frag_recv() using the macro sixlowpan_frag_is(). This check inspects the dispatch byte but does not validate that the packet snip is large enough to hold a full sixlowpan_frag_t or sixlowpan_frag_n_t header. A packet containing only the dispatch byte, or a truncated fragment, passes the check and triggers downstream parsing of memory beyond the allocated snip.

Root Cause

The root cause is missing length validation prior to structure interpretation. The original call site cast the dispatch pointer directly to sixlowpan_frag_t * without checking snip->size. Subsequent field accesses, such as fragment size and datagram tag reads, dereference memory outside the packet buffer. This produces an out-of-bounds read that may leak adjacent heap or stack contents and corrupt stack state enough to crash the GNRC thread.

Attack Vector

Exploitation requires only network reachability to the IoT device over any link-layer that carries 6LoWPAN traffic, such as IEEE 802.15.4. No authentication or user interaction is needed. An attacker crafts a malformed 6LoWPAN fragment with a valid fragment dispatch but an undersized payload and transmits it to the target. The malformed frame drives the receive path into the vulnerable fragment handler.

The upstream fix introduces a new inline helper, sixlowpan_frag_is_snip(), that validates snip->size against the required header type before classification:

static inline bool sixlowpan_frag_is_snip(const gnrc_pktsnip_t *snip)
{
    if (snip->size < sizeof(sixlowpan_frag_t)) {
        return false;
    }

    if (sixlowpan_frag_n_is(snip->data)) {
        return snip->size >= sizeof(sixlowpan_frag_n_t);
    }

    return sixlowpan_frag_1_is(snip->data);
}

The dispatcher in gnrc_sixlowpan.c is updated to call the snip-aware helper instead of the unchecked macro:

#ifdef MODULE_GNRC_SIXLOWPAN_FRAG
    else if (sixlowpan_frag_is_snip(payload)) {
        DEBUG("6lo: received 6LoWPAN fragment\n");
        gnrc_sixlowpan_frag_recv(pkt, NULL, 0);
        return;
    }
#endif

Source: RIOT-OS Commit 0cae205

Detection Methods for CVE-2026-107837

Indicators of Compromise

Unexpected restarts, watchdog resets, or crash dumps on RIOT-based nodes correlated with inbound 6LoWPAN traffic.
6LoWPAN frames whose payload length is below sizeof(sixlowpan_frag_t) yet carry a fragment dispatch byte.
Repeated malformed fragment packets originating from a single border router neighbor or mesh peer.

Detection Strategies

Inspect 6LoWPAN traffic at the border router and flag fragments with payload sizes smaller than the minimum fragment header.
Enable RIOT debug logging for the gnrc_sixlowpan module and alert on repeated "received 6LoWPAN fragment" events paired with device reboots.
Monitor for anomalous network stack termination or GNRC thread panics in field-deployed telemetry.

Monitoring Recommendations

Capture link-layer traces at the 6LoWPAN border router using tools such as Wireshark with 6LoWPAN dissectors.
Track per-neighbor packet-loss and crash counters to identify nodes under malformed-fragment pressure.
Forward device syslog or CoAP management telemetry to a central SIEM for correlation across the mesh.

How to Mitigate CVE-2026-107837

Immediate Actions Required

Rebuild affected firmware against RIOT master including commit 0cae205cd1d110e0b8fce094714598322d56ce2d and redeploy to field devices.
Audit device inventories to identify nodes running RIOT 2026.07 or earlier with gnrc_sixlowpan_frag enabled.
Restrict 6LoWPAN segments to trusted neighbors by enabling link-layer encryption and access control at the border router.

Patch Information

No tagged RIOT release includes the fix as of this review. The upstream patch is tracked in GitHub Pull Request #22504 and merged via commit 0cae205. Full advisory details are in GitHub Security Advisory GHSA-m8mc-q4p7-p8j3. Vendors shipping RIOT-based products should cherry-pick the commit onto their maintained branches.

Workarounds

Disable the gnrc_sixlowpan_frag module where fragmentation is not required by the application profile.
Deploy filtering at the 6LoWPAN border router to drop frames shorter than the minimum fragment header length.
Isolate IoT segments on dedicated VLANs or PANs and require authenticated link-layer peers to limit attacker reachability.

bash

# Cherry-pick the upstream fix into a local RIOT branch
git clone https://github.com/RIOT-OS/RIOT.git
cd RIOT
git checkout -b sixlowpan-frag-fix 2026.07
git cherry-pick 0cae205cd1d110e0b8fce094714598322d56ce2d
make -C examples/gnrc_networking BOARD=<your_board>