CVE-2026-15891 in Zephyr
Summary
by MITRE • 09/14/2026
The MQTT-SN client keepalive handler process_ping() in subsys/net/lib/mqtt_sn/mqtt_sn.c removes the gateway record after PINGREQ retries are exhausted. It invoked SYS_SLIST_PEEK_HEAD_CONTAINER(&client->gateways, gw, next) but discarded the result. That macro is a pure expression that does not assign to gw, so gw retained its NULL initializer regardless of the list contents.
The code then dereferences the NULL gw (gw->gw_id) and passes it to mqtt_sn_gw_destroy(), reaching k_mem_slab_free(&gateways, NULL). With CONFIG_MEM_SLAB_POINTER_VALIDATE enabled this triggers k_panic(); in the default configuration it performs a write through the NULL pointer ((char )mem = slab->free_list;) and corrupts the slab free list. The outcome is a crash/kernel panic or, on targets where address 0 is writable, silent memory-allocator corruption.
The vulnerable branch runs whenever the connected MQTT-SN gateway fails to answer keepalive PINGREQs for the configured number of retries. This condition is controlled by the remote peer: a malicious or compromised gateway, or an on-path/adjacent attacker that advertises itself as a gateway and then stops responding (or blackholes the real gateway's PINGRESPs), forces the client into the defect. MQTT-SN runs over UDP and no authentication is required.
The impact is a remotely triggerable denial of service (availability) of the affected MQTT-SN client; there is no attacker-controlled data written. The sibling remover process_advertise() uses SYS_SLIST_FOR_EACH_CONTAINER_SAFE and is not affected. The fix assigns the macro's return value to gw.
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability in question resides within the keepalive handling mechanism of the MQTT-SN client implementation, specifically inside the process_ping function located in the subsys/net/lib/mqtt_sn directory. This component is responsible for maintaining connectivity with an MQTT-SN gateway by periodically sending PINGREQ messages and awaiting corresponding PINGRESP acknowledgments. The core technical flaw stems from a critical programming error involving the misuse of the SYS_SLIST_PEEK_HEAD_CONTAINER macro. In C, this macro evaluates to a pointer but does not perform assignment; however, the vulnerable code invokes it without capturing its return value into the gw variable. Consequently, regardless of whether gateway records exist in the client's list, the gw variable retains its initial NULL state. This logical error leads directly to a null pointer dereference when the subsequent lines attempt to access members such as gw->gw_id or pass the invalid reference to mqtt_sn_gw_destroy for cleanup operations involving k_mem_slab_free.
The operational impact of this flaw is severe, primarily manifesting as a denial of service against the affected MQTT-SN client device. When the configured number of PINGREQ retries is exhausted due to non-responsive gateways, the system attempts to remove the gateway record but instead triggers undefined behavior. If the kernel configuration includes CONFIG_MEM_SLAB_POINTER_VALIDATE, which is common in development and debugging environments, this null pointer access immediately causes a k_panic(), resulting in an immediate device crash or reboot. In production configurations where memory validation is disabled, the outcome depends on the target architecture's memory mapping. On systems where address zero is writable, the code performs a write through the NULL pointer to update slab free list pointers, leading to silent heap corruption that can cause delayed crashes, data integrity issues, or further exploitation vectors. This represents a classic CWE-476 null pointer dereference vulnerability compounded by improper resource management logic.
From an attacker perspective, this vulnerability is remotely triggerable and does not require authentication, aligning with the characteristics of availability-focused attacks often categorized under MITRE ATT&CK techniques related to service disruption or resource exhaustion. Since MQTT-SN operates over UDP without mandatory encryption or strong mutual authentication in many deployments, a malicious actor can exploit this by advertising themselves as a valid gateway using standard discovery mechanisms and then ceasing to respond to keepalive requests. Alternatively, an on-path attacker could blackhole the PINGRESP packets from legitimate gateways, forcing the client into the retry exhaustion state that triggers the bug. The attack vector is straightforward: simply stop responding to pings after establishing initial connectivity or advertising presence.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary fix involves correcting the code logic by assigning the return value of SYS_SLIST_PEEK_HEAD_CONTAINER directly to the gw variable, ensuring that valid pointers are passed to cleanup functions rather than null references. Beyond this specific patch, developers should enforce strict static analysis rules against discarding macro returns in pointer manipulation contexts and consider enabling CONFIG_MEM_SLAB_POINTER_VALIDATE in production builds where feasible to catch such errors early. Furthermore, implementing robust gateway authentication mechanisms using DTLS or other secure transport layers would prevent unauthorized devices from advertising themselves as gateways, thereby reducing the attack surface for triggering this denial-of-service condition. Regular code reviews focusing on list traversal macros and pointer validation are essential to prevent similar defects in network stack implementations.