CVE-2026-18746 NULL pointer dereference in Zephyr LwM2M client when the CoAP Block1 context pool is exhausted
An attacker can crash a Zephyr LwM2M client by exhausting the CoAP Block1 context pool, causing a null pointer dereference and device reset.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18746?
CVE-2026-18746 is a medium-severity vulnerability in zephyrproject zephyr, published on 28 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
parse_write_op() in subsys/net/lib/lwm2m/lwm2m_message_handling.c handles inbound CoAP WRITE/CREATE requests that carry a Block1 option. For the first block of a transfer it called init_block_ctx() and then immediately stored the peer-selected block size with block_ctx->ctx.block_size = block_size before inspecting the return code. init_block_ctx() sets the caller's pointer to NULL and returns -ENOMEM when no entry of the static block1_contexts[] pool is free or timed out, so that store dereferences a NULL pointer.
The pool holds CONFIG_LWM2M_NUM_BLOCK1_CONTEXT entries (default 3) and an entry is only reclaimed once its transfer completes, fails, or ages past 30 seconds. A peer that reaches the client's LwM2M socket can therefore start three block-wise writes on three distinct object paths with the CoAP More bit set and leave them incomplete, then send the first block of a fourth write on a new path to reach the unguarded dereference. Reachability is gated only by the connected UDP socket's source-address filter unless CONFIG_LWM2M_DTLS_SUPPORT is enabled — which has no default — so in a NoSec deployment an on-path or address-spoofing attacker needs no credentials; the same sequence is also reachable from a bootstrap or lower-trust server, and can be hit accidentally by a legitimate server running four concurrent block transfers.
The write targets a fixed low address with a value between 0 and 7, so the consequence is a fatal memory fault (BusFault or corrupted low memory leading to a fault) rather than a usable memory-corruption primitive: the device crashes or resets. Confidentiality and integrity are not affected. The fix moves the store below the guard and validates the context pointer itself instead of the return code, so the context is only touched once it is known to be valid.
Weakness: CWE-476
Which versions are affected by CVE-2026-18746?
How do I fix CVE-2026-18746?
No fix has been published for CVE-2026-18746 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-18746 being exploited?
There is no known exploitation of CVE-2026-18746 in the wild and no public exploit code.
Sources
Also known as GHSA-4q9g-2mhh-m7w7.