# CVE-2026-18746: Zephyr LwM2M parse_write_op null pointer dereference in block handling

Source: https://junglewise.ai/threats/cve-2026-18746-parse-write-op-in-subsys-net-lib-lwm2m-lwm2m-message-handling-c  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-29. Updated: 2026-09-29  
License: CC BY 4.0 with a link back

CVE-2026-18746 · Severity: medium · CVSS 5.9 · Published 2026-09-29

Zephyr's lightweight M2M (LwM2M) implementation contains a null pointer dereference in CoAP block transfer handling that allows an unauthenticated attacker on the network to crash or reset an LwM2M client device. By initiating multiple incomplete block-wise write operations, an attacker can exhaust the fixed pool of transfer contexts and trigger a crash when processing the fourth concurrent transfer. In deployments without DTLS security, this requires only network access to the device's UDP socket.

Technologies: [Zephyr Project Zephyr RTOS](https://junglewise.ai/threats/technologies/zephyr). Vendors: [Zephyr Project](https://junglewise.ai/threats/vendors/zephyr-project).

## Quick answers

**Is CVE-2026-18746 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-18746 being exploited in the wild.

**How severe is CVE-2026-18746?** Medium severity, with a CVSS score of 5.9, published 29 September 2026.

**Which products and versions are affected?** Zephyr Project Zephyr RTOS All versions with LwM2M block transfer support.

**Is there a fix for CVE-2026-18746?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-18746 attacked?** It is attacked over the network, in an operating system.

**Am I affected by CVE-2026-18746?** Check whether you run Zephyr Project Zephyr RTOS at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The parse_write_op() function in lwm2m_message_handling.c fails to validate the return code of init_block_ctx() before dereferencing the returned context pointer. When the block context pool is exhausted, init_block_ctx() returns -ENOMEM with the context pointer set to NULL, but the subsequent assignment block_ctx->ctx.block_size = block_size proceeds unconditionally, causing a null pointer dereference. The attack requires an attacker with network access to the LwM2M socket (reachable from any peer in NoSec mode or from bootstrap/lower-trust servers) and works by exhausting the three-entry static pool with incomplete transfers before sending a fourth write request.

## Affected products

- Zephyr Project Zephyr RTOS All versions with LwM2M block transfer support

## Timeline

- 2026-09-29: disclosed

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