# CVE-2026-5855: Contiki-NG LwM2M TLV parser buffer over-read in lwm2m_tlv_read()

Source: https://junglewise.ai/threats/cve-2026-5855-contiki-ng-lwm2m-tlv-parser-buffer-over-read-in-lwm2m-tlv-read  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-06. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-5855 · Severity: high · CVSS 7.5 · Published 2026-08-06

Contiki-NG is an operating system for Internet-of-Things devices that implements the LwM2M (Lightweight M2M) remote management protocol. A flaw in the TLV (Tag-Length-Value) parser causes it to read beyond the supplied buffer boundary, allowing an attacker to send a specially crafted CoAP message that leaks sensitive data from device memory—such as encryption keys and network peer addresses—without requiring authentication in the default NoSec mode.

Technologies: [Contiki-NG](https://junglewise.ai/threats/technologies/contiki-ng). Vendors: [Contiki-NG](https://junglewise.ai/threats/vendors/contiki-ng).

## Quick answers

**Is CVE-2026-5855 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5855 being exploited in the wild.

**How severe is CVE-2026-5855?** High severity, with a CVSS score of 7.5, published 6 August 2026.

**Which products and versions are affected?** Contiki-NG versions prior to commit f1673b5.

**Is there a fix for CVE-2026-5855?** A patch was published on 2026-08-06: Fix merged as commit f1673b5 implementing bounds-check in lwm2m_tlv_read() Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-5855 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-5855?** Check whether you run Contiki-NG 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 vulnerability is a classic buffer over-read in the lwm2m_tlv_read() function (os/services/lwm2m/lwm2m-tlv.c), which ignores the caller-supplied buffer length argument and unconditionally reads up to six bytes from the input buffer. When the CoAP message ends with a TLV element containing exactly one byte, the parser reads five additional bytes from adjacent heap memory. The out-of-bounds data is returned to the caller (lwm2m-engine.c) via tlv->id, tlv->length, and tlv->value fields, disclosing memory contents including cryptographic key material and peer addresses. The attack requires only network access to the LwM2M endpoint and no authentication in NoSec mode (the default for constrained devices). A patch implementing proper bounds checking has been merged (commit f1673b5).

## Affected products

- Contiki-NG Contiki-NG versions prior to commit f1673b5

## Timeline

- 2026-08-06: disclosed
- 2026-08-06: patched: Fix merged as commit f1673b5 implementing bounds-check in lwm2m_tlv_read()

## References

- [https://github.com/contiki-ng/contiki-ng](https://github.com/contiki-ng/contiki-ng)
- [https://github.com/contiki-ng/contiki-ng/commit/f1673b5766d4d4d514cefb8a0350f43653574997](https://github.com/contiki-ng/contiki-ng/commit/f1673b5766d4d4d514cefb8a0350f43653574997)

## Related threats

- [Contiki-NG MQTT client buffer overflow in topic parsing](https://junglewise.ai/threats/cve-2026-5857-contiki-ng-mqtt-client-buffer-overflow-in-topic-parsing)
- [Contiki-NG DNS resolver out-of-bounds read in skip_name()](https://junglewise.ai/threats/cve-2026-5856-contiki-ng-dns-resolver-out-of-bounds-read-in-skip-name)