CVE-2026-12633 Out-of-bounds write in IPv6 6LoWPAN Context Option handling via unauthenticated Router Advertisement
An attacker on the same network link can cause a denial of service and memory corruption in Zephyr RTOS by sending a crafted IPv6 Router Advertisement.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-12633?
CVE-2026-12633 is a high-severity memory corruption vulnerability in zephyrproject zephyr, published on 19 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
The IPv6 neighbor-discovery code in subsys/net/ip/ipv6_nbr.c processes the 6LoWPAN Context Option (6CO, RFC 6775) carried inside ICMPv6 Router Advertisements. In handle_ra_6co() the 8-bit context_len field is taken directly from the packet and was never bounded to the RFC maximum of 128. The function computes context->context_len / 8 and then performs memset(context->prefix + context_len, 0, sizeof(context->prefix) - context_len), where context->prefix is a fixed 16-byte array.
With context_len between 136 and 255 (and the option length field set to 3, which the pre-fix validation accepts), context_len / 8 evaluates to 17..31, so the memset length 16 - context_len/8 underflows the unsigned size_t argument to roughly SIZE_MAX. This produces an unbounded out-of-bounds memset that zeroes kernel memory well past the 6lo context structure.
The defect is reachable from unauthenticated, link-local input: any host on the same link can send a crafted Router Advertisement with a 6CO option. The RA handler validates only the option length field before calling handle_ra_6co(), so a single packet triggers the wild write. The code is compiled when CONFIG_NET_6LO_CONTEXT is enabled.
The impact is a reliable remote (adjacent) denial of service via memory corruption, with collateral integrity loss as the memset zeroes contiguous memory before the system faults. Router Advertisements are link-scoped and not forwarded, so the attacker must be on the same link (AV:A). The fix rejects any context_len greater than 128 before the length computation.
Weakness: CWE-787
Which versions are affected by CVE-2026-12633?
How do I fix CVE-2026-12633?
No fix has been published for CVE-2026-12633 yet. Limit exposure of the affected software until one is available.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-12633 being exploited?
There is no known exploitation of CVE-2026-12633 in the wild and no public exploit code.
Sources
Also known as GHSA-h5m5-hm6j-cgpf.