CVE-2026-12633: Out-of-bounds write in IPv6 6LoWPAN Context Option handling via unauthenticated Router Advertisement
The IPv6 neighbor-discovery code in subsys/net/ip/ipv6nbr.c processes the 6LoWPAN Context Option (6CO, RFC 6775) carried inside ICMPv6 Router Advertisements. In handlera6co() the 8-bit contextlen field is taken directly from the packet and was never bounded to the RFC maximum of 128. The function computes context->contextlen / 8 and then performs memset(context->prefix + contextlen, 0, sizeof(context->prefix) - contextlen), where context->prefix is a fixed 16-byte array.
With contextlen between 136 and 255 (and the option length field set to 3, which the pre-fix validation accepts), contextlen / 8 evaluates to 17..31, so the memset length 16 - contextlen/8 underflows the unsigned sizet argument to roughly SIZEMAX. 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 handlera6co(), so a single packet triggers the wild write. The code is compiled when CONFIGNET6LOCONTEXT 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 contextlen greater than 128 before the length computation.
Affected Software
Event History
Frequently Asked Questions
Which systems are exposed to this issue?
Systems are exposed when their IPv6 neighbor-discovery implementation includes the affected code and CONFIG_NET_6LO_CONTEXT is enabled. An attacker must be on the same network link as the target.
Does exploitation require authentication or prior access to the target?
No. Any host on the same link can send an unauthenticated crafted ICMPv6 Router Advertisement containing a 6LoWPAN Context Option.
How many packets are needed to trigger the memory corruption?
A single crafted Router Advertisement can trigger the out-of-bounds memset. The malicious option uses an accepted option length of 3 and a context_len value from 136 through 255.
What is the practical impact of successful exploitation?
The malformed option causes an unsigned length underflow and an unbounded memset that zeroes kernel memory beyond the fixed 6Lo context structure. This can compromise integrity and availability.