CVE-2026-53263

In the Linux kernel, the following vulnerability has been resolved:

6lowpan: fix off-by-one in multicast context address compression

The second memcpy in lowpan_iphc_mcast_ctx_addr_compress() uses
&data[1] as destination and &ipaddr->s6_addr[11] as source, but
both should be offset by one: &data[2] and &ipaddr->s6_addr[12]
respectively.

This off-by-one has two consequences:
1. data[1] is overwritten with s6_addr[11], corrupting the RIID
   field in the compressed multicast address
2. data[5] is never written, so uninitialized kernel stack memory
   is transmitted over the network via lowpan_push_hc_data(),
   leaking kernel stack contents

The correct inline data layout must match what the decompression
function lowpan_uncompress_multicast_ctx_daddr() expects:
  data[0..1] = s6_addr[1..2]  (flags/scope + RIID)
  data[2..5] = s6_addr[12..15] (group ID)

Also zero-initialize the data array as a defensive measure against
similar bugs in the future.