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CVE-2026-63920 is an out-of-bounds read in the Linux kernel IPv6 receive path. The flaw is in ip6_datagram_recv_specific_ctl(), which constructs ancillary data for IPv6 extension headers such as IPV6_HOPOPTS, IPV6_DSTOPTS, IPV6_RTHDR, and legacy IPV6_2292* control messages. The function trusted the on-wire hdrlen field when calculating the length passed to put_cmsg(), even though that length had only been validated earlier during extension-header parsing. An attacker able to modify the extension header after parsing but before recvmsg processing can cause the kernel to use a corrupted hdrlen value and copy far more data than is actually present in the header. In the demonstrated case, a payload-write expression in nftables can rewrite the hdrlen byte in-bounds, after which put_cmsg() may read up to 2040 bytes from what is only an 8-byte header, producing a slab out-of-bounds read. The fix introduces ipv6_get_exthdr_len() to validate accessibility of the hdrlen field and ensure the computed extension-header length remains within skb bounds before copying to cmsg, and also adds additional bounds checks in the extension-header walk loop.
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