{"cve":"CVE-2026-63920","nvd":{"published":"2026-07-19T16:17:10.153","modified":"2026-10-19T21:17:11.257","status":"Undergoing Analysis","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: validate extension header length before copying to cmsg\n\nip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}\ncmsgs (and their IPV6_2292* legacy counterparts) by trusting the\non-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.\nThe length was validated only at parse time (ipv6_parse_hopopts(),\netc.).  An nftables payload-write expression can rewrite hdrlen after\nparsing and before the skb reaches recvmsg; the write itself is\nin-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040\nbytes from an 8-byte header.  nftables is reachable from an\nunprivileged user namespace, so this is an unprivileged\nslab-out-of-bounds read:\n\n  BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540\n   put_cmsg+0x3ac/0x540\n   udpv6_recvmsg+0xca0/0x1250\n   sock_recvmsg+0xdf/0x190\n   ____sys_recvmsg+0x1b1/0x620\n\nAdd ipv6_get_exthdr_len() which validates that at least two bytes\nare accessible before reading the hdrlen field, then checks the\ncomputed length against skb_tail_pointer(skb), returning 0 on\nfailure.  Extension headers are kept in the linear skb area by\npskb_may_pull() during input, so skb_tail_pointer() is the correct\nbound.\n\nUse ipv6_get_exthdr_len() at all non-AH call sites: the five\nstandalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)\nand the three standard cases in the extension-header walk loop\n(DSTOPTS, ROUTING, default).  AH retains an inline bounds check\nbecause its length formula differs ((ptr[1]+2)<<2).\n\nThe walk loop also gets a pre-read bounds check at the top to\nvalidate ptr before any case accesses ptr[0] or ptr[1].\n\nWhen the walk loop detects a corrupted header, return from the\nfunction instead of continuing to process later socket options.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H","score":7.1},"cwes":["CWE-125"],"references":["https://git.kernel.org/stable/c/08464413e628803bd10cb1df68d0138665f2f885","https://git.kernel.org/stable/c/0d330eff318c0f44d4fb0ad2c2aef38f87f24c90","https://git.kernel.org/stable/c/81394827dfb72772c50d0ae3bdfa094428a5d76d","https://git.kernel.org/stable/c/931b4a1f13408c2507719890f78f7227c34a0282","https://git.kernel.org/stable/c/a29768d56eb3798c052ad3281b05596e695a17af"],"products":[],"source":"nvd","synced_at":"2026-10-21T03:43:09.190223697+00:00"},"epss":{"cve":"CVE-2026-63920","score":0.00172,"percentile":0.06511,"date":"2026-10-25"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}