{"cve":"CVE-2026-80840","nvd":{"published":"2026-09-04T16:18:12.440","modified":"2026-10-19T09:17:19.333","status":"Received","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: seg6: clear IPv4 control block on IPIP decapsulation\n\nEnd.DX4 and End.DT4 decapsulate an IPv4 packet through\ndecap_and_validate() and send it directly to IPv4 routing. The inner\npacket therefore bypasses ip_rcv_core(), which normally clears IPCB\nbefore IPv4 interprets skb->cb.\n\nThe skb instead retains IP6CB data from the outer packet. IP6CB and\nIPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps\nIPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and\nts.\n\nThe sender can make the stale optlen byte nonzero with a valid outer\nextension-header chain. The reproducers put an eight-byte Destination\nOptions header immediately after the 40-byte IPv6 header and before the\nSegment Routing Header. ipv6_destopt_rcv() records the sender-controlled\nDestination Options offset in both lastopt and nhoff, setting them to\n40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees\noptlen = 40 and rr = 40.\n\nBoth tcp_v4_save_options() and __ip_options_echo() skip option copying\nwhen optlen is zero. Here optlen is 40, so the TCP SYN path allocates\nroom for 40 bytes of option data and calls __ip_options_echo(). The\nstale rr value makes that function read inner packet byte 41 as the\nRecord Route option length. The reproducers set that sender-controlled\nbyte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte\noption-data area.\n\nSeparate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5\nkernel both produced:\n\n  BUG: KASAN: slab-out-of-bounds in __ip_options_echo()\n  Write of size 255\n\nThe relevant End.DX4 call path is:\n\n  __ip_options_echo\n  tcp_v4_route_req\n  tcp_conn_request\n  tcp_v4_conn_request\n  tcp_rcv_state_process\n  tcp_v4_do_rcv\n  tcp_v4_rcv\n  ip_protocol_deliver_rcu\n  ip_local_deliver_finish\n  ip_local_deliver\n  input_action_end_dx4_finish\n  input_action_end_dx4\n\nThe relevant End.DT4 call path is:\n\n  __ip_options_echo\n  tcp_v4_route_req\n  tcp_conn_request\n  tcp_v4_conn_request\n  tcp_rcv_state_process\n  tcp_v4_do_rcv\n  tcp_v4_rcv\n  ip_protocol_deliver_rcu\n  ip_local_deliver_finish\n  ip_local_deliver\n  input_action_end_dt4\n\ntcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so\nit does not appear as a separate frame.\n\nWhen decap_and_validate() handles IPPROTO_IPIP, save the ingress\ninterface from IP6CB, clear IPCB, and restore the saved value. Doing\nthis in the common decapsulation path covers End.DX4, End.DT4, and\nEnd.DT46's IPv4 arm.\n\nUse IP6CB(skb)->iif rather than skb->skb_iif. These actions run after\nl3mdev processing, which can replace skb_iif with the L3 master;\nIP6CB iif still records the receiving interface set at IPv6 ingress.","cvss_v4":null,"cvss_v3":null,"cwes":[],"references":["https://git.kernel.org/stable/c/0e3f01fe2e704e76af4385b8a1742641885a191c","https://git.kernel.org/stable/c/10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667","https://git.kernel.org/stable/c/3e4476e58343fb8f2fffced9e22d935376b17aaf","https://git.kernel.org/stable/c/44930446dde45a7a90fe1446fa38eb0e2c561646","https://git.kernel.org/stable/c/9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:52:03.409947763+00:00"},"epss":{"cve":"CVE-2026-80840","score":0.00205,"percentile":0.10345,"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."]}