{"id":"CVE-2026-80840","state":"PUBLISHED","title":"ipv6: seg6: clear IPv4 control block on IPIP decapsulation","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.","publishedAt":"2026-09-04T15:54:50.403Z","updatedAt":"2026-10-19T08:44:33.475Z","severity":null,"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"eb0f422487228e140f3d609b032ac61aedcab8fa","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"f52f1e75716d2ee49e013edf204ac92337c72fd8","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"0e3f01fe2e704e76af4385b8a1742641885a191c","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"3e4476e58343fb8f2fffced9e22d935376b17aaf","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"bf1c1151560d11036a144d917fa4c131831342d7","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"f4be3b391265e24c7720fc867c50062b436acf33","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"44930446dde45a7a90fe1446fa38eb0e2c561646","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/ipv6/seg6_local.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.14","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.14","versionType":"semver"},{"version":"5.10.269","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.220","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.187","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.156","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.108","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.49","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.1.13","status":"unaffected","lessThanOrEqual":"7.1.*","versionType":"semver"},{"version":"7.2.3","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/ipv6/seg6_local.c"]}],"references":["https://git.kernel.org/stable/c/10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667","https://git.kernel.org/stable/c/eb0f422487228e140f3d609b032ac61aedcab8fa","https://git.kernel.org/stable/c/9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c","https://git.kernel.org/stable/c/f52f1e75716d2ee49e013edf204ac92337c72fd8","https://git.kernel.org/stable/c/0e3f01fe2e704e76af4385b8a1742641885a191c","https://git.kernel.org/stable/c/3e4476e58343fb8f2fffced9e22d935376b17aaf","https://git.kernel.org/stable/c/bf1c1151560d11036a144d917fa4c131831342d7","https://git.kernel.org/stable/c/f4be3b391265e24c7720fc867c50062b436acf33","https://git.kernel.org/stable/c/44930446dde45a7a90fe1446fa38eb0e2c561646"],"weaknesses":[],"ingestedAt":"2026-10-29T20:13:07.220Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/80xxx/CVE-2026-80840.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-80840","state":"PUBLISHED","datePublished":"2026-09-04T15:54:50.403Z","dateUpdated":"2026-10-19T08:44:33.475Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-08-26T14:34:25.796Z"},"containers":{"cna":{"title":"ipv6: seg6: clear IPv4 control block on IPIP decapsulation","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"eb0f422487228e140f3d609b032ac61aedcab8fa","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"f52f1e75716d2ee49e013edf204ac92337c72fd8","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"0e3f01fe2e704e76af4385b8a1742641885a191c","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"3e4476e58343fb8f2fffced9e22d935376b17aaf","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"bf1c1151560d11036a144d917fa4c131831342d7","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"f4be3b391265e24c7720fc867c50062b436acf33","versionType":"git"},{"version":"891ef8dd2a8d14e4e73a81dcdb135b574c57f556","status":"affected","lessThan":"44930446dde45a7a90fe1446fa38eb0e2c561646","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/ipv6/seg6_local.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.14","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.14","versionType":"semver"},{"version":"5.10.269","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.220","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.187","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.156","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.108","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.49","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.1.13","status":"unaffected","lessThanOrEqual":"7.1.*","versionType":"semver"},{"version":"7.2.3","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/ipv6/seg6_local.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667"},{"url":"https://git.kernel.org/stable/c/eb0f422487228e140f3d609b032ac61aedcab8fa"},{"url":"https://git.kernel.org/stable/c/9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c"},{"url":"https://git.kernel.org/stable/c/f52f1e75716d2ee49e013edf204ac92337c72fd8"},{"url":"https://git.kernel.org/stable/c/0e3f01fe2e704e76af4385b8a1742641885a191c"},{"url":"https://git.kernel.org/stable/c/3e4476e58343fb8f2fffced9e22d935376b17aaf"},{"url":"https://git.kernel.org/stable/c/bf1c1151560d11036a144d917fa4c131831342d7"},{"url":"https://git.kernel.org/stable/c/f4be3b391265e24c7720fc867c50062b436acf33"},{"url":"https://git.kernel.org/stable/c/44930446dde45a7a90fe1446fa38eb0e2c561646"}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-19T08:44:33.475Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"5.10.269"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"5.15.220"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"6.1.187"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"6.6.156"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"6.12.108"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"6.18.49"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"7.1.13"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"7.2.3"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.14","versionEndExcluding":"7.3"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}}}}}