{"id":"CVE-2026-89563","state":"PUBLISHED","title":"ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()\n\nip6_tnl_xmit() may need to expand headroom before it can push the\nouter IPv6 and optional encap headers. It currently does that with\nskb_realloc_headroom(), copies skb->sk ownership, consumes the original\nskb, and then continues processing with the replacement skb kept only in\nits local variable.\n\nThat is safe only if the helper cannot fail afterwards. But this helper\nstill has post-reallocation error exits. collect_md tunnels reject\nnon-NONE encap after the replacement, and ip6_tnl_encap() can also fail\nlater. In those cases the helper returns an error to its callers while\nthe caller still only has the original skb pointer.\n\nBoth ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on\nerror, so they can end up freeing an skb that ip6_tnl_xmit() already\nconsumed.\n\nUse skb_cow_head() instead. It provides the required headroom and\nwritability without privately replacing the caller-owned skb, so later\nerror returns cannot leave callers with a stale pointer.\n\nThe Ethernet users, ip6gretap and ip6erspan, clear IFF_TX_SKB_SHARING\nand already call skb_cow_head() before entering ip6_tnl_xmit(). They do\nnot rely on the removed skb_shared() reallocation. This also makes the\nIPv6 tunnel path consistent with ip_tunnel_xmit().","publishedAt":"2026-09-11T19:44:34.375Z","updatedAt":"2026-10-19T08:48:56.784Z","severity":{"score":7.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"6e8eff8ed13b14708bc39f170144eacde494aac6","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"dc1a319a4815c074914e2812e0e9cd0164ca682b","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"629d5557e635cdbc109a14ed913ba5a540e3f79a","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"defa0adeff132cc90ed66915b9a104d9393e1e54","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"37df5bc6b5ccad88c2faa0f5466f150178882b74","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"b36dfd6e8cff00cea81c11cc0af88d1564763461","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"8fc56ca49fc0ee1725b55c67bb43ab0dd01f3f52","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"87f21b59ddc618eff9670c174842964ad65fdade","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/ipv6/ip6_tunnel.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.7","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.7","versionType":"semver"},{"version":"5.10.270","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.221","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.188","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.157","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.109","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.50","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.4","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/ip6_tunnel.c"]}],"references":["https://git.kernel.org/stable/c/6e8eff8ed13b14708bc39f170144eacde494aac6","https://git.kernel.org/stable/c/dc1a319a4815c074914e2812e0e9cd0164ca682b","https://git.kernel.org/stable/c/629d5557e635cdbc109a14ed913ba5a540e3f79a","https://git.kernel.org/stable/c/defa0adeff132cc90ed66915b9a104d9393e1e54","https://git.kernel.org/stable/c/37df5bc6b5ccad88c2faa0f5466f150178882b74","https://git.kernel.org/stable/c/b36dfd6e8cff00cea81c11cc0af88d1564763461","https://git.kernel.org/stable/c/8fc56ca49fc0ee1725b55c67bb43ab0dd01f3f52","https://git.kernel.org/stable/c/87f21b59ddc618eff9670c174842964ad65fdade"],"weaknesses":[],"ingestedAt":"2026-10-29T20:13:26.108Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/89xxx/CVE-2026-89563.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-89563","state":"PUBLISHED","datePublished":"2026-09-11T19:44:34.375Z","dateUpdated":"2026-10-19T08:48:56.784Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-09-11T19:38:34.725Z"},"containers":{"cna":{"title":"ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()\n\nip6_tnl_xmit() may need to expand headroom before it can push the\nouter IPv6 and optional encap headers. It currently does that with\nskb_realloc_headroom(), copies skb->sk ownership, consumes the original\nskb, and then continues processing with the replacement skb kept only in\nits local variable.\n\nThat is safe only if the helper cannot fail afterwards. But this helper\nstill has post-reallocation error exits. collect_md tunnels reject\nnon-NONE encap after the replacement, and ip6_tnl_encap() can also fail\nlater. In those cases the helper returns an error to its callers while\nthe caller still only has the original skb pointer.\n\nBoth ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on\nerror, so they can end up freeing an skb that ip6_tnl_xmit() already\nconsumed.\n\nUse skb_cow_head() instead. It provides the required headroom and\nwritability without privately replacing the caller-owned skb, so later\nerror returns cannot leave callers with a stale pointer.\n\nThe Ethernet users, ip6gretap and ip6erspan, clear IFF_TX_SKB_SHARING\nand already call skb_cow_head() before entering ip6_tnl_xmit(). They do\nnot rely on the removed skb_shared() reallocation. This also makes the\nIPv6 tunnel path consistent with ip_tunnel_xmit()."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"6e8eff8ed13b14708bc39f170144eacde494aac6","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"dc1a319a4815c074914e2812e0e9cd0164ca682b","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"629d5557e635cdbc109a14ed913ba5a540e3f79a","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"defa0adeff132cc90ed66915b9a104d9393e1e54","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"37df5bc6b5ccad88c2faa0f5466f150178882b74","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"b36dfd6e8cff00cea81c11cc0af88d1564763461","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"8fc56ca49fc0ee1725b55c67bb43ab0dd01f3f52","versionType":"git"},{"version":"058214a4d1dfefed9f01a277fadd3590acb5f990","status":"affected","lessThan":"87f21b59ddc618eff9670c174842964ad65fdade","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/ipv6/ip6_tunnel.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.7","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.7","versionType":"semver"},{"version":"5.10.270","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.221","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.188","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.157","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.109","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.50","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.4","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/ip6_tunnel.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/6e8eff8ed13b14708bc39f170144eacde494aac6"},{"url":"https://git.kernel.org/stable/c/dc1a319a4815c074914e2812e0e9cd0164ca682b"},{"url":"https://git.kernel.org/stable/c/629d5557e635cdbc109a14ed913ba5a540e3f79a"},{"url":"https://git.kernel.org/stable/c/defa0adeff132cc90ed66915b9a104d9393e1e54"},{"url":"https://git.kernel.org/stable/c/37df5bc6b5ccad88c2faa0f5466f150178882b74"},{"url":"https://git.kernel.org/stable/c/b36dfd6e8cff00cea81c11cc0af88d1564763461"},{"url":"https://git.kernel.org/stable/c/8fc56ca49fc0ee1725b55c67bb43ab0dd01f3f52"},{"url":"https://git.kernel.org/stable/c/87f21b59ddc618eff9670c174842964ad65fdade"}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","baseScore":7.8,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:L - The double-free is in ip6_tnl_xmit() on the ip6tnl/ip6gre transmit path. The reliable trigger is local: create a collect-metadata tunnel with FOU/GUE encap via rtnetlink, then push a packet through ip6_tnl_start_xmit() with tc tunnel_key+mirred. Ordinary remote packets on a working ip6tnl do not hit the post-realloc error path.\nAC:L - Once the collect-md device is created with non-NONE encap, any skb that needs headroom expansion takes skb_realloc_headroom(), consume_skb()s the original, then deterministically fails the collect_md/encap check so the caller kfree_skb()s the stale pointer. The published PoC reproduces this without a race or lucky layout.\nPR:L - Creating ip6tnl/ip6gre and tc filters needs CAP_NET_ADMIN, which an unprivileged user gets in their own user and network namespaces (unshare -Urn). The reporter confirmed this userns path; the resulting skb double-free is not confined to that namespace.\nUI:N - The attacker creates the tunnel, installs the tc redirect, and generates the packet themselves. No other user must mount a filesystem, open a file, or otherwise interact.\nS:U - The skb double-free corrupts kernel heap (skbuff_head_cache) on the same host. This is standard in-kernel memory corruption, not a VM escape, IOMMU/DMA bypass, or other cross-authority impact.\nC:H - consume_skb() frees the original skb and the caller later kfree_skb()s that stale pointer, a slab use-after-free/double-free. Reclaiming the skbuff_head_cache object yields an arbitrary kernel read primitive, so confidentiality impact is High.\nI:H - The same skb double-free/UAF is a heap primitive: the freed sk_buff can be sprayed and the second free or a later dereference used for an arbitrary write or control-flow hijack, so integrity impact is High.\nA:H - KASAN reports slab-use-after-free in ip6_tnl_start_xmit()/kfree_skb and the reproducer panics the kernel. Any skb double-free can oops or panic even if not fully exploited, so availability impact is High."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-19T08:48:56.784Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"5.10.270"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"5.15.221"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"6.1.188"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"6.6.157"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"6.12.109"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"6.18.50"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"7.2.4"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.7","versionEndExcluding":"7.3"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"id":"CVE-2026-89563","role":"CISA Coordinator","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"total"}],"version":"2.0.3","timestamp":"2026-10-16T19:24:12.096672Z"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-16T19:26:44.202Z"}}]}}}