{"id":"CVE-2026-31415","title":"In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: avoid overflows in ip6_datagram_send_ctl()\n\nYiming Qian reported :\n<quote>\n I believe I found a locally triggerable kernel bug in the IPv6 sendmsg\n ancillary-data…","summary":"In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: avoid overflows in ip6_datagram_send_ctl()\n\nYiming Qian reported :\n<quote>\n I believe I found a locally triggerable kernel bug in the IPv6 sendmsg\n ancillary-data…","severity":"medium","cvss":5.5,"cvssVector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","cwe":["CWE-617"],"vendor":"linux","product":"linux_kernel","affected":["linux_kernel >= 2.6.14, < 5.10.253","linux_kernel >= 5.11, < 5.15.203","linux_kernel >= 5.16, < 6.1.168","linux_kernel >= 6.2, < 6.6.134","linux_kernel >= 6.7, < 6.12.81","linux_kernel >= 6.13, < 6.18.22","linux_kernel >= 6.19, < 6.19.12","linux_kernel = 7.0"],"patched":["linux_kernel 6.19.12"],"published":"2026-04-13","updated":"2026-10-15","sourceUpdated":"2026-10-15T02:19:03.030","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-31415","references":[{"url":"https://git.kernel.org/stable/c/0bdaf54d3aaddfe8df29371260fa8d4939b4fd6f","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/2dbfb003bbf3fc0e94f07efefab0ebcf83029a2a","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/4082f9984a694829153115d28c956a3534f52f29","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/4e453375561fc60820e6b9d8ebeb6b3ee177d42e","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/5e4ee5dbea134e9257f205e31a96040bed71e83f","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/63fda74885555e6bd1623b5d811feec998740ba4","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/872b74900d5daa37067ac676d9001bb929fc6a2a","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/9ed81d692758dfb9471d7799b24bfa7a08224c31","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://cert-portal.siemens.com/productcert/html/ssa-019113.html","label":"0b142b55-0307-4c5a-b3c9-f314f3fb7c5e"},{"url":"https://cert-portal.siemens.com/productcert/html/ssa-082556.html","label":"0b142b55-0307-4c5a-b3c9-f314f3fb7c5e"},{"url":"https://cert-portal.siemens.com/productcert/html/ssa-457936.html","label":"0b142b55-0307-4c5a-b3c9-f314f3fb7c5e"}],"tags":["nvd","cve.org"],"epss":0.00114,"epssPercentile":0.01456,"ingestedAt":"2026-09-08T15:33:26.991Z","slug":"CVE-2026-31415","body":"## Overview\n\nIn the Linux kernel, the following vulnerability has been resolved:\n\nipv6: avoid overflows in ip6_datagram_send_ctl()\n\nYiming Qian reported :\n<quote>\n I believe I found a locally triggerable kernel bug in the IPv6 sendmsg\n ancillary-data path that can panic the kernel via `skb_under_panic()`\n (local DoS).\n\n The core issue is a mismatch between:\n\n - a 16-bit length accumulator (`struct ipv6_txoptions::opt_flen`, type\n `__u16`) and\n - a pointer to the *last* provided destination-options header (`opt->dst1opt`)\n\n when multiple `IPV6_DSTOPTS` control messages (cmsgs) are provided.\n\n - `include/net/ipv6.h`:\n   - `struct ipv6_txoptions::opt_flen` is `__u16` (wrap possible).\n (lines 291-307, especially 298)\n - `net/ipv6/datagram.c:ip6_datagram_send_ctl()`:\n   - Accepts repeated `IPV6_DSTOPTS` and accumulates into `opt_flen`\n without rejecting duplicates. (lines 909-933)\n - `net/ipv6/ip6_output.c:__ip6_append_data()`:\n   - Uses `opt->opt_flen + opt->opt_nflen` to compute header\n sizes/headroom decisions. (lines 1448-1466, especially 1463-1465)\n - `net/ipv6/ip6_output.c:__ip6_make_skb()`:\n   - Calls `ipv6_push_frag_opts()` if `opt->opt_flen` is non-zero.\n (lines 1930-1934)\n - `net/ipv6/exthdrs.c:ipv6_push_frag_opts()` / `ipv6_push_exthdr()`:\n   - Push size comes from `ipv6_optlen(opt->dst1opt)` (based on the\n pointed-to header). (lines 1179-1185 and 1206-1211)\n\n 1. `opt_flen` is a 16-bit accumulator:\n\n - `include/net/ipv6.h:298` defines `__u16 opt_flen; /* after fragment hdr */`.\n\n 2. `ip6_datagram_send_ctl()` accepts *repeated* `IPV6_DSTOPTS` cmsgs\n and increments `opt_flen` each time:\n\n - In `net/ipv6/datagram.c:909-933`, for `IPV6_DSTOPTS`:\n   - It computes `len = ((hdr->hdrlen + 1) << 3);`\n   - It checks `CAP_NET_RAW` using `ns_capable(net->user_ns,\n CAP_NET_RAW)`. (line 922)\n   - Then it does:\n     - `opt->opt_flen += len;` (line 927)\n     - `opt->dst1opt = hdr;` (line 928)\n\n There is no duplicate rejection here (unlike the legacy\n `IPV6_2292DSTOPTS` path which rejects duplicates at\n `net/ipv6/datagram.c:901-904`).\n\n If enough large `IPV6_DSTOPTS` cmsgs are provided, `opt_flen` wraps\n while `dst1opt` still points to a large (2048-byte)\n destination-options header.\n\n In the attached PoC (`poc.c`):\n\n - 32 cmsgs with `hdrlen=255` => `len = (255+1)*8 = 2048`\n - 1 cmsg with `hdrlen=0` => `len = 8`\n - Total increment: `32*2048 + 8 = 65544`, so `(__u16)opt_flen == 8`\n - The last cmsg is 2048 bytes, so `dst1opt` points to a 2048-byte header.\n\n 3. The transmit path sizes headers using the wrapped `opt_flen`:\n\n- In `net/ipv6/ip6_output.c:1463-1465`:\n  - `headersize = sizeof(struct ipv6hdr) + (opt ? opt->opt_flen +\n opt->opt_nflen : 0) + ...;`\n\n With wrapped `opt_flen`, `headersize`/headroom decisions underestimate\n what will be pushed later.\n\n 4. When building the final skb, the actual push length comes from\n `dst1opt` and is not limited by wrapped `opt_flen`:\n\n - In `net/ipv6/ip6_output.c:1930-1934`:\n   - `if (opt->opt_flen) proto = ipv6_push_frag_opts(skb, opt, proto);`\n - In `net/ipv6/exthdrs.c:1206-1211`, `ipv6_push_frag_opts()` pushes\n `dst1opt` via `ipv6_push_exthdr()`.\n - In `net/ipv6/exthdrs.c:1179-1184`, `ipv6_push_exthdr()` does:\n   - `skb_push(skb, ipv6_optlen(opt));`\n   - `memcpy(h, opt, ipv6_optlen(opt));`\n\n With insufficient headroom, `skb_push()` underflows and triggers\n `skb_under_panic()` -> `BUG()`:\n\n - `net/core/skbuff.c:2669-2675` (`skb_push()` calls `skb_under_panic()`)\n - `net/core/skbuff.c:207-214` (`skb_panic()` ends in `BUG()`)\n\n - The `IPV6_DSTOPTS` cmsg path requires `CAP_NET_RAW` in the target\n netns user namespace (`ns_capable(net->user_ns, CAP_NET_RAW)`).\n - Root (or any task with `CAP_NET_RAW`) can trigger this without user\n namespaces.\n - An unprivileged `uid=1000` user can trigger this if unprivileged\n user namespaces are enabled and it can create a userns+netns to obtain\n namespaced `CAP_NET_RAW` (the attached PoC does this).\n\n - Local denial of service: kernel BUG/panic (system crash).\n -\n---truncated---\n\n## Affected\n\n- `linux_kernel >= 2.6.14, < 5.10.253`\n- `linux_kernel >= 5.11, < 5.15.203`\n- `linux_kernel >= 5.16, < 6.1.168`\n- `linux_kernel >= 6.2, < 6.6.134`\n- `linux_kernel >= 6.7, < 6.12.81`\n- `linux_kernel >= 6.13, < 6.18.22`\n- `linux_kernel >= 6.19, < 6.19.12`\n- `linux_kernel = 7.0`\n\n## Remediation\n\nUpgrade past the affected range:\n\n- `linux_kernel 6.19.12`","depth":"sunlit","depthScore":30,"depthScoreParts":{"impact":30.3,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[{"seq":221691,"id":"CVE-2026-31415","ts":1792035327893,"field":"cvss","old":null,"new":"5.5"},{"seq":221690,"id":"CVE-2026-31415","ts":1792035327893,"field":"severity","old":"none","new":"medium"}]}