# cveboard — Historical CVE brief

## Instructions for the operator's AI
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Fix information unknown; no fixed version is derived here. Missing information does not prove that no fix exists.
Exploitation unknown; this catalogue does not assess exploitation.

## Source facts (JSON-quoted data)
CNA source text (English)
id: "CVE-2026-31415"
idYear: 2026
status: "PUBLISHED"
publishedAt: "2026-04-13T13:21:03.284Z"
updatedAt: "2026-10-15T01:15:51.840Z"
importedAt: "2026-10-24T01:58:02.025Z"
snapshot: {"id":"cve_2026-10-24_midnight_baseline","at":"2026-10-24T00:00:00.000Z","source":"https://github.com/CVEProject/cvelistV5/releases/tag/cve_2026-10-24_0100Z"}
url: "https://cveboard.org/cve/CVE-2026-31415"
json: "https://cveboard.org/cve/CVE-2026-31415.json"
sourceURL: "https://www.cve.org/CVERecord?id=CVE-2026-31415"
title: "ipv6: avoid overflows in ip6_datagram_send_ctl()"
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\u003cquote\u003e\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-\u003edst1opt`)\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-\u003eopt_flen + opt-\u003eopt_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-\u003eopt_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-\u003edst1opt)` (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-\u003ehdrlen + 1) \u003c\u003c 3);`\n   - It checks `CAP_NET_RAW` using `ns_capable(net-\u003euser_ns,\n CAP_NET_RAW)`. (line 922)\n   - Then it does:\n     - `opt-\u003eopt_flen += len;` (line 927)\n     - `opt-\u003edst1opt = 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` =\u003e `len = (255+1)*8 = 2048`\n - 1 cmsg with `hdrlen=0` =\u003e `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-\u003eopt_flen +\n opt-\u003eopt_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-\u003eopt_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()` -\u003e `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-\u003euser_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---"
affected: [{"versions":[{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"2dbfb003bbf3fc0e94f07efefab0ebcf83029a2a"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"4082f9984a694829153115d28c956a3534f52f29"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"0bdaf54d3aaddfe8df29371260fa8d4939b4fd6f"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"5e4ee5dbea134e9257f205e31a96040bed71e83f"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"63fda74885555e6bd1623b5d811feec998740ba4"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"9ed81d692758dfb9471d7799b24bfa7a08224c31"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"872b74900d5daa37067ac676d9001bb929fc6a2a"},{"version":"333fad5364d6b457c8d837f7d05802d2aaf8a961","status":"affected","versionType":"git","lessThan":"4e453375561fc60820e6b9d8ebeb6b3ee177d42e"}],"vendor":"Linux","product":"Linux","defaultStatus":"unaffected"},{"versions":[{"version":"2.6.14","status":"affected"},{"version":"0","status":"unaffected","versionType":"semver","lessThan":"2.6.14"},{"version":"5.10.253","status":"unaffected","versionType":"semver","lessThanOrEqual":"5.10.*"},{"version":"5.15.203","status":"unaffected","versionType":"semver","lessThanOrEqual":"5.15.*"},{"version":"6.1.168","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.1.*"},{"version":"6.6.134","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.6.*"},{"version":"6.12.81","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.12.*"},{"version":"6.18.22","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.18.*"},{"version":"6.19.12","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.19.*"},{"version":"7.0","status":"unaffected","versionType":"original_commit_for_fix","lessThanOrEqual":"*"}],"vendor":"Linux","product":"Linux","defaultStatus":"affected"}]

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