{"id":"CVE-2026-98393","summary":"net: ipv4: Fix UDP length overflow with PMTU discover and big MTU","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nnet: ipv4: Fix UDP length overflow with PMTU discover and big MTU\n\nThis commit bounds cork-\u003ebase.fragsize to IP_MAX_MTU to avoid a\npossible overflow of UDP length that triggers a WARN in\nudp_set_len_short when setsockopt IP_MTU_DISCOVER is set to\nIP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an\nunusually large MTU.\n\nSteps to reproduce:\n\n1. Set device MTU bigger than IP_MAX_MTU + 20. cork-\u003ebase.fragsize will\n   be set to that MTU in ip_setup_cork.\n2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be\n   set to device MTU (cork-\u003efragsize) in __ip_append_data, rather than\n   to IP_MAX_MTU.\n3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of\n   IPv4 header). Device MTU allows it (it's only one byte bigger than\n   IP_MAX_MTU + IPv4 header, and the device MTU is bigger than that).\n4. The UDP length in the built packet is 65536, which overflows the\n   16-bit length field and triggers the WARN in udp_set_len_short.\n\nNote: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward\nalways clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.\n\nThe Fixes tag points at the first commit where I could reproduce the\noverflow with IPv4 and IP_PMTUDISC_PROBE.","modified":"2026-10-15T02:47:42.023477493Z","published":"2026-10-14T13:25:56.697Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98393.json"},"references":[{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"},{"type":"WEB","url":"https://git.kernel.org/stable/c/81f540827d243b8dad76c1f44e70ea31719f393c"},{"type":"WEB","url":"https://git.kernel.org/stable/c/83fedc4741210f75be8ca5b487985a242bf6f18d"},{"type":"WEB","url":"https://git.kernel.org/stable/c/b83641e0ab8b20eefcc4cdc5a059f897375291a2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98393.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98393"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"daba287b299ec7a2c61ae3a714920e90e8396ad5"},{"fixed":"83fedc4741210f75be8ca5b487985a242bf6f18d"},{"fixed":"81f540827d243b8dad76c1f44e70ea31719f393c"},{"fixed":"b83641e0ab8b20eefcc4cdc5a059f897375291a2"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98393.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.13.0"},{"fixed":"6.18.53"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.2.7"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98393.json"}}],"schema_version":"1.9.0"}