{"id":"CVE-2026-68124","summary":"mctp: serial: handle zero-length frames to prevent rx buffer overflow","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nmctp: serial: handle zero-length frames to prevent rx buffer overflow\n\nThe MCTP serial receive state machine reads a frame length byte in\nmctp_serial_push_header() case 2 and validates it upper-bound-only:\n\n\tif (c \u003e MCTP_SERIAL_FRAME_MTU) {\n\t\tdev-\u003erxstate = STATE_ERR;\n\t} else {\n\t\tdev-\u003erxlen = c;\n\t\tdev-\u003erxpos = 0;\n\t\tdev-\u003erxstate = STATE_DATA;\n\t\t...\n\t}\n\nA length of zero passes this check, so rxlen is set to 0 and the state\nmachine advances to STATE_DATA. In mctp_serial_push() STATE_DATA, the\nincoming byte is stored and rxpos incremented before the terminator is\n\n\tdev-\u003erxbuf[dev-\u003erxpos] = c;\n\tdev-\u003erxpos++;\n\tdev-\u003erxstate = STATE_DATA;\n\tif (dev-\u003erxpos == dev-\u003erxlen) {\n\t\tdev-\u003erxpos = 0;\n\t\tdev-\u003erxstate = STATE_TRAILER;\n\t}\n\nWith rxlen == 0 the \"rxpos == rxlen\" terminator can never fire (rxpos is\nalready 1 on the first data byte), so subsequent bytes are written past\nthe end of the fixed 74-byte rxbuf, which is the last member of the\nnetdev private area. Every following data byte is an attacker-controlled\n1-byte out-of-bounds heap write, and the overflow continues until a\nframe (0x7e) or escape byte resets the parser -- effectively unbounded.\n\nReaching this requires CAP_NET_ADMIN to attach the N_MCTP line\ndiscipline and bring the resulting mctpserialN netdev up, after which\nthe bytes arrive via the tty receive path.\n\nRoute a zero-length frame straight to STATE_TRAILER instead of\nSTATE_DATA. The trailer/framing bytes are still consumed, and the frame\nresolves to a zero-length skb that the MCTP core rejects; the parser\nnever enters STATE_DATA with rxlen == 0, so the out-of-bounds write can\nno longer occur.\n\nKASAN, on a frame of 0x7e 0x01 0x00 followed by data bytes (before this\nchange):\n\n  UBSAN: array-index-out-of-bounds in drivers/net/mctp/mctp-serial.c:370\n  index 74 is out of range for type 'u8 [74]'\n  BUG: KASAN: slab-out-of-bounds in mctp_serial_tty_receive_buf\n  Write of size 1 at addr ... by task kworker/u16:0\n   mctp_serial_tty_receive_buf\n   tty_ldisc_receive_buf\n   flush_to_ldisc\n  Allocated by task 152:\n   alloc_netdev_mqs\n   mctp_serial_open\n\nv2: route zero-length frames to STATE_TRAILER instead of STATE_ERR so\n    the trailer/framing bytes are still consumed (Jeremy Kerr).\n\nFound by 0sec automated security-research tooling (https://0sec.ai).","modified":"2026-08-12T04:19:21.408348759Z","published":"2026-08-10T11:58:44.997Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/68xxx/CVE-2026-68124.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/06a6b606129c8a25cd457760f5370f3ff01fe05d"},{"type":"WEB","url":"https://git.kernel.org/stable/c/36dc6d6964a3b90411cc7944cd9b8b6f67b9807b"},{"type":"WEB","url":"https://git.kernel.org/stable/c/68819427bc07eca7963a9e8be19e5272cc29186c"},{"type":"WEB","url":"https://git.kernel.org/stable/c/793b9b729f1e8de57be8c8daf1a9838be96cabed"},{"type":"WEB","url":"https://git.kernel.org/stable/c/f80ba170d7b3a44e3d244a2c8e06031d61bf3b23"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/68xxx/CVE-2026-68124.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-68124"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"a0c2ccd9b5ad0a9e838158404e041b5a8ff762dd"},{"fixed":"36dc6d6964a3b90411cc7944cd9b8b6f67b9807b"},{"fixed":"68819427bc07eca7963a9e8be19e5272cc29186c"},{"fixed":"f80ba170d7b3a44e3d244a2c8e06031d61bf3b23"},{"fixed":"06a6b606129c8a25cd457760f5370f3ff01fe05d"},{"fixed":"793b9b729f1e8de57be8c8daf1a9838be96cabed"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-68124.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.17.0"},{"fixed":"6.6.148"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.101"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.42"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.1.6"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-68124.json"}}],"schema_version":"1.9.0"}