{"id":"CVE-2026-98550","summary":"tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow","details":"In the Linux kernel, the following vulnerability has been resolved:\n\ntipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow\n\nCommit 48a5fe38772b (\"tipc: fix bc_ackers underflow on duplicate\nGRP_ACK_MSG\") rejected duplicate/stale ACKs in tipc_group_proto_rcv()\nby returning early when less_eq(acked, m-\u003ebc_acked).\n\nHowever, that check remains incomplete in two ways:\n\n1. When grp-\u003ebc_ackers is zero (e.g. on a quiet group, when replicast\n   ACKs were not requested, or after all expected members have already\n   acknowledged), an unexpected GRP_ACK_MSG with acked \u003e m-\u003ebc_acked\n   passes less_eq() and unconditionally decrements grp-\u003ebc_ackers.\n   Because bc_ackers is a u16, this wraps to 65535, causing\n   tipc_group_bc_cong() to permanently report congestion and blocking\n   all future group broadcasts on the socket.\n\n2. During an active broadcast round (grp-\u003ebc_ackers \u003e 0), the sender\n   transmits packet S and advances grp-\u003ebc_snd_nxt to S + 1. Receivers\n   increment their expected counter to S + 1 upon consuming packet S,\n   so the only valid ACK value for the current round is strictly\n   acked == grp-\u003ebc_snd_nxt.\n\n   However, tipc_group_update_bc_members() initializes each member's\n   m-\u003ebc_acked to prev = grp-\u003ebc_snd_nxt - 1 (S - 1 before increment).\n   This leaves a 2-sequence gap (S - 1 to S + 1) in sequence space.\n   An incoming ACK is therefore neither rejected as duplicate nor\n   prevented from decrementing grp-\u003ebc_ackers if an unexpected or stale\n   value (such as S) is received. A member sending acked = S followed\n   by acked = S + 1 could decrement grp-\u003ebc_ackers twice in the same\n   round, prematurely clearing bc_ackers or underflowing it.\n\nFix this by:\n- Dropping GRP_ACK_MSG immediately if grp-\u003ebc_ackers is zero.\n- Requiring acked == grp-\u003ebc_snd_nxt and rejecting duplicates where\n  m-\u003ebc_acked == acked. Because replicast broadcast rounds are strictly\n  sequential, only grp-\u003ebc_snd_nxt can be acknowledged, and each member\n  can acknowledge at most once per round.\n\nNote that a related pre-existing issue in tipc_group_delete_member()\n(where grp-\u003ebc_ackers decrementing to zero upon member departure does\nnot restore *grp-\u003eopen or trigger a socket wakeup) will be addressed\nin a separate patch.","modified":"2026-10-20T02:30:52.654405042Z","published":"2026-10-18T15:18:18.173Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98550.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/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b"},{"type":"WEB","url":"https://git.kernel.org/stable/c/478fb08d0c2a53002a721d22b8d6681ec971d90f"},{"type":"WEB","url":"https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b"},{"type":"WEB","url":"https://git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7"},{"type":"WEB","url":"https://git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d"},{"type":"WEB","url":"https://git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0"},{"type":"WEB","url":"https://git.kernel.org/stable/c/b0e517961f09b9f60d338d1ea4e17aacdbf206f0"},{"type":"WEB","url":"https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98550.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98550"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"2f487712b89376fce267223bbb0db93d393d4b09"},{"fixed":"edc6593c442278fceff0120e8cc58bc592168c35"},{"fixed":"9b94a90651e1fa573c6ac6a5a6b269d5874df2c0"},{"fixed":"507a27435b2bb1f22e3a3674f6a6553eb815021b"},{"fixed":"80a9ccfc24d3776ef6753a57315e726475fac7d7"},{"fixed":"478fb08d0c2a53002a721d22b8d6681ec971d90f"},{"fixed":"12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b"},{"fixed":"b0e517961f09b9f60d338d1ea4e17aacdbf206f0"},{"fixed":"99cc2a62e07a44a22254d7beca9ef1f8ad886d0d"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98550.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.15.0"},{"fixed":"5.10.271"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.222"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.189"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.158"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.112"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.55"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.2.9"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98550.json"}}],"schema_version":"1.9.0"}