{"schema":"threatline.record/2","record":{"id":"CVE-2026-98550","aliases":[],"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->bc_acked).\n\nHowever, that check remains incomplete in two ways:\n\n1. When grp->bc_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 > m->bc_acked\n   passes less_eq() and unconditionally decrements grp->bc_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->bc_ackers > 0), the sender\n   transmits packet S and advances grp->bc_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->bc_snd_nxt.\n\n   However, tipc_group_update_bc_members() initializes each member's\n   m->bc_acked to prev = grp->bc_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->bc_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->bc_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->bc_ackers is zero.\n- Requiring acked == grp->bc_snd_nxt and rejecting duplicates where\n  m->bc_acked == acked. Because replicast broadcast rounds are strictly\n  sequential, only grp->bc_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->bc_ackers decrementing to zero upon member departure does\nnot restore *grp->open or trigger a socket wakeup) will be addressed\nin a separate patch.","categories":["Operating systems"],"product":"Linux","vendor":"Linux","ecosystem":null,"affected_ranges":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98550","source_modified_at":"2026-10-19T09:21:33.349Z","fetched_at":"2026-10-23T18:45:31.882637Z","product":"Linux","vendor":"Linux","versions":[{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"edc6593c442278fceff0120e8cc58bc592168c35","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"9b94a90651e1fa573c6ac6a5a6b269d5874df2c0","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"507a27435b2bb1f22e3a3674f6a6553eb815021b","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"80a9ccfc24d3776ef6753a57315e726475fac7d7","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"478fb08d0c2a53002a721d22b8d6681ec971d90f","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"b0e517961f09b9f60d338d1ea4e17aacdbf206f0","status":"affected","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","lessThan":"99cc2a62e07a44a22254d7beca9ef1f8ad886d0d","status":"affected","versionType":"git"}],"defaultStatus":"unaffected"},{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98550","source_modified_at":"2026-10-19T09:21:33.349Z","fetched_at":"2026-10-23T18:45:31.882637Z","product":"Linux","vendor":"Linux","versions":[{"version":"4.15","status":"affected"},{"version":"0","lessThan":"4.15","status":"unaffected","versionType":"semver"},{"version":"5.10.271","lessThanOrEqual":"5.10.*","status":"unaffected","versionType":"semver"},{"version":"5.15.222","lessThanOrEqual":"5.15.*","status":"unaffected","versionType":"semver"},{"version":"6.1.189","lessThanOrEqual":"6.1.*","status":"unaffected","versionType":"semver"},{"version":"6.6.158","lessThanOrEqual":"6.6.*","status":"unaffected","versionType":"semver"},{"version":"6.12.112","lessThanOrEqual":"6.12.*","status":"unaffected","versionType":"semver"},{"version":"6.18.55","lessThanOrEqual":"6.18.*","status":"unaffected","versionType":"semver"},{"version":"7.2.9","lessThanOrEqual":"7.2.*","status":"unaffected","versionType":"semver"},{"version":"7.3","lessThanOrEqual":"*","status":"unaffected","versionType":"original_commit_for_fix"}],"defaultStatus":"affected"}],"fixed_versions":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98550","source_modified_at":"2026-10-19T09:21:33.349Z","fetched_at":"2026-10-23T18:45:31.882637Z","package":{"name":"Linux","ecosystem":null},"version":"9b94a90651e1fa573c6ac6a5a6b269d5874df2c0","basis":"first unaffected version stated in the CVE record"},{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98550","source_modified_at":"2026-10-19T09:21:33.349Z","fetched_at":"2026-10-23T18:45:31.882637Z","package":{"name":"Linux","ecosystem":null},"version":"507a27435b2bb1f22e3a3674f6a6553eb815021b","basis":"first unaffected version stated in the CVE record"},{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-98550","source_modified_at":"2026-10-19T09:21:33.349Z","fetched_at":"2026-10-23T18:45:31.882637Z","package":{"name":"Linux","ecosystem":null},"version":"80a9ccfc24d3776ef6753a57315e726475fac7d7","basis":"first unaffected version stated in the CVE record"},{"source":"CVE 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record"}],"cvss":null,"severity":null,"kev":{"listed":false},"epss":{"probability":0.0021,"percentile":0.107,"score_date":"2026-10-23"},"published_at":"2026-10-18T15:18:18.173Z","modified_at":"2026-10-19T09:21:33.349Z","provenance":[{"source":"CVE List","url":"https://www.cve.org/CVERecord?id=CVE-2026-98550","fetched_at":"2026-10-23T18:45:31.882637Z","source_modified_at":"2026-10-19T09:21:33.349Z"},{"source":"EPSS","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98550","fetched_at":"2026-10-23T18:42:43.628194Z","source_modified_at":"2026-10-23"}],"sources":["CVE List","EPSS"],"references":[{"source":"CVE List","url":"https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35"},{"source":"CVE List","url":"https://git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0"},{"source":"CVE List","url":"https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b"},{"source":"CVE 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