{"identifier":"CVE-2026-98550","cve":{"id":"CVE-2026-98550","description":"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.","published":"2026-10-18T16:17:07.030000+00:00","modified":"2026-10-24T14:16:47.290000+00:00","status":"Received","score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","cvss_version":"3.1","affected":[],"weaknesses":[],"references":[{"url":"https://git.kernel.org/stable/c/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/478fb08d0c2a53002a721d22b8d6681ec971d90f","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/b0e517961f09b9f60d338d1ea4e17aacdbf206f0","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"}],"kev":null,"kev_checked":true,"metrics":{"cvssMetricV31":[{"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","type":"Secondary","cvssData":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","baseScore":7.5,"baseSeverity":"HIGH","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"NONE","integrityImpact":"NONE","availabilityImpact":"HIGH"},"exploitabilityScore":3.9,"impactScore":3.6}]},"configurations":[],"source":"nvd","retrieved_at":"2026-10-28T10:23:21.412233+00:00","exploited":false,"kev_stale":false,"products":[],"priority":{"label":"Assess exposure and vendor guidance","reasons":["No higher-priority signal is available in the current stored evidence."]},"epss":{"probability":0.00677,"percentile":0.52095,"date":"2026-10-27","stale":false},"version_assessment":null},"news":[],"campaigns":[],"domains":[{"domain":"git.kernel.org","source_url":"https://git.kernel.org/stable/c/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b","investigate_url":"https://investigate.aeglixmind.com/#git.kernel.org"}],"arsenal_url":"https://arsenal.aeglixmind.com/?cve=CVE-2026-98550","catalog_url":"https://cves.aeglixmind.com/cve/CVE-2026-98550","research_url":"https://investigate.aeglixmind.com/research/cve/CVE-2026-98550","coverage":{"news_candidates_checked":0,"campaign_candidates_checked":0,"bounded":false,"notice":"Exact CVE mentions in stored public excerpts and reviewed campaign content only. A mention is not proof of exploitation, attribution or tool suitability. Missing matches do not establish absence."}}