{"id":"AZL-109167","summary":"CVE-2026-98550 affecting package kernel 6.6.157.1-1","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":"2026-10-19T14:16:31.678064202Z","published":"2026-10-18T16:17:07Z","upstream":["CVE-2026-98550"],"references":[{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98550"}],"affected":[{"package":{"name":"kernel","ecosystem":"Azure Linux:3","purl":"pkg:rpm/azure-linux/kernel"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"6.6.157.1-1"}]}],"database_specific":{"source":"https://github.com/microsoft/AzureLinuxVulnerabilityData/blob/main/osv/AZL-109167.json"}}],"schema_version":"1.9.0"}