{"id":"DEBIAN-CVE-2026-98550","details":"In the Linux kernel, the following vulnerability has been resolved:  tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow  Commit 48a5fe38772b (\"tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG\") rejected duplicate/stale ACKs in tipc_group_proto_rcv() by returning early when less_eq(acked, m-\u003ebc_acked).  However, that check remains incomplete in two ways:  1. When grp-\u003ebc_ackers is zero (e.g. on a quiet group, when replicast    ACKs were not requested, or after all expected members have already    acknowledged), an unexpected GRP_ACK_MSG with acked \u003e m-\u003ebc_acked    passes less_eq() and unconditionally decrements grp-\u003ebc_ackers.    Because bc_ackers is a u16, this wraps to 65535, causing    tipc_group_bc_cong() to permanently report congestion and blocking    all future group broadcasts on the socket.  2. During an active broadcast round (grp-\u003ebc_ackers \u003e 0), the sender    transmits packet S and advances grp-\u003ebc_snd_nxt to S + 1. Receivers    increment their expected counter to S + 1 upon consuming packet S,    so the only valid ACK value for the current round is strictly    acked == grp-\u003ebc_snd_nxt.     However, tipc_group_update_bc_members() initializes each member's    m-\u003ebc_acked to prev = grp-\u003ebc_snd_nxt - 1 (S - 1 before increment).    This leaves a 2-sequence gap (S - 1 to S + 1) in sequence space.    An incoming ACK is therefore neither rejected as duplicate nor    prevented from decrementing grp-\u003ebc_ackers if an unexpected or stale    value (such as S) is received. A member sending acked = S followed    by acked = S + 1 could decrement grp-\u003ebc_ackers twice in the same    round, prematurely clearing bc_ackers or underflowing it.  Fix this by: - Dropping GRP_ACK_MSG immediately if grp-\u003ebc_ackers is zero. - Requiring acked == grp-\u003ebc_snd_nxt and rejecting duplicates where   m-\u003ebc_acked == acked. Because replicast broadcast rounds are strictly   sequential, only grp-\u003ebc_snd_nxt can be acknowledged, and each member   can acknowledge at most once per round.  Note that a related pre-existing issue in tipc_group_delete_member() (where grp-\u003ebc_ackers decrementing to zero upon member departure does not restore *grp-\u003eopen or trigger a socket wakeup) will be addressed in a separate patch.","modified":"2026-10-19T05:00:26.205561397Z","published":"2026-10-18T17:47:36.519790568Z","upstream":["CVE-2026-98550"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2026-98550"}],"affected":[{"package":{"name":"linux","ecosystem":"Debian:12","purl":"pkg:deb/debian/linux?arch=source&distro=bookworm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.1.190-1"}]}],"versions":["6.1.106-1","6.1.106-2","6.1.106-3","6.1.112-1","6.1.115-1","6.1.119-1","6.1.123-1","6.1.124-1","6.1.128-1","6.1.129-1","6.1.133-1","6.1.135-1","6.1.137-1","6.1.139-1","6.1.140-1","6.1.147-1","6.1.148-1","6.1.153-1","6.1.158-1","6.1.159-1","6.1.162-1","6.1.164-1","6.1.170-1","6.1.170-2","6.1.170-3","6.1.172-1","6.1.174-1","6.1.176-1","6.1.177-1","6.1.180-1","6.1.187-1","6.1.27-1","6.1.37-1","6.1.38-1","6.1.38-2","6.1.38-2~bpo11+1","6.1.38-3","6.1.38-4","6.1.38-4~bpo11+1","6.1.52-1","6.1.55-1","6.1.55-1~bpo11+1","6.1.64-1","6.1.66-1","6.1.67-1","6.1.69-1","6.1.69-1~bpo11+1","6.1.76-1","6.1.76-1~bpo11+1","6.1.82-1","6.1.85-1","6.1.90-1","6.1.90-1~bpo11+1","6.1.94-1","6.1.94-1~bpo11+1","6.1.98-1","6.1.99-1"],"ecosystem_specific":{"urgency":"not 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