{"id":"CVE-2026-98550","kind":"vulnerability","aliases":["CVE-2026-98550"],"related":[],"title":"Linux kernel TIPC: stale group ACK can wrap counter and block broadcasts","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.","summary":"A flaw in the Linux kernel's TIPC group messaging accepts unexpected GRP_ACK_MSG packets, which can decrement a 16-bit acknowledgement counter below zero. The counter then wraps and makes the socket report permanent congestion, blocking future group broadcasts.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["kernel","linux","package","tipc"],"not_affected_if":[],"how_to_check":null,"fix":null,"status":"open","published_at":"2026-10-18T16:17:07+00:00","first_seen_at":"2026-10-23T21:57:15+00:00","triaged_at":"2026-10-25T05:23:31+00:00","updated_at":"2026-10-25T05:23:31+00:00","category":"package","confidence":0.5,"triage":{"at":"2026-10-25T05:23:31Z","model":"claude-haiku-5-5","output":{"fix":"unknown","reach":"network","title":"Linux kernel TIPC: stale group ACK can wrap counter and block broadcasts","impact":"low","summary":"A flaw in the Linux kernel's TIPC group messaging accepts unexpected GRP_ACK_MSG packets, which can decrement a 16-bit acknowledgement counter below zero. The counter then wraps and makes the socket report permanent congestion, blocking future group broadcasts.","category":"package","nicknames":[],"who_cares":["linux","kernel","tipc"],"confidence":0.5,"needs_auth":"unknown","worst_case":"Denial of service: group broadcasts on the socket are blocked","analyst_note":"Check the kernel.org stable commit referenced by NVD for the fixed kernel versions and whether TIPC group messaging is in use.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":[],"who_is_affected":"unknown","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"low"},"cost_usd":0.000599,"input_tokens":4149,"output_tokens":368,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':14A,17A '-98550':15A,18A '0':207C '1':125C,221C,229C,270C,272C,282C,285C,325C '16':43B '2':198C,278C '48a5fe38772b':88C '65535':179C 'accept':33B 'ack':6A,36B,80C,97C,101C,113C,116C,139C,154C,157C,160C,238C,246C,263C,291C,319C,323C,348C,358C,369C,370C 'acker':85C,92C,129C,169C,172C,206C,304C,330C,339C,354C,412C 'acknowledg':45B,150C,385C,390C 'activ':201C 'address':431C 'advanc':214C 'alreadi':149C 'bc':84C,91C,115C,128C,159C,168C,171C,183C,205C,216C,248C,255C,262C,267C,303C,329C,338C,353C,360C,368C,380C,411C 'bit':44B 'block':11A,60B,190C 'broadcast':12A,63B,194C,202C,373C 'caus':180C 'check':119C 'clear':337C 'commit':87C 'cong':184C 'congest':59B,188C 'consum':231C 'could':326C 'counter':9A,46B,50B,226C 'current':242C 'cve':13A,16A 'decrement':41B,166C,301C,327C,413C 'delet':407C 'departur':418C 'drop':346C 'duplic':95C,297C,365C 'duplicate/stale':100C 'e.g':132C 'earli':109C 'eq':112C,163C 'exist':402C 'expect':146C,225C 'fix':90C,343C 'flaw':24B 'follow':69C,321C 'futur':61B,192C 'gap':280C 'group':5A,31B,62B,104C,136C,182C,193C,253C,406C 'grp':35B,79C,96C,127C,153C,167C,204C,215C,247C,266C,302C,328C,347C,352C,359C,379C,410C,422C 'howev':117C,251C 'immedi':350C 'incom':290C 'incomplet':121C 'increment':223C,274C 'initi':257C 'invalid':76C 'issu':403C 'kernel':2A,19B,28B,67C 'leav':276C 'less':111C,162C 'linux':1A,20B,27B,66C 'm':114C,158C,261C,367C 'make':54B 'member':147C,256C,259C,317C,388C,408C,417C 'messag':32B 'msg':37B,81C,98C,155C,349C 'neither':294C 'note':396C 'nxt':218C,250C,269C,362C,382C 'open':423C 'packag':21B 'packet':38B,211C,232C 'pass':161C 'patch':435C 'per':394C 'perman':58B,186C 'pre':401C 'pre-exist':400C 'prematur':336C 'prev':265C 'prevent':83C,299C 'proto':105C 'quiet':135C 'rcv':106C 'receiv':222C,315C 'reject':75C,99C,295C,364C 'relat':399C 'remain':120C 'replicast':138C,372C 'report':57B,187C 'request':142C 'requir':357C 'resolv':73C 'restor':421C 'return':108C 'round':203C,243C,335C,374C,395C 'send':318C 'sender':209C 'separ':434C 'sequenc':279C,287C 'sequenti':377C 'snd':217C,249C,268C,361C,381C 'socket':56B,197C,427C 'space':288C 'stale':4A,309C 'strict':245C,376C 'therefor':293C 'tipc':3A,22B,30B,74C,89C,103C,181C,252C,405C 'transmit':210C 'trigger':425C 'twice':331C 'two':123C 'u16':175C 'uncondit':165C 'underflow':86C,93C,341C 'unexpect':34B,78C,152C,307C 'updat':254C 'upon':230C,416C 'valid':237C 'valu':239C,310C 'vulner':70C 'wakeup':428C 'way':124C 'wrap':8A,52B,177C 'zero':48B,131C,356C,415C","nicknames":[],"mitigations":[],"who_affected":null,"tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-18T16:17:07+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98550","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98550","severity":null,"published_at":"2026-10-18T16:17:07+00:00","modified_at":"2026-10-19T10:19:11+00:00","first_seen_at":"2026-10-23T21:57:15+00:00","fetched_at":"2026-10-23T22:28:56+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}