{"id":"CVE-2026-98550","state":"PUBLISHED","title":"tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow","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.","publishedAt":"2026-10-18T15:18:18.173Z","updatedAt":"2026-10-24T14:09:23.470Z","severity":{"score":7.5,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"edc6593c442278fceff0120e8cc58bc592168c35","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"9b94a90651e1fa573c6ac6a5a6b269d5874df2c0","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"507a27435b2bb1f22e3a3674f6a6553eb815021b","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"80a9ccfc24d3776ef6753a57315e726475fac7d7","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"478fb08d0c2a53002a721d22b8d6681ec971d90f","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"b0e517961f09b9f60d338d1ea4e17aacdbf206f0","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"99cc2a62e07a44a22254d7beca9ef1f8ad886d0d","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/tipc/group.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.15","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.15","versionType":"semver"},{"version":"5.10.271","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.222","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.189","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.158","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.112","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.55","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.9","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/tipc/group.c"]}],"references":["https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35","https://git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0","https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b","https://git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7","https://git.kernel.org/stable/c/478fb08d0c2a53002a721d22b8d6681ec971d90f","https://git.kernel.org/stable/c/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b","https://git.kernel.org/stable/c/b0e517961f09b9f60d338d1ea4e17aacdbf206f0","https://git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d"],"weaknesses":[],"ingestedAt":"2026-10-29T20:13:43.249Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/98xxx/CVE-2026-98550.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-98550","state":"PUBLISHED","datePublished":"2026-10-18T15:18:18.173Z","dateUpdated":"2026-10-24T14:09:23.470Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-09-25T10:25:14.356Z"},"containers":{"cna":{"title":"tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"edc6593c442278fceff0120e8cc58bc592168c35","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"9b94a90651e1fa573c6ac6a5a6b269d5874df2c0","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"507a27435b2bb1f22e3a3674f6a6553eb815021b","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"80a9ccfc24d3776ef6753a57315e726475fac7d7","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"478fb08d0c2a53002a721d22b8d6681ec971d90f","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"b0e517961f09b9f60d338d1ea4e17aacdbf206f0","versionType":"git"},{"version":"2f487712b89376fce267223bbb0db93d393d4b09","status":"affected","lessThan":"99cc2a62e07a44a22254d7beca9ef1f8ad886d0d","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/tipc/group.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"4.15","status":"affected"},{"version":"0","status":"unaffected","lessThan":"4.15","versionType":"semver"},{"version":"5.10.271","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.222","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.189","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.158","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.112","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.55","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.9","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["net/tipc/group.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35"},{"url":"https://git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0"},{"url":"https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b"},{"url":"https://git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7"},{"url":"https://git.kernel.org/stable/c/478fb08d0c2a53002a721d22b8d6681ec971d90f"},{"url":"https://git.kernel.org/stable/c/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b"},{"url":"https://git.kernel.org/stable/c/b0e517961f09b9f60d338d1ea4e17aacdbf206f0"},{"url":"https://git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d"}],"metrics":[{"cvssV3_1":{"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"},"scenarios":[{"lang":"en","value":"AV:N - A crafted UDP/IP TIPC GROUP_PROTOCOL/GRP_ACK_MSG carrying attacker-controlled msg_grp_bc_acked() reaches tipc_group_proto_rcv() through tipc_udp_recv(), tipc_rcv(), link delivery, and socket filtering. The UDP bearer is routable.\nAC:L - After becoming a recognized group member, the attacker can send one ACK that compares newer than m->bc_acked while grp->bc_ackers is zero, deterministically causing the underflow. No race or external condition is required.\nPR:N - The GRP_ACK_MSG case in tipc_group_proto_rcv() verifies only that the packet's origin node and port match a tipc_member; it performs no credential or capability check. TIPC wire encryption is optional, so no victim-system privilege is required.\nUI:N - Once a cluster-scope TIPC group socket exists, the remote member triggers tipc_group_proto_rcv() solely by sending the crafted GRP_ACK_MSG; no victim action is needed.\nS:U - The malformed ACK changes grp->bc_ackers and availability of the same kernel-managed TIPC group socket; it does not cross a VM, sandbox, or other security-authority boundary.\nC:N - The faulty operation is a decrement of the u16 grp->bc_ackers field from zero to 65535. It neither reads outside an object nor returns kernel or application data to the sender.\nI:N - The bug alters only TIPC ACK-accounting state through the intended grp->bc_ackers field; it provides no out-of-bounds write, control-flow primitive, or modification of application data.\nA:H - After grp->bc_ackers wraps to 65535, tipc_group_bc_cong() continually reports congestion and tipc_send_group_bcast() blocks or fails all later broadcasts on that socket until the group is recreated."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-24T14:09:23.470Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"5.10.271"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"5.15.222"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"6.1.189"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"6.6.158"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"6.12.112"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"6.18.55"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"7.2.9"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"4.15","versionEndExcluding":"7.3"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}}}}}