CVE-2026-98550 (GCVE-0-2026-98550)

Vulnerability from cvelistv5 – Published: 2026-10-18 15:18 – Updated: 2026-10-19 09:21
VLAI
Title
tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow
Summary
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->bc_acked). However, that check remains incomplete in two ways: 1. When grp->bc_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 > m->bc_acked passes less_eq() and unconditionally decrements grp->bc_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->bc_ackers > 0), the sender transmits packet S and advances grp->bc_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->bc_snd_nxt. However, tipc_group_update_bc_members() initializes each member's m->bc_acked to prev = grp->bc_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->bc_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->bc_ackers twice in the same round, prematurely clearing bc_ackers or underflowing it. Fix this by: - Dropping GRP_ACK_MSG immediately if grp->bc_ackers is zero. - Requiring acked == grp->bc_snd_nxt and rejecting duplicates where m->bc_acked == acked. Because replicast broadcast rounds are strictly sequential, only grp->bc_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->bc_ackers decrementing to zero upon member departure does not restore *grp->open or trigger a socket wakeup) will be addressed in a separate patch.
Severity
No CVSS data available.
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 6856 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < edc6593c442278fceff0120e8cc58bc592168c35 (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < 9b94a90651e1fa573c6ac6a5a6b269d5874df2c0 (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < 507a27435b2bb1f22e3a3674f6a6553eb815021b (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < 80a9ccfc24d3776ef6753a57315e726475fac7d7 (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < 478fb08d0c2a53002a721d22b8d6681ec971d90f (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < 12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < b0e517961f09b9f60d338d1ea4e17aacdbf206f0 (git)
Affected: 2f487712b89376fce267223bbb0db93d393d4b09 , < 99cc2a62e07a44a22254d7beca9ef1f8ad886d0d (git)
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Linux Linux Affected: 4.15
Unaffected: 0 , < 4.15 (semver)
Unaffected: 5.10.271 , ≤ 5.10.* (semver)
Unaffected: 5.15.222 , ≤ 5.15.* (semver)
Unaffected: 6.1.189 , ≤ 6.1.* (semver)
Unaffected: 6.6.158 , ≤ 6.6.* (semver)
Unaffected: 6.12.112 , ≤ 6.12.* (semver)
Unaffected: 6.18.55 , ≤ 6.18.* (semver)
Unaffected: 7.2.9 , ≤ 7.2.* (semver)
Unaffected: 7.3 , ≤ * (original_commit_for_fix)
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          "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-\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\u0027s\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."
        }
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          "url": "https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35"
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          "url": "https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b"
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          "url": "https://git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7"
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        {
          "url": "https://git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d"
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    "cveId": "CVE-2026-98550",
    "datePublished": "2026-10-18T15:18:18.173Z",
    "dateReserved": "2026-09-25T10:25:14.356Z",
    "dateUpdated": "2026-10-19T09:21:33.349Z",
    "state": "PUBLISHED"
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      "date": "2026-10-21",
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    "microsoft_vex": {
      "current_release_date": "2026-10-21T18:39:34.000Z",
      "cve": "CVE-2026-98550",
      "id": "msrc_CVE-2026-98550",
      "initial_release_date": "2026-10-20T01:11:21.000Z",
      "product_status:known_affected": "1",
      "product_status:under_investigation": "1",
      "source": "Microsoft CSAF VEX",
      "status": "final",
      "title": "tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow",
      "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-98550.json",
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        "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-\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\u0027s\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."
          }
        ],
        "id": "CVE-2026-98550",
        "lastModified": "2026-10-19T10:19:11.200",
        "metrics": {},
        "published": "2026-10-18T16:17:07.030",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "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"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Received"
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-10-20T20:58:21+00:00",
      "cve": "CVE-2026-98550",
      "id": "CVE-2026-98550",
      "initial_release_date": "2026-10-18T00:00:00+00:00",
      "product_status:known_affected": "235",
      "product_status:known_not_affected": "42",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-98550.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "not set",
      "current_release_date": "2026-10-18T23:45:19Z",
      "cve": "CVE-2026-98550",
      "id": "CVE-2026-98550",
      "initial_release_date": "2026-10-18T23:45:19Z",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-98550",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/2026/cve-2026-98550.json",
      "version": "2"
    }
  }
}



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