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tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow

Type: advisorySources: OSV

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 =

Products
Kernel
Affected versions
Not specified
Fixed versions
5.10.271, 5.15.222, 6.1.189, 6.12.112, 6.18.55, 6.6.158, 7.2.9
CVEs
CVE-2026-98550
Confirmed fixed CVEs
Not specified
Updated in this feed
3 Nov 2026, 17:04 UTC · Revision 1

Source evidence

tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow

core-osv · Original source

Published 18 Oct 2026, 15:18 UTC

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 =

osv:CVE-2026-98550

Kernel

Fixed versions: 5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.112, 6.18.55, 7.2.9

Complete source record
{
  "id": "56858cdf2ebfacc899a8a5d95f58c79d8bdf3a712bc7a97a69eca4f4464af006",
  "url": "https://osv.dev/vulnerability/CVE-2026-98550",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "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 =",
    "evidence": "osv:CVE-2026-98550",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "5.10.271",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "5.15.222",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.1.189",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.6.158",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.12.112",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.18.55",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "7.2.9",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-98550",
      "aliases": [
        "CVE-2026-98550"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "2f487712b89376fce267223bbb0db93d393d4b09"
                },
                {
                  "fixed": "edc6593c442278fceff0120e8cc58bc592168c35"
                },
                {
                  "fixed": "9b94a90651e1fa573c6ac6a5a6b269d5874df2c0"
                },
                {
                  "fixed": "507a27435b2bb1f22e3a3674f6a6553eb815021b"
                },
                {
                  "fixed": "80a9ccfc24d3776ef6753a57315e726475fac7d7"
                },
                {
                  "fixed": "478fb08d0c2a53002a721d22b8d6681ec971d90f"
                },
                {
                  "fixed": "12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b"
                },
                {
                  "fixed": "b0e517961f09b9f60d338d1ea4e17aacdbf206f0"
                },
                {
                  "fixed": "99cc2a62e07a44a22254d7beca9ef1f8ad886d0d"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98550.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "4.15.0"
                },
                {
                  "fixed": "5.10.271"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.11.0"
                },
                {
                  "fixed": "5.15.222"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.16.0"
                },
                {
                  "fixed": "6.1.189"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.2.0"
                },
                {
                  "fixed": "6.6.158"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.7.0"
                },
                {
                  "fixed": "6.12.112"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.13.0"
                },
                {
                  "fixed": "6.18.55"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.19.0"
                },
                {
                  "fixed": "7.2.9"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98550.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:30.475280581Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b",
        "https://git.kernel.org/stable/c/478fb08d0c2a53002a721d22b8d6681ec971d90f",
        "https://git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b",
        "https://git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7",
        "https://git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d",
        "https://git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0",
        "https://git.kernel.org/stable/c/b0e517961f09b9f60d338d1ea4e17aacdbf206f0",
        "https://git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98550.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-98550",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98550.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-98550",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 497686,
  "published_at": "2026-10-18T15:18:18.173+00:00"
}