---
id: CVE-2026-98550
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 …
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    edc6593c442278fceff0120e8cc58bc592168c35
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    9b94a90651e1fa573c6ac6a5a6b269d5874df2c0
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    507a27435b2bb1f22e3a3674f6a6553eb815021b
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    80a9ccfc24d3776ef6753a57315e726475fac7d7
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    478fb08d0c2a53002a721d22b8d6681ec971d90f
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    b0e517961f09b9f60d338d1ea4e17aacdbf206f0
  - >-
    Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 <
    99cc2a62e07a44a22254d7beca9ef1f8ad886d0d
  - Linux 4.15
published: '2026-10-18'
updated: '2026-10-18'
sourceUpdated: '2026-10-18T15:18:18.173Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-98550'
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'
tags:
  - cve.org
ingestedAt: '2026-10-18T15:45:12.264Z'
---

## Overview

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.

## Affected

- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < edc6593c442278fceff0120e8cc58bc592168c35`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < 9b94a90651e1fa573c6ac6a5a6b269d5874df2c0`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < 507a27435b2bb1f22e3a3674f6a6553eb815021b`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < 80a9ccfc24d3776ef6753a57315e726475fac7d7`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < 478fb08d0c2a53002a721d22b8d6681ec971d90f`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < 12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < b0e517961f09b9f60d338d1ea4e17aacdbf206f0`
- `Linux >= 2f487712b89376fce267223bbb0db93d393d4b09 < 99cc2a62e07a44a22254d7beca9ef1f8ad886d0d`
- `Linux 4.15`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.