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Response snapshot Oct 26, 2026, 23:49 UTC

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VulnerabilityCVE-2026-98550

Linux kernel: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow

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).

Published Updated

At a glance

Exploited
Not known
Severity
7.5 High (CVSS 3.1)
Chance of exploitation
0.7% in the next 30 days
Sources
1 source

What to do

Check the vendor advisory for affected versions and fixes.

Read NVD CVE-2026-98550 (opens a new tab)

What happened

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:

Continue reading

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.

Scores and how they were measured

CVSS base severity

7.5

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.7%

Scored Percentile 52.0%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    NVD CVE-2026-98550 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  2. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-98550, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 24). Linux kernel: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow (CVE-2026-98550). https://kevwire.com/record/CVE-2026-98550

MLA

"Linux kernel: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow (CVE-2026-98550)." KEVWire, 24 Oct. 2026, kevwire.com/record/CVE-2026-98550.

Chicago

KEVWire. "Linux kernel: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow (CVE-2026-98550)." Last modified October 24, 2026. https://kevwire.com/record/CVE-2026-98550.

Evidence summary (plain text)

Linux kernel: tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow (Oct 18, 2026).
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
CVSS 7.5 (version 3.1; evidence as of Oct 24, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-98550).
EPSS 0.7% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-98550&date=2026-10-26).
Sources:
NVD CVE-2026-98550: https://nvd.nist.gov/vuln/detail/CVE-2026-98550
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?pubStartDate=2026-10-12T13:35:55.142Z&pubEndDate=2026-10-26T13:35:55.142Z&resultsPerPage=500&startIndex=4500
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-98550&date=2026-10-26
Record ID: CVE-2026-98550.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

NVD status: Received. Publication in the NVD is not evidence of exploitation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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