High

CVE-2026-98550 — 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). 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.

Published Oct 18, 2026 · Updated Oct 24, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: Linux

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.7% (30-day exploitation probability)+8
Technical severity (high)+6

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.68%

ahead of 52% of scored CVEs · Oct 30, 2026

Affects

Linux

CVSS base

7.5

High · 2 source(s)

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Linux Linux).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
No fix documented in the sources: reduce exposure and monitor the vendor advisory.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Linux	Linux	≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < edc6593c442278fceff0120e8cc58bc592168c35≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < 9b94a90651e1fa573c6ac6a5a6b269d5874df2c0≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < 507a27435b2bb1f22e3a3674f6a6553eb815021b≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < 80a9ccfc24d3776ef6753a57315e726475fac7d7≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < 478fb08d0c2a53002a721d22b8d6681ec971d90f≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < 12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < b0e517961f09b9f60d338d1ea4e17aacdbf206f0≥ 2f487712b89376fce267223bbb0db93d393d4b09 and < 99cc2a62e07a44a22254d7beca9ef1f8ad886d0d and 1 more version 4.15	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
7.5	CVSS 3.1	High	3.9	3.6	416baaa9-dc9f-4396-8d5f-8c081fb06d67 via NIST National Vulnerability Database
7.5	CVSS 3.1	High	—	—	Linux via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (score shown at the top: 416baaa9-dc9f-4396-8d5f-8c081fb06d67)

EPSS over time FIRST

Oct 19, 2026 → Oct 30, 2026 · 12 reading(s) · max 0.68%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
CVSS changed · 7.5
Oct 24, 2026 · NIST National Vulnerability Database
Record published
Oct 18, 2026
Affected products added · linux/linux
Oct 18, 2026 · CVE Program (cve.org) e CISA ADP
Source status changed · → Received
Oct 18, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 18, 2026 · NIST National Vulnerability Database
CVE published
Oct 18, 2026 · Linux

Source activity

Repeated readings are counted rather than listed.

1× References added · Oct 19, 2026 – Oct 19, 2026
1× Official description updated · Oct 18, 2026 – Oct 18, 2026

References

10 on the record

git.kernel.org/stable/c/12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b
NIST National Vulnerability Database
git.kernel.org/stable/c/507a27435b2bb1f22e3a3674f6a6553eb815021b
NIST National Vulnerability Database
git.kernel.org/stable/c/9b94a90651e1fa573c6ac6a5a6b269d5874df2c0
NIST National Vulnerability Database
git.kernel.org/stable/c/478fb08d0c2a53002a721d22b8d6681ec971d90f
NIST National Vulnerability Database
github.com/advisories/GHSA-xqp5-j42w-5fq8
GitHub Security Advisories
git.kernel.org/stable/c/80a9ccfc24d3776ef6753a57315e726475fac7d7
NIST National Vulnerability Database
git.kernel.org/stable/c/b0e517961f09b9f60d338d1ea4e17aacdbf206f0
NIST National Vulnerability Database
nvd.nist.gov/vuln/detail/CVE-2026-98550
GitHub Security Advisories
git.kernel.org/stable/c/99cc2a62e07a44a22254d7beca9ef1f8ad886d0d
NIST National Vulnerability Database
git.kernel.org/stable/c/edc6593c442278fceff0120e8cc58bc592168c35
NIST National Vulnerability Database

Elsewhere on this site

Linux
every CVE for this vendor

Related CVEs (same products)

High CVE-2026-99216 Drivers: hv: Don't reset VMBus connection state on error/exit paths 3d
Unknown CVE-2026-99215 power: supply: core: prevent unregistering a power supply while a callback runs 3d
Unknown CVE-2026-99207 scsi: target: Disable interrupts while holding delayed_cmd_lock 3d
Unknown CVE-2026-99208 drm/xe/oa: Zero OA buffer at allocation 3d
Unknown CVE-2026-99209 drm/xe: skip eviction of a BO with no resource 3d
Unknown CVE-2026-99210 accel/rocket: clear rdev on device init failure 3d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No fixed version, patch or mitigation documented.
No CISA SSVC assessment.
No CWE classification.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-98550 is "tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow", rated High. It affects Linux (Linux); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.5 describes technical severity. EPSS 0.68% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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