Bug 2461491 (CVE-2026-31662)

Summary: CVE-2026-31662 kernel: tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's Transparent Inter-Process Communication (TIPC) protocol. An attacker can exploit an integer underflow vulnerability by sending duplicate group acknowledgment messages (GRP_ACK_MSG). This causes the `bc_ackers` counter to wrap around, leading to persistent network congestion. Consequently, group broadcasts on the affected socket become blocked, resulting in a denial of service until the group is manually recreated.
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Description OSIDB Bzimport 2026-04-24 15:04:40 UTC 
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.
Comment 1 Keith Grant 2026-04-24 22:19:59 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026042404-CVE-2026-31662-3728@gregkh/T