High [CVE-2026-81665] Corosync: corosync: heap-based buffer overflow in totempg assembly buffer during fragmented message reassembly
This high-severity Red Hat Linux advisory covers CVE-2026-81665 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8.
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Summary
A heap-based buffer overflow was found in Corosync's Totem Process Group (totempg) message reassembly. When processing fragmented multicast messages, the buffer used to reassemble fragments lacks a runtime bounds check in release builds.
A network-adjacent attacker able to send crafted multicast protocol messages to the cluster could cause a heap buffer overflow with attacker-controlled data. This can crash the Corosync daemon, causing a denial of service to the entire cluster, and may potentially allow further exploitation given sufficient heap-corruption control.
This vulnerability is rated as Important. The published Red Hat CVSS score is 7.5 (CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H), matching the unmitigated upstream baseline.
RHEL High Availability Add-On clusters are configured via the pcs (Pacemaker/Corosync configuration system) tool, whose 'pcs cluster setup' command has defaulted to generating a corosync.conf with crypto_hash=sha256 and crypto_cipher=aes256 for multiple RHEL 8.x and 9.x releases, which meaningfully reduces real-world risk by requiring an attacker to already possess the cluster's shared authentication key before the vulnerable code is ever reached.
This has been confirmed directly with upstream corosync maintainer Jan Friesse (2026-08-31).
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 58 minutes ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 58 minutes ago·verify at source
Mitigation checklist
- There is no complete mitigation for this vulnerability short of a fix, but the following steps significantly reduce risk: 1. Ensure cluster message authentication is enabled in corosync.conf: set crypto_cipher to aes256 and crypto_hash to sha256 (or stronger) in the totem {} block, with a shared key generated by corosync-keygen. This is the default when clusters are configured via 'pcs cluster setup', and disabling it (cipher=none/hash=none) is an explicitly unsupported configuration per upstream maintainer guidance. 2. Restrict network access to the cluster communication ports (default 5405-5412/UDP) to trusted cluster node addresses only, using firewalld or other firewall rules. 3. Ensure SELinux is running in enforcing mode on cluster nodes to limit the impact of any successful exploitation.
Official advisory · high-confidence parse· fetched 58 minutes ago·verify at source
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