# CVE-2026-81665: Corosync: corosync: heap-based buffer overflow in totempg assembly buffer during fragmented message reassembly Status: published. Published: 2026-09-04. Updated: 2026-11-02. 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. ## Remediation - workaround: 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. (source: CNA: redhat) ## Affected products and versions - Red Hat / Red Hat Enterprise Linux 10 (corosync): ≥ 0:3.1.10-1.el10_2.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 10.0 Extended Update Support (corosync): ≥ 0:3.1.9-1.el10_0.3; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 7 Extended Lifecycle Support (corosync): ≥ 0:2.4.5-7.el7_9.4; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8 (corosync): ≥ 0:3.1.8-1.el8_10.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Support (corosync): ≥ 0:3.1.0-3.el8_4.3; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8.4 Extended Update Support Long-Life Add-On (corosync): ≥ 0:3.1.0-3.el8_4.3; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8.6 Advanced Mission Critical Update Support (corosync): ≥ 0:3.1.5-2.el8_6.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8.6 Extended Update Support Long-Life Add-On (corosync): ≥ 0:3.1.5-2.el8_6.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8.8 Telecommunications Update Service (corosync): ≥ 0:3.1.7-1.el8_8.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 8.8 Update Services for SAP Solutions (corosync): ≥ 0:3.1.7-1.el8_8.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 9 (corosync): ≥ 0:3.1.10-1.el9_8.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 9.2 Update Services for SAP Solutions (corosync): ≥ 0:3.1.7-1.el9_2.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 9.4 Update Services for SAP Solutions (corosync): ≥ 0:3.1.8-1.el9_4.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat Enterprise Linux 9.6 Extended Update Support (corosync): ≥ 0:3.1.9-2.el9_6.2; < * (unaffected, rpm). Source: CNA: redhat - Red Hat / Red Hat OpenShift Container Platform 4 (openshift/ose-rhel-coreos-9): Version range not reported. Source: CNA: redhat - Red Hat / Red Hat OpenShift Container Platform 4 (rhcos): Version range not reported. Source: CNA: redhat - Red Hat / Red Hat OpenShift Container Platform 5 (openshift/ose-rhel-coreos-9): Version range not reported. Source: CNA: redhat - Red Hat / Red Hat OpenShift Container Platform 5 (rhcos): Version range not reported. Source: CNA: redhat ## Severity and prioritization - CVSS 3.1: 7.5 (high); CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H; source: CNA: redhat - CVSS 3.1: 7.5 (high); CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H; source: NVD: secalert@redhat.com - CISA KEV: No listing in collected sources - EPSS: 0.42% as of 2026-11-02; percentile 35.88% - SSVC: exploitation none; automatable no; technical impact total; source ADP: CISA-ADP - Weaknesses: CWE-122 ## Exploit evidence No exploit evidence in collected sources. This is not proof of no exploitation. ## References - https://access.redhat.com/errata/RHSA-2026:67872 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:67873 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:67878 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:67879 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:67880 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:67881 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:68574 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:69277 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:69278 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:69279 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:69280 (CNA: redhat; vendor-advisory, x_refsource_REDHAT) - https://access.redhat.com/security/cve/CVE-2026-81665 (CNA: redhat; vdb-entry, x_refsource_REDHAT) - https://bugzilla.redhat.com/show_bug.cgi?id=2524910 (CNA: redhat; issue-tracking, x_refsource_REDHAT) - https://access.redhat.com/errata/RHSA-2026:67872 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:67873 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:67878 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:67879 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:67880 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:67881 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:68574 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:69277 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:69278 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:69279 (secalert@redhat.com) - https://access.redhat.com/errata/RHSA-2026:69280 (secalert@redhat.com) - https://access.redhat.com/security/cve/CVE-2026-81665 (secalert@redhat.com) - https://bugzilla.redhat.com/show_bug.cgi?id=2524910 (secalert@redhat.com) ## Provenance - CNA: redhat: https://www.cve.org/CVERecord?id=CVE-2026-81665; retrieved 2026-11-02; source updated 2026-11-02 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81665; retrieved 2026-11-02; source updated 2026-11-02 - FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-81665; retrieved 2026-11-02; source updated Not reported Revision: 2e27f3400b88284040350ac3786745c5fc179e34d007fa821ad353d2eb6920ab Canonical: https://y2.dev/cve/CVE-2026-81665 ## Agent and API access Public CVE pages, Markdown, and JSON are free to read without an account or API key. - Public catalog: https://y2.dev/cve/index.json. 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