CVE-2026-81666
Description
An integer overflow was found in Corosync's handling of membership commit token messages. The length-validation check for these messages can be bypassed on 32-bit systems due to an integer overflow in the calculation of the expected message length, allowing a crafted network packet to trigger an out-of-bounds memory access that crashes the Corosync daemon. This results in a denial of service for the affected cluster node. The overflow does not occur on 64-bit systems, where the length calculation is correctly performed in 64-bit arithmetic.
Statement
This vulnerability is rated as Moderate. The published Red Hat CVSS score is 6.5 (CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), matching the unmitigated upstream baseline for 32-bit deployments.
The integer overflow described only affects 32-bit builds, because C integer-promotion rules cause the vulnerable multiplication to be evaluated in 64-bit arithmetic on 64-bit systems, which prevents the overflow entirely. Red Hat Enterprise Linux 8, 9, and 10 ship primarily 64-bit architectures (x86_64, aarch64), so this specific overflow mechanism does not apply to the vast majority of Red Hat deployments; this remains true regardless of the encryption scoring decision below.
As with the related totempg heap overflow (see CVE-2026-81665), RHEL High Availability Add-On clusters configured via 'pcs cluster setup' default to cluster message authentication enabled (crypto_hash=sha256, crypto_cipher=aes256), which meaningfully reduces real-world risk. This has been confirmed directly with upstream corosync maintainer Jan Friesse (2026-08-31), who also confirmed this protection is a configurable default rather than an enforced control for RHEL <=10: an administrator can disable it via the officially supported 'pcs' interface (cipher=none, hash=none), not just by unsupported manual editing. Because this protection can be turned off through a supported configuration path, Red Hat is publishing the CVSS score for the worst-case supported configuration (encryption disabled) rather than crediting the default as a permanent mitigating factor.
Administrators who have not explicitly disabled cluster message authentication benefit from a reduced practical risk, equivalent to CVSS 4.5 (CVSS:3.1/AV:A/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H). See the Mitigation section for guidance on confirming and enforcing this configuration.
Corosync is shipped in Red Hat Enterprise Linux 7 through 10 and RHIVOS as part of the High Availability Add-On.
Note: upstream corosync HEAD and current Fedora (which will become RHEL 11) now enforce encryption at compile time, removing the ability to disable it entirely without recompiling from source. Once this reaches RHEL as a supported release, this compensating control will become a true enforced guarantee rather than a configurable default, and the published score is expected to be revisited at that time.
Mitigation
There is no complete mitigation for this vulnerability short of a fix, but the following steps reduce risk:
1. Ensure cluster message authentication is enabled in corosync.conf (crypto_cipher=aes256, crypto_hash=sha256 with a shared key from corosync-keygen), which 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.
3. Where architecturally feasible, prefer 64-bit deployments, since the integer overflow described here does not occur on 64-bit systems.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.5 | N/A | 6.5 |
| Attack Vector | Adjacent Network | N/A | Adjacent Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat CVSS v3 Score Explanation
Published at the unmitigated baseline (matching upstream, for 32-bit deployments). RHEL HA clusters configured via 'pcs cluster setup' default to cluster message authentication enabled, which reduces real-world risk to an equivalent of 4.5 (PR:H) for administrators who have not disabled it. However, per confirmation from upstream maintainer Jan Friesse (2026-08-31), this protection is a configurable default rather than an enforced control on RHEL <=10 -- it can be turned off via the officially supported pcs interface itself (cipher=none/hash=none), not only via unsupported manual editing. Since a supported configuration path exists to disable it, Red Hat publishes the worst-case supported configuration score rather than crediting the default. See statement for full detail.
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (Memory); DoS: Instability
This weakness can generally lead to undefined behavior and therefore crashes. When the calculated result is used for resource allocation, this weakness can cause too many (or too few) resources to be allocated, possibly enabling crashes if the product requests more resources than can be provided.
Integrity
Technical Impact: Modify Memory
If the value in question is important to data (as opposed to flow), simple data corruption has occurred. Also, if the overflow/wraparound results in other conditions such as buffer overflows, further memory corruption may occur.
Confidentiality,Availability,Access Control
Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism
This weakness can sometimes trigger buffer overflows, which can be used to execute arbitrary code. This is usually outside the scope of the product's implicit security policy.
Availability,Other
Technical Impact: Alter Execution Logic; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU)
If the overflow/wraparound occurs in a loop index variable, this could cause the loop to terminate at the wrong time - too early, too late, or not at all (i.e., infinite loops). With too many iterations, some loops could consume too many resources such as memory, file handles, etc., possibly leading to a crash or other DoS.
Access Control
Technical Impact: Bypass Protection Mechanism
If integer values are used in security-critical decisions, such as calculating quotas or allocation limits, integer overflows can be used to cause an incorrect security decision.
Acknowledgements
Red Hat would like to thank Tristan Madani (Talence Security) for reporting this issue.
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