Medium [CVE-2026-21723] Denial of Service via uncontrolled memory usage in alertmanager templates
This medium-severity Red Hat Linux advisory covers CVE-2026-21723 affecting Red Hat Hardened Images, Multicluster Global Hub, Red Hat Advanced Cluster Management for Kubernetes 2.
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Summary
The alertmanager templates test endpoint (/api/alertmanager/grafana/config/api/v1/templates/test) can execute templates with no memory limits. Mass-executing templates in a short period causes OOM and crashes the Grafana service.
The endpoint requires very low privileges and is exploitable with anonymous access enabled. A flaw was found in Grafana.
This can lead to an Out-Of-Memory (OOM) error, causing the Grafana service to crash and resulting in a Denial of Service (DoS). This Moderate flaw in Grafana allows a remote attacker to trigger a Denial of Service by repeatedly executing alertmanager templates.
While requiring low privileges, or anonymous access if enabled, the high attack complexity limits the immediate threat, as it relies on exhausting memory through mass template execution. Red Hat severity: Moderate — CVSS 5.3 (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H).
Weakness: CWE-770. Affected Red Hat products: Red Hat Hardened Images; Multicluster Global Hub; Red Hat Advanced Cluster Management for Kubernetes 2; Red Hat Ceph Storage 5; Red Hat Ceph Storage 6; Red Hat Ceph Storage 7; Red Hat Ceph Storage 8; Red Hat Ceph Storage 9; Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9.
Red Hat lists Red Hat Hardened Images as not affected. Red Hat fixing advisory: RHSA-2026:63164, RHSA-2026:63165, RHSA-2026:65157.
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 8 days ago·verify at source
- grafana13-2-main-13.2.1-0.1.hum1
- grafana13-1-main-13.1.3-0.5.hum1
- grafana12-4-main-12.4.10-0.1.hum1
- RHSA-2026:63164
- RHSA-2026:63165
- RHSA-2026:65157
Official advisory · high-confidence parse· fetched 8 days ago·verify at source
Mitigation checklist
- If not required, disable anonymous access to prevent unauthenticated exploitation of the Alertmanager templates test endpoint. Refer to Grafana’s official documentation for configuration details. To protect against low-privileged authenticated users triggering this flaw, configure a reverse proxy or WAF to block or strictly rate-limit traffic to /api/alertmanager/grafana/config/api/v1/templates/test.
Official advisory · high-confidence parse· fetched 8 days ago·verify at source
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