What is CVE-2026-81938 in Grafana Mimir?

CVE-2026-81938 is a high-severity security vulnerability rated with a CVSS score of 7.7 that affects Grafana Mimir and Grafana Enterprise Metrics. The core of the problem lies in the Alertmanager component, which failed to properly sanitize specific HTTP client settings against local path traversal patterns. Similar vulnerabilities often appear in complex data processing pipelines, much like how developers review other security bugs such as vulnerabilidad crítica CVE-2026-115905 or investigate local privileges like CVE-2026-58946: Vulnerabilidad Crítica in enterprise setups.

Although Alertmanager successfully rejected general receiver configurations pointing to local files, it omitted checks for http_headers.<name>.files and basic_auth.username_file settings. This oversight opened a path classified under CWE-22, allowing unauthorized data access vectors if permissions were mishandled.

Who is affected and how does the exploit work?

The vulnerability impacts any deployment where untrusted tenants possess permission to upload custom Alertmanager receiver configurations. By targeting vulnerable parameters, an attacker could point these fields to any file readable by the Alertmanager process on the host machine. When notifications fired, Alertmanager automatically ingested the file contents and appended them to outbound HTTP headers, leaking sensitive configuration files and system credentials to an external server controlled by the attacker.

Grafana Enterprise Metrics (GEM), built directly on top of Mimir, inherits this exact vulnerability architecture. For broader context on permission management bugs in software stacks, security teams frequently compare these flaws with incidents like CVE-2026-58957: Vulnerabilidad Crítica affecting local authorization boundaries.

How do I protect my system against this vulnerability?

Mitigating CVE-2026-81938 requires immediate action from system administrators and DevOps engineers managing multi-tenant monitoring clusters. Reviewing tenant permissions and applying the official vendor patches is critical to block potential data exfiltration attempts.

Follow these steps to secure your environment:

  1. Identify all active instances of Grafana Mimir and Grafana Enterprise Metrics across your cluster infrastructure.
  2. Upgrade Grafana Mimir to version 3.1.7 or version 3.2.2 immediately.
  3. Upgrade Grafana Enterprise Metrics (GEM) to version 2.17.20.
  4. Verify that older GEM versions between 2.11 and 2.16 are updated, keeping in mind they will not receive direct backports and must move to 2.17.20 or newer.
  5. Audit tenant configurations for any unauthorized use of http_headers or basic_auth file references.