---
id: "CVE-2026-81938"
kind: "vulnerability"
aliases: ["CVE-2026-81938"]
title: "Grafana Mimir: Alertmanager tenants can read arbitrary files from the host"
severity: "high"
cvss: 7.7
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["alertmanager", "grafana-enterprise-metrics", "grafana-mimir", "infrastructure", "self-hosted"]
not_affected_if: ["no-untrusted-alertmanager-config", "single-tenant"]
published_at: "2026-11-02T19:18:11+00:00"
first_seen_at: "2026-11-02T19:24:40+00:00"
updated_at: "2026-11-02T19:26:37+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81938", "first_seen_at": "2026-11-02T19:24:40+00:00"}]
---

# Grafana Mimir: Alertmanager tenants can read arbitrary files from the host

Grafana Mimir's Alertmanager did not block tenant configurations that point HTTP header or basic auth username fields at local files. A tenant who can write an Alertmanager configuration can send those file contents to a server they control, exposing credentials and configuration from the Alertmanager host.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the Mimir version with `mimir --version` and confirm whether it is below `3.1.7` or `3.2.2`.
- For Grafana Enterprise Metrics, check whether the version is between `2.11` and `2.16`, or below `2.17.20`.
- Review which tenants are allowed to upload Alertmanager configurations.

## Fix

- Grafana Mimir: upgrade to `3.1.7` or `3.2.2`.
- Grafana Enterprise Metrics: upgrade to `2.17.20` or later; versions `2.11` through `2.16` will not receive a fix.

## Timeline

- 2026-11-02T19:24:40+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-81938

## From the sources

Grafana Mimir's Alertmanager rejects tenant-supplied receiver configurations that reference local files, but the check did not cover two HTTP client settings: http_headers.<name>.files and basic_auth.username_file. A tenant permitted to upload an Alertmanager configuration could point these fields at any file readable by the Alertmanager process. When a notification was sent, Alertmanager read the file and put its contents in a header of the outgoing request. By directing the receiver at a server they control, the tenant could read files from the Alertmanager host, such as credentials and configuration.

Exploitation requires permission to write the tenant's Alertmanager configuration. Grafana Enterprise Metrics (GEM), which is built on Mimir, is also affected. The issue is fixed in Mimir 3.1.7 and 3.2.2 and in GEM 2.17.20. GEM 2.11 through 2.16 will not receive a fix; users on those versions should upgrade to 2.17.20 or later.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_