{"id":"CVE-2026-81938","kind":"vulnerability","aliases":["CVE-2026-81938"],"related":[],"title":"Grafana Mimir: Alertmanager tenants can read arbitrary files from the host","description":"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.\n\nExploitation 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.","summary":"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.","severity":"high","cvss":7.7,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["alertmanager","grafana-enterprise-metrics","grafana-mimir","infrastructure","self-hosted"],"not_affected_if":["no-untrusted-alertmanager-config","single-tenant"],"how_to_check":"- Check the Mimir version with `mimir --version` and confirm whether it is below `3.1.7` or `3.2.2`.\n- For Grafana Enterprise Metrics, check whether the version is between `2.11` and `2.16`, or below `2.17.20`.\n- Review which tenants are allowed to upload Alertmanager configurations.","fix":"- Grafana Mimir: upgrade to `3.1.7` or `3.2.2`.\n- Grafana Enterprise Metrics: upgrade to `2.17.20` or later; versions `2.11` through `2.16` will not receive a fix.","status":"open","published_at":"2026-11-02T19:18:11+00:00","first_seen_at":"2026-11-02T19:24:40+00:00","triaged_at":"2026-11-02T19:26:37+00:00","updated_at":"2026-11-02T19:26:37+00:00","category":"infrastructure","confidence":0.7,"triage":{"at":"2026-11-02T19:26:37Z","model":"claude-haiku-5-5","output":{"fix":"- Grafana Mimir: upgrade to `3.1.7` or `3.2.2`.\n- Grafana Enterprise Metrics: upgrade to `2.17.20` or later; versions `2.11` through `2.16` will not receive a fix.","reach":"network","title":"Grafana Mimir: Alertmanager tenants can read arbitrary files from the host","impact":"high","summary":"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.","audience":"businesses","category":"infrastructure","nicknames":[],"who_cares":["grafana-mimir","grafana-enterprise-metrics","alertmanager","self-hosted"],"confidence":0.7,"fix_status":"available","needs_auth":"yes","worst_case":"tenant reads arbitrary files from the Alertmanager host, including credentials","analyst_note":"Confirm the exact affected version ranges and whether Grafana Cloud is affected via the Grafana security advisory.","how_to_check":"- Check the Mimir version with `mimir --version` and confirm whether it is below `3.1.7` or `3.2.2`.\n- For Grafana Enterprise Metrics, check whether the version is between `2.11` and `2.16`, or below `2.17.20`.\n- Review which tenants are allowed to upload Alertmanager configurations.","plain_action":"","needs_analyst":true,"not_affected_if":["single-tenant","no-untrusted-alertmanager-config"],"who_is_affected":"Teams running Grafana Mimir or Grafana Enterprise Metrics with multiple tenants who can upload their own Alertmanager configurations are exposed. Single-tenant deployments or setups where no one outside operators can write Alertmanager configs are less exposed. unknown whether managed Grafana Cloud is affected.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"high"},"cost_usd":0.000799,"input_tokens":4355,"output_tokens":727,"severity_before":"high"},"analyst_status":null,"analyst_note":null,"search":"'-2026':13A,16A '-81938':14A,17A '2.11':206C '2.16':208C '2.17.20':204C,221C '3.1.7':198C '3.2.2':200C 'affect':191C 'alertmanag':3A,18B,33B,57B,75B,80C,113C,125C,132C,163C,178C 'also':190C 'arbitrari':7A 'auth':45B 'basic':44B 'basic_auth.username':105C 'block':36B 'built':186C 'check':93C 'client':99C 'configur':38B,58B,72B,86C,114C,169C,179C 'content':63B,139C 'control':68B,155C 'could':115C,158C 'cover':96C 'credenti':70B,167C 'cve':12A,15A 'direct':148C 'enterpris':21B,181C 'exploit':170C 'expos':69B 'field':47B,118C 'file':8A,50B,62B,90C,103C,106C,121C,135C,160C 'fix':195C,213C 'gem':183C,203C,205C 'grafana':1A,20B,24B,30B,77C,180C 'grafana-enterprise-metr':19B 'grafana-mimir':23B 'header':42B,102C,142C 'host':11A,29B,76B,164C 'http':41B,98C,101C 'infrastructur':26B 'issu':193C 'later':223C 'local':49B,89C 'metric':22B,182C 'mimir':2A,25B,31B,78C,188C,197C 'notif':129C 'outgo':145C 'permiss':172C 'permit':109C 'point':40B,116C 'process':126C 'put':137C 'read':6A,133C,159C 'readabl':122C 'receiv':85C,150C,211C 'refer':88C 'reject':81C 'request':146C 'requir':171C 'self':28B 'self-host':27B 'send':60B 'sent':131C 'server':66B,153C 'set':100C 'suppli':84C 'tenant':4A,37B,52B,83C,108C,157C,176C 'tenant-suppli':82C 'two':97C 'upgrad':219C 'upload':111C 'user':214C 'usernam':46B 'version':217C 'write':55B,174C","nicknames":[],"mitigations":[],"who_affected":"Teams running Grafana Mimir or Grafana Enterprise Metrics with multiple tenants who can upload their own Alertmanager configurations are exposed. Single-tenant deployments or setups where no one outside operators can write Alertmanager configs are less exposed. unknown whether managed Grafana Cloud is affected.","tier":"B","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"businesses","plain_action":null,"pkeys":[],"ptokens":[],"labels":["alertmanager","grafanaenterprisemetrics","grafanamimir","high","infrastructure","selfhosted"],"first_public_at":"2026-11-02T19:18:11+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N","cvss_by":"security@grafana.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-81938","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-81938","severity":null,"published_at":"2026-11-02T19:18:11+00:00","modified_at":"2026-11-02T19:18:11+00:00","first_seen_at":"2026-11-02T19:24:40+00:00","fetched_at":"2026-11-02T19:24:40+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}