BUG: Heartbeat goes into a state of writing billions of error logs

Elastic Stack Beats
1

Hello,

Since version 9.3.1 of our deployment, that now uses 9.4.2 we observe the following behaviour in Heartbeat:

  • Heartbeat is deployed with Elastic Agent, that controlled by the ECK operator
  • Heartbeat is used for running about 200 synthetics tests, which are configured with Kibana
  • After a few days of normal operation, Heartbeat starts writing billions of log entries (apx. 500 entries/second) with the following content:
      {
  "log.level": "warn",
  "component.dataset": "elastic_agent.heartbeat",
  "component.binary": "heartbeat",
  "log.source": "synthetics/browser-default",
  "@timestamp": "2026-06-04T09:36:54.289Z",
  "component.id": "synthetics/browser-default",
  "message": "error decoding json for test json results: unexpected EOF",
  "component.type": "synthetics/browser",
  "log.origin": {
    "file.line": 223,
    "file.name": "synthexec/synthexec.go",
    "function": "github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec.runCmd.func3"
  },
  "ecs.version": "1.6.0",
  "service.name": "heartbeat"
}
  • CPU consumption becomes twice as much
  • Restart of the Elastic Agent's container remediates the issue immediately, without any configuration adjustments

On Github, I found the same issue had reported, but had not been answered: Heartbeat Browser Monitor can lock up with EOF error · Issue #34560 · elastic/beats · GitHub