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Logstash is used to gather, transform, and dispatch logs through a system known as pipelines. These pipelines are made up of input, filter, and output stages. An interesting aspect arises when Logstash operates on a compromised machine.

Pipeline Configuration

Pipelines are configured in the file /etc/logstash/pipelines.yml, which lists the locations of the pipeline configurations:

# Define your pipelines here. Multiple pipelines can be defined.
# For details on multiple pipelines, refer to the documentation:
# https://www.elastic.co/guide/en/logstash/current/multiple-pipelines.html

- pipeline.id: main
  path.config: "/etc/logstash/conf.d/*.conf"
- pipeline.id: example
  path.config: "/usr/share/logstash/pipeline/1*.conf"
  pipeline.workers: 6

This file reveals where the .conf files, containing pipeline configurations, are located. When employing an Elasticsearch output module, it's common for pipelines to include Elasticsearch credentials, which often possess extensive privileges due to Logstash's need to write data to Elasticsearch. Wildcards in configuration paths allow Logstash to execute all matching pipelines in the designated directory.

Privilege Escalation via Writable Pipelines

To attempt privilege escalation, first identify the user under which the Logstash service is running, typically the logstash user. Ensure you meet one of these criteria:

  • Possess write access to a pipeline .conf file or

  • The /etc/logstash/pipelines.yml file uses a wildcard, and you can write to the target folder

Additionally, one of these conditions must be fulfilled:

  • Capability to restart the Logstash service or

  • The /etc/logstash/logstash.yml file has config.reload.automatic: true set

Given a wildcard in the configuration, creating a file that matches this wildcard allows for command execution. For instance:

input {
  exec {
    command => "whoami"
    interval => 120

output {
  file {
    path => "/tmp/output.log"
    codec => rubydebug

Here, interval determines the execution frequency in seconds. In the given example, the whoami command runs every 120 seconds, with its output directed to /tmp/output.log.

With config.reload.automatic: true in /etc/logstash/logstash.yml, Logstash will automatically detect and apply new or modified pipeline configurations without needing a restart. If there's no wildcard, modifications can still be made to existing configurations, but caution is advised to avoid disruptions.


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