Splunk Email Security Risk: How to Fix allowedDomainList Warning in alert_actions.conf

Splunk allowedDomainList configuration

Learn how to fix the Splunk allowedDomainList security warning in alert_actions.conf. Prevent unauthorized alert emails and protect sensitive data by configuring domain restrictions correctly.

Table of Contents

🔈Introduction

Splunk administrators often encounter a critical security warning:

⚠️ Found an empty value for ‘allowedDomainList’ in the alert_actions.conf configuration file.

This alert points to a significant security misconfiguration — one that could allow users to send sensitive data outside your organization via email alerts. This article provides a comprehensive guide to understanding, mitigating, and properly configuring the allowedDomainList setting to secure your Splunk instance.

✅ What Is allowedDomainList in Splunk?

The allowedDomainList setting in Splunk’s alert_actions.conf file specifies which email domains are allowed to receive alert messages. If not configured, Splunk permits any domain, creating a potential data exfiltration path.

🚨 Why This Matters

Leaving allowedDomainList empty means:

  • No domain restrictions on email alerts
  • Users can send search results externally (e.g., to Gmail, Outlook.com)
  • Risk of data leaks and compliance violations
  • Easy target for insider threats

🔍 Security Warning Message Breakdown

Here’s the actual warning message administrators might see:

				
					Security risk warning: Found an empty value for 'allowedDomainList' in the alert_actions.conf configuration file. If you do not configure this setting, then users can send email alerts with search results to any domain.
Splunk allowedDomainList configuration

Photo by admingeek from Infotechys

✅ How to Fix the allowedDomainList Warning

You can resolve this misconfiguration in two ways:

🔄 Method 1: Configure in alert_actions.conf (CLI)

First, switch to the root user and define the SPLUNK_HOME environment variable before executing the following commands:

				
					sudo -i
				
			
				
					export SPLUNK_HOME=`cat /etc/passwd | grep splunk | awk -F: '{ print $6 }'`

Navigate to your Splunk configuration directory:

				
					cd $SPLUNK_HOME/etc/system/local

Open or create the alert_actions.conf file:

				
					vim alert_actions.conf

Add your organization’s approved email domains:

				
					[email]
allowedDomainList = yourcompany.com, partnercompany.com

Save the file and restart Splunk:

				
					$SPLUNK_HOME/bin/splunk restart
				
			
				
					Stopping splunkd...
Shutting down.  Please wait, as this may take a few minutes.
.............
Stopping splunk helpers...

Done.
...omitted for brevity...

🔄 Method 2: Configure in Splunk Web UI

  • Go to:
    Settings > Server Settings > Email Settings
  • Scroll down to the “Email Domains” section.
  • Enter approved domains, separated by commas (e.g., yourcompany.com, vendor.org).
  • Click Save.
Splunk allowedDomainList configuration

Photo by admingeek from Infotechys

💡NOTE: Splunk Web changes take effect immediately, no restart needed.

🧪 Verifying the Configuration

After applying your settings, test the domain restriction:

  • Try to send a test alert to an unauthorized domain (e.g., gmail.com).
  • Splunk should reject the alert and return an error:
				
					Error sending email: Email domain not allowed by 'allowedDomainList'.

📊 Comparison: Configured vs. Unconfigured

Configuration StateBehaviorRisk Level
allowedDomainList EmptyUsers can send alerts to any domain🔴 High
Configured with DomainsOnly specified domains can receive alerts🟢 Low
Misconfigured (typo, etc.)Could block legitimate alert delivery or allow leakage🟡 Medium–High

🛡️ Security Best Practices

To ensure your Splunk environment remains secure, follow these practices:

▶️ Enforce Domain Restrictions

Restrict emails to internal and trusted partner domains.

▶️ Use RBAC (Role-Based Access Control)

Limit who can create or modify alerts. Assign alerting permissions to only trusted roles.

▶️ Log and Audit Alert Actions

Enable auditing of alert actions. Monitor who sends what alerts to which domains.

▶️ Restrict Free-Text Entry of Email Addresses

Avoid open input fields where users can enter arbitrary email addresses.

▶️ Update alert_actions.conf on All Cluster Nodes

In a clustered deployment (SHC), sync this configuration across all search heads.

				
					# Example: Deploy to all search heads using deployment server or Ansible
scp alert_actions.conf user@searchhead1:/opt/splunk/etc/system/local/
scp alert_actions.conf user@searchhead2:/opt/splunk/etc/system/local/

🖥️ Example: Full alert_actions.conf Configuration

				
					[email]
mailserver = smtp.yourcompany.com
from = splunk-alerts@yourcompany.com
use_tls = 1
allowedDomainList = yourcompany.com, trustedpartner.com
sendemail = 1

💬 Frequently Asked Questions (FAQ)

❓ What happens if I leave allowedDomainList empty?

You open up your Splunk instance to unrestricted email alerting, which can result in data exfiltration.

❓ Can I use wildcards in domain names?

No. allowedDomainList does not support wildcards like *.yourcompany.com. Use specific domains only.

❓ How do I block all external emails?

Simply include only your internal domains in the list:

				
					allowedDomainList = yourcompany.com

Any email sent outside of yourcompany.com will be blocked.

❓ Does this apply to all Splunk versions?

Yes, this applies to Splunk Enterprise and Splunk Cloud Platform, although GUI and file locations may vary slightly in managed environments.

🔗 Related Configuration Files

FilePurpose
alert_actions.confDefines alert actions (like email)
authentication.confSets authentication methods and roles
authorize.confRole-based access control
outputs.confSets up data forwarding

🧩 Integrating with Email Gateways

If you use an internal SMTP relay (e.g., Microsoft Exchange or Postfix), make sure to:

  • Restrict relay access to Splunk IPs
  • Block external domains at the MTA level for an added layer of security
Example Postfix rule:
				
					smtpd_recipient_restrictions =
    check_recipient_access hash:/etc/postfix/allowed_domains

📌 Final Thoughts

Ignoring the allowedDomainList setting in Splunk can lead to critical data leakage. Fortunately, fixing this security hole is straightforward. Whether through the Splunk Web interface or direct config file edits, setting this parameter ensures that alert emails only go to trusted domains — maintaining your organization’s privacy, compliance, and security posture.

⏱️ Take the time to review your configurations today. A few lines of config can save you from a major incident tomorrow.

Did you find this article helpful? Your feedback is invaluable to us! Feel free to share this post with those who may benefit, and let us know your thoughts in the comments section below.

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