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Mailgun API Key

Mailgun is a popular email automation service that provides APIs for sending, receiving, and tracking emails. The Mailgun API Key is a credential used to authenticate requests to Mailgun's services, allowing applications to interact with Mailgun's email infrastructure. Exposure of this API Key is a significant security concern because it can grant unauthorized access to email sending capabilities, potentially leading to spam, phishing attacks, or unauthorized access to sensitive email data.

How Does It Look​

Mailgun API Keys can appear in various contexts, such as:

  • Environment variables:

    export MAILGUN_API_KEY="key-XXXXXXXXXXXXXXXXXXXXXX"

  • Configuration files (JSON):

    {
      "mailgun": {
        "apiKey": "key-XXXXXXXXXXXXXXXXXXXXXX"
      }
    }

  • Code snippets:

    import requests

    api_key = "key-XXXXXXXXXXXXXXXXXXXXXX"
    response = requests.get("https://api.mailgun.net/v3/domains", auth=("api", api_key))

  • Connection strings:

    https://api:key-XXXXXXXXXXXXXXXXXXXXXX@api.mailgun.net/v3/domains

Severity​

  • 🟠 High

The Mailgun API Key provides access to email sending and receiving capabilities. If exposed, an attacker can send emails on behalf of the organization, potentially leading to spam or phishing attacks. The blast radius includes unauthorized access to email data and potential misuse of email sending quotas, impacting both security and reputation.

What Can an Attacker Do?​

With immediate access to a Mailgun API Key, an attacker can:

  • Send unauthorized emails: Send spam or phishing emails (if the key has sending permissions).
  • Access email logs: View email logs and metadata (if the key has read permissions).
  • Modify email settings: Change email routing or settings (if the key has write permissions).
  • Consume email quotas: Exhaust email sending limits, impacting legitimate email operations.

An attacker can escalate their access by using the API Key to gather more information about the email infrastructure, potentially leading to further exploitation or lateral movement within the organization's systems.

Real-World Impact​

Exposure of a Mailgun API Key poses significant business risks:

  • Data Exposure: Email metadata and logs (if the key has read access to email data).
  • Financial Loss: Costs associated with sending unauthorized emails (if billing is linked to email usage).
  • Operational Disruption: Legitimate email services could be disrupted (if sending limits are exhausted).
  • Reputational Damage: Trust and brand impact due to spam or phishing emails sent from the organization's domain.

In worst-case scenarios, the exposure could lead to blacklisting of the organization's email domain, severely impacting communication capabilities.

Prerequisites for Exploitation​

  • Network access requirements: Internet access to Mailgun's API endpoints.
  • Additional context needed: Knowledge of the Mailgun domain or account details.
  • Rate limits or restrictions: Mailgun's API rate limits may apply, but they do not prevent initial exploitation.

How to Verify If It's Active​

To verify if a Mailgun API Key is active, use the following command:

curl -s --user "api:[API_KEY]" https://api.mailgun.net/v3/domains

Valid credential response: A list of domains associated with the Mailgun account.

Invalid/expired credential response: An error message indicating authentication failure.

Detection Patterns​

Common Variable Names:

  • MAILGUN_API_KEY
  • MG_API_KEY
  • MAILGUN_KEY
  • API_KEY_MAILGUN
  • MG_KEY
  • MAILGUN_SECRET

File Locations:

  • .env
  • config.json
  • settings.yaml
  • mailgun_config.py
  • appsettings.json

Regex Pattern:

key-[a-zA-Z0-9]{32}

Remediation Steps​

  1. Revoke immediately - Go to Mailgun Dashboard > Security > API Keys and delete the compromised key.
  2. Audit access logs - Review Mailgun logs for unauthorized email sending or access during the exposure window.
  3. Assess blast radius - Identify all systems, applications, and environments that used the exposed API Key.
  4. Rotate credential - Generate a new API Key in the Mailgun Dashboard with least-privilege permissions.
  5. Update dependent systems - Deploy the new API Key to all applications and update CI/CD pipelines securely.
  6. Harden access controls - Enable IP allowlisting in Mailgun and require TLS connections for API requests.
  7. Implement secrets management - Migrate API Keys to a secrets manager (HashiCorp Vault, AWS Secrets Manager) to prevent hardcoding.
  8. Add detection controls - Set up pre-commit hooks and repository scanning to catch credential leaks before they reach production.

Credential exposures often go undetected for extended periods, increasing the window for exploitation. As a long-term strategy, plan to establish an internal process or engage an external vendor for continuous external exposure monitoring. This helps identify leaked secrets across public repositories, paste sites, dark web forums, and other external sources before attackers can leverage them. Proactive detection and rapid response are essential to minimizing the impact of credential leaks.

References​