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Mailchimp API Key

Mailchimp API keys are used to authenticate requests to the Mailchimp API, which provides access to email marketing services, audience management, and campaign analytics. Exposure of these keys is a significant security concern because they can grant unauthorized access to sensitive customer data, email lists, and campaign configurations. Unauthorized access could lead to data breaches, spam campaigns, and reputational damage.

How Does It Look​

Mailchimp API keys can appear in various contexts, such as:

  • Environment variables:

    export MAILCHIMP_API_KEY="1234567890abcdef-us1"

  • Configuration files (JSON, YAML, .env):

    {
      "mailchimp": {
        "apiKey": "1234567890abcdef-us1"
      }
    }

  • Code snippets:

    mailchimp = MailchimpMarketing.Client()
    mailchimp.set_config({
        "api_key": "1234567890abcdef-us1",
        "server": "us1"
    })

  • Connection strings (if applicable): Not typically used in connection strings.

Severity​

  • 🟠 High

This severity level is due to the potential access to sensitive customer data and the ability to manipulate email campaigns. The blast radius includes unauthorized access to email lists, campaign data, and potentially sending unauthorized emails, which could lead to data breaches and reputational harm.

What Can an Attacker Do?​

With immediate access to a Mailchimp API key, an attacker can perform several actions:

  • Access and export email lists (if the credential has read permissions)
  • Send unauthorized emails (if the credential has send permissions)
  • Modify campaign settings (if the credential has write permissions)
  • Access account analytics (if the credential has analytics permissions)

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

Real-World Impact​

Exposure of a Mailchimp API key poses significant business risks:

  • Data Exposure: Email addresses and associated metadata (if the credential has read access to lists)
  • Financial Loss: Costs associated with unauthorized email sends and potential fines (if billing/resource creation is permitted)
  • Operational Disruption: Unauthorized changes to campaigns could disrupt marketing efforts (if the attacker has modify permissions)
  • Reputational Damage: Loss of customer trust due to spam or unauthorized emails

In a worst-case scenario, the exposure could lead to a large-scale data breach, affecting customer trust and resulting in long-term reputational damage.

Prerequisites for Exploitation​

  • Network access requirements: Internet access to reach Mailchimp API endpoints
  • Additional context needed: Knowledge of the Mailchimp account structure and API usage
  • Rate limits or restrictions: Mailchimp API rate limits may slow down exploitation but not prevent it

How to Verify If It's Active​

To verify if a Mailchimp API key is active, use the following command:

curl -X GET "https://<dc>.api.mailchimp.com/3.0/" \
--user "anystring:[API_KEY]"

Valid credential response: A successful response will return account details in JSON format.

Invalid/expired credential response: An error message indicating authentication failure or invalid API key.

Detection Patterns​

Common Variable Names:

  • MAILCHIMP_API_KEY
  • MC_API_KEY
  • MAILCHIMP_KEY
  • API_KEY_MAILCHIMP
  • MC_KEY
  • MAILCHIMP_SECRET

File Locations:

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

Regex Pattern:

[a-f0-9]{32}-us[0-9]{1,2}

Remediation Steps​

  1. Revoke immediately - Go to Mailchimp Account > Extras > API Keys and delete the compromised key.
  2. Audit access logs - Review Mailchimp account activity logs for unauthorized access or actions 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 Mailchimp and ensure it has 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 Mailchimp and require two-factor authentication for account access.
  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​