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SendGrid API Key

SendGrid API keys are used to authenticate requests to SendGrid's email delivery service, which allows applications to send emails programmatically. These keys are critical for maintaining secure communication between your application and SendGrid's servers. Exposure of a SendGrid API key can lead to unauthorized access to your email sending capabilities, potentially resulting in spam or phishing attacks being sent from your domain, damaging your reputation and trustworthiness.

How Does It Look​

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

  • Environment variables:

    export SENDGRID_API_KEY='SG.xxxxxxxx.yyyyyyyy.zzzzzzzz'

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

    {
      "sendgrid": {
        "apiKey": "SG.xxxxxxxx.yyyyyyyy.zzzzzzzz"
      }
    }
    sendgrid:
      apiKey: SG.xxxxxxxx.yyyyyyyy.zzzzzzzz

  • Code snippets:

    import sendgrid
    sg = sendgrid.SendGridAPIClient(api_key='SG.xxxxxxxx.yyyyyyyy.zzzzzzzz')

Severity​

  • 🔴 Critical

The severity of a SendGrid API key exposure is critical because it grants full access to your email sending capabilities. An attacker could send emails on your behalf, potentially leading to spam or phishing attacks. The blast radius includes potential blacklisting of your domain and damage to your brand's reputation.

What Can an Attacker Do?​

With immediate access to a SendGrid API key, an attacker can:

  • Send unauthorized emails: Send spam or phishing emails from your domain (if the key has full send permissions).
  • Access email logs: View email activity and logs (if logging permissions are enabled).
  • Modify email templates: Change or delete email templates (if template management permissions are granted).
  • Access account settings: Alter account configurations (if the key has account management permissions).

An attacker could escalate their access by using the compromised key to gather information about your email infrastructure, potentially leading to further attacks on your systems or lateral movement within your network.

Real-World Impact​

Exposure of a SendGrid API key poses significant business risks:

  • Data Exposure: Email content and recipient information (if the key has access to email logs).
  • Financial Loss: Costs associated with sending unauthorized emails and potential fines (if billing permissions are enabled).
  • Operational Disruption: Email services could be disrupted or blacklisted (if the attacker sends spam).
  • Reputational Damage: Loss of trust and credibility with customers and partners.

In the worst-case scenario, your domain could be blacklisted, severely impacting your ability to communicate with customers and partners, leading to long-term reputational and financial damage.

Prerequisites for Exploitation​

  • Network access requirements: None, as the API is accessible over the internet.
  • Additional context needed: Knowledge of the SendGrid API endpoints.
  • Rate limits or restrictions: SendGrid may have rate limits, but these can be circumvented by spreading requests over time.

How to Verify If It's Active​

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

curl -X GET "https://api.sendgrid.com/v3/user/account" -H "Authorization: Bearer [API_KEY]"

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

Invalid/expired credential response: An error message indicating unauthorized access or invalid credentials.

Detection Patterns​

Common Variable Names:

  • SENDGRID_API_KEY
  • SG_API_KEY
  • SENDGRID_KEY
  • SG_KEY
  • API_KEY_SENDGRID
  • SENDGRID_SECRET

File Locations:

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

Regex Pattern:

SG\.[A-Za-z0-9\-_]{22}\.[A-Za-z0-9\-_]{43}

Remediation Steps​

  1. Revoke immediately - Go to SendGrid Dashboard > Settings > API Keys and delete the compromised key.
  2. Audit access logs - Review SendGrid activity logs for unauthorized email sends or changes during the exposure window.
  3. Assess blast radius - Identify all systems, applications, and environments that used the exposed API key.
  4. Rotate credential - Create a new API key in SendGrid 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 whitelisting in SendGrid 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 API key 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​